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Operations on strings (or arrays of characters) are an important part of
many programs. The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings. Many of these functions can also
operate on arbitrary regions of storage; for example, the memcpy
function can be used to copy the contents of any kind of array.
It's fairly common for beginning C programmers to “reinvent the wheel” by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.
For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in strcmp function,
you're less likely to make a mistake. And, since these library
functions are typically highly optimized, your program may run faster
too.
| 5.1 Representation of Strings | Introduction to basic concepts. | |
| 5.2 String and Array Conventions | Whether to use a string function or an arbitrary array function. | |
| 5.3 String Length | Determining the length of a string. | |
| 5.4 Copying and Concatenation | Functions to copy the contents of strings and arrays. | |
| 5.5 String/Array Comparison | Functions for byte-wise and character-wise comparison. | |
| 5.6 Collation Functions | Functions for collating strings. | |
| 5.7 Search Functions | Searching for a specific element or substring. | |
| 5.8 Finding Tokens in a String | Splitting a string into tokens by looking for delimiters. | |
| 5.9 strfry | Function for flash-cooking a string. | |
| 5.10 Trivial Encryption | Obscuring data. | |
| 5.11 Encode Binary Data | Encoding and Decoding of Binary Data. | |
| 5.12 Argz and Envz Vectors | Null-separated string vectors. |
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This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.
A string is an array of char objects. But string-valued
variables are usually declared to be pointers of type char *.
Such variables do not include space for the text of a string; that has
to be stored somewhere else—in an array variable, a string constant,
or dynamically allocated memory (see section Allocating Storage For Program Data). It's up to
you to store the address of the chosen memory space into the pointer
variable. Alternatively you can store a null pointer in the
pointer variable. The null pointer does not point anywhere, so
attempting to reference the string it points to gets an error.
“string” normally refers to multibyte character strings as opposed to
wide character strings. Wide character strings are arrays of type
wchar_t and as for multibyte character strings usually pointers
of type wchar_t * are used.
By convention, a null character, '\0', marks the end of a
multibyte character string and the null wide character,
L'\0', marks the end of a wide character string. For example, in
testing to see whether the char * variable p points to a
null character marking the end of a string, you can write
!*p or *p == '\0'.
A null character is quite different conceptually from a null pointer,
although both are represented by the integer 0.
String literals appear in C program source as strings of
characters between double-quote characters (‘"’) where the initial
double-quote character is immediately preceded by a capital ‘L’
(ell) character (as in L"foo"). In ISO C, string literals
can also be formed by string concatenation: "a" "b" is the
same as "ab". For wide character strings one can either use
L"a" L"b" or L"a" "b". Modification of string literals is
not allowed by the GNU C compiler, because literals are placed in
read-only storage.
Character arrays that are declared const cannot be modified
either. It's generally good style to declare non-modifiable string
pointers to be of type const char *, since this often allows the
C compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.
The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocated size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.
A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.
Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see Introduction to Extended Characters). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.
But since there is no separate interface taking care of these
differences the byte-based string functions are sometimes hard to use.
Since the count parameters of these functions specify bytes a call to
strncpy could cut a multibyte character in the middle and put an
incomplete (and therefore unusable) byte sequence in the target buffer.
To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on wide characters (see section Introduction to Extended Characters). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied.
The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available.
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This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters.
Functions that operate on arbitrary blocks of memory have names
beginning with ‘mem’ and ‘wmem’ (such as memcpy and
wmemcpy) and invariably take an argument which specifies the size
(in bytes and wide characters respectively) of the block of memory to
operate on. The array arguments and return values for these functions
have type void * or wchar_t. As a matter of style, the
elements of the arrays used with the ‘mem’ functions are referred
to as “bytes”. You can pass any kind of pointer to these functions,
and the sizeof operator is useful in computing the value for the
size argument. Parameters to the ‘wmem’ functions must be of type
wchar_t *. These functions are not really usable with anything
but arrays of this type.
In contrast, functions that operate specifically on strings and wide
character strings have names beginning with ‘str’ and ‘wcs’
respectively (such as strcpy and wcscpy) and look for a
null character to terminate the string instead of requiring an explicit
size argument to be passed. (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.) The array arguments and return values for these
functions have type char * and wchar_t * respectively, and
the array elements are referred to as “characters” and “wide
characters”.
In many cases, there are both ‘mem’ and ‘str’/‘wcs’ versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the ‘mem’ functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the ‘str’/‘wcs’ functions, unless you already know the length of the string in advance. The ‘wmem’ functions should be used for wide character arrays with known size.
Some of the memory and string functions take single characters as
arguments. Since a value of type char is automatically promoted
into an value of type int when used as a parameter, the functions
are declared with int as the type of the parameter in question.
In case of the wide character function the situation is similarly: the
parameter type for a single wide character is wint_t and not
wchar_t. This would for many implementations not be necessary
since the wchar_t is large enough to not be automatically
promoted, but since the ISO C standard does not require such a
choice of types the wint_t type is used.
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You can get the length of a string using the strlen function.
This function is declared in the header file ‘string.h’.
The strlen function returns the length of the null-terminated
string s in bytes. (In other words, it returns the offset of the
terminating null character within the array.)
For example,
strlen ("hello, world")
⇒ 12
|
When applied to a character array, the strlen function returns
the length of the string stored there, not its allocated size. You can
get the allocated size of the character array that holds a string using
the sizeof operator:
char string[32] = "hello, world";
sizeof (string)
⇒ 32
strlen (string)
⇒ 12
|
But beware, this will not work unless string is the character array itself, not a pointer to it. For example:
char string[32] = "hello, world";
char *ptr = string;
sizeof (string)
⇒ 32
sizeof (ptr)
⇒ 4 /* (on a machine with 4 byte pointers) */
|
This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.
It must also be noted that for multibyte encoded strings the return
value does not have to correspond to the number of characters in the
string. To get this value the string can be converted to wide
characters and wcslen can be used or something like the following
code can be used:
/* The input is in |
This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.
The wide character equivalent is declared in ‘wchar.h’.
The wcslen function is the wide character equivalent to
strlen. The return value is the number of wide characters in the
wide character string pointed to by ws (this is also the offset of
the terminating null wide character of ws).
Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters.
This function was introduced in Amendment 1 to ISO C90.
The strnlen function returns the length of the string s in
bytes if this length is smaller than maxlen bytes. Otherwise it
returns maxlen. Therefore this function is equivalent to
(strlen (s) < n ? strlen (s) : maxlen) but it
is more efficient and works even if the string s is not
null-terminated.
char string[32] = "hello, world";
strnlen (string, 32)
⇒ 12
strnlen (string, 5)
⇒ 5
|
This function is a GNU extension and is declared in ‘string.h’.
wcsnlen is the wide character equivalent to strnlen. The
maxlen parameter specifies the maximum number of wide characters.
This function is a GNU extension and is declared in ‘wchar.h’.
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You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The ‘str’ and ‘mem’ functions are declared in the header file ‘string.h’ while the ‘wstr’ and ‘wmem’ functions are declared in the file ‘wchar.h’.
A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.
Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.
All functions that have problems copying between overlapping arrays are
explicitly identified in this manual. In addition to functions in this
section, there are a few others like sprintf (see section Formatted Output Functions) and scanf (see section Formatted Input Functions).
The memcpy function copies size bytes from the object
beginning at from into the object beginning at to. The
behavior of this function is undefined if the two arrays to and
from overlap; use memmove instead if overlapping is possible.
The value returned by memcpy is the value of to.
Here is an example of how you might use memcpy to copy the
contents of an array:
struct foo *oldarray, *newarray; int arraysize; … memcpy (new, old, arraysize * sizeof (struct foo)); |
The wmemcpy function copies size wide characters from the object
beginning at wfrom into the object beginning at wto. The
behavior of this function is undefined if the two arrays wto and
wfrom overlap; use wmemmove instead if overlapping is possible.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
}
|
The value returned by wmemcpy is the value of wto.
This function was introduced in Amendment 1 to ISO C90.
The mempcpy function is nearly identical to the memcpy
function. It copies size bytes from the object beginning at
from into the object pointed to by to. But instead of
returning the value of to it returns a pointer to the byte
following the last written byte in the object beginning at to.
I.e., the value is ((void *) ((char *) to + size)).
This function is useful in situations where a number of objects shall be copied to consecutive memory positions.
void *
combine (void *o1, size_t s1, void *o2, size_t s2)
{
void *result = malloc (s1 + s2);
if (result != NULL)
mempcpy (mempcpy (result, o1, s1), o2, s2);
return result;
}
|
This function is a GNU extension.
The wmempcpy function is nearly identical to the wmemcpy
function. It copies size wide characters from the object
beginning at wfrom into the object pointed to by wto. But
instead of returning the value of wto it returns a pointer to the
wide character following the last written wide character in the object
beginning at wto. I.e., the value is wto + size.
This function is useful in situations where a number of objects shall be copied to consecutive memory positions.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
|
This function is a GNU extension.
memmove copies the size bytes at from into the
size bytes at to, even if those two blocks of space
overlap. In the case of overlap, memmove is careful to copy the
original values of the bytes in the block at from, including those
bytes which also belong to the block at to.
The value returned by memmove is the value of to.
wmemmove copies the size wide characters at wfrom
into the size wide characters at wto, even if those two
blocks of space overlap. In the case of overlap, memmove is
careful to copy the original values of the wide characters in the block
at wfrom, including those wide characters which also belong to the
block at wto.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
|
The value returned by wmemmove is the value of wto.
This function is a GNU extension.
This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.
This function copies the value of c (converted to an
unsigned char) into each of the first size bytes of the
object beginning at block. It returns the value of block.
This function copies the value of wc into each of the first size wide characters of the object beginning at block. It returns the value of block.
This copies characters from the string from (up to and including
the terminating null character) into the string to. Like
memcpy, this function has undefined results if the strings
overlap. The return value is the value of to.
This copies wide characters from the string wfrom (up to and
including the terminating null wide character) into the string
wto. Like wmemcpy, this function has undefined results if
the strings overlap. The return value is the value of wto.
This function is similar to strcpy but always copies exactly
size characters into to.
If the length of from is more than size, then strncpy
copies just the first size characters. Note that in this case
there is no null terminator written into to.
If the length of from is less than size, then strncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is specified by the ISO C standard.
The behavior of strncpy is undefined if the strings overlap.
Using strncpy as opposed to strcpy is a way to avoid bugs
relating to writing past the end of the allocated space for to.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, strncpy will
waste a considerable amount of time copying null characters.
This function is similar to wcscpy but always copies exactly
size wide characters into wto.
If the length of wfrom is more than size, then
wcsncpy copies just the first size wide characters. Note
that in this case there is no null terminator written into wto.
If the length of wfrom is less than size, then
wcsncpy copies all of wfrom, followed by enough null wide
characters to add up to size wide characters in all. This
behavior is rarely useful, but it is specified by the ISO C
standard.
The behavior of wcsncpy is undefined if the strings overlap.
Using wcsncpy as opposed to wcscpy is a way to avoid bugs
relating to writing past the end of the allocated space for wto.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, wcsncpy will
waste a considerable amount of time copying null wide characters.
This function copies the null-terminated string s into a newly
allocated string. The string is allocated using malloc; see
Unconstrained Allocation. If malloc cannot allocate space
for the new string, strdup returns a null pointer. Otherwise it
returns a pointer to the new string.
This function copies the null-terminated wide character string ws
into a newly allocated string. The string is allocated using
malloc; see Unconstrained Allocation. If malloc
cannot allocate space for the new string, wcsdup returns a null
pointer. Otherwise it returns a pointer to the new wide character
string.
This function is a GNU extension.
This function is similar to strdup but always copies at most
size characters into the newly allocated string.
If the length of s is more than size, then strndup
copies just the first size characters and adds a closing null
terminator. Otherwise all characters are copied and the string is
terminated.
This function is different to strncpy in that it always
terminates the destination string.
strndup is a GNU extension.
This function is like strcpy, except that it returns a pointer to
the end of the string to (that is, the address of the terminating
null character to + strlen (from)) rather than the beginning.
For example, this program uses stpcpy to concatenate ‘foo’
and ‘bar’ to produce ‘foobar’, which it then prints.
#include <string.h>
#include <stdio.h>
int
main (void)
{
char buffer[10];
char *to = buffer;
to = stpcpy (to, "foo");
to = stpcpy (to, "bar");
puts (buffer);
return 0;
}
|
This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.
Its behavior is undefined if the strings overlap. The function is declared in ‘string.h’.
This function is like wcscpy, except that it returns a pointer to
the end of the string wto (that is, the address of the terminating
null character wto + strlen (wfrom)) rather than the beginning.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
The behavior of wcpcpy is undefined if the strings overlap.
wcpcpy is a GNU extension and is declared in ‘wchar.h’.
This function is similar to stpcpy but copies always exactly
size characters into to.
If the length of from is more then size, then stpncpy
copies just the first size characters and returns a pointer to the
character directly following the one which was copied last. Note that in
this case there is no null terminator written into to.
If the length of from is less than size, then stpncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is implemented to be useful in contexts where this behavior of the
strncpy is used. stpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
Its behavior is undefined if the strings overlap. The function is declared in ‘string.h’.
This function is similar to wcpcpy but copies always exactly
wsize characters into wto.
If the length of wfrom is more then size, then
wcpncpy copies just the first size wide characters and
returns a pointer to the wide character directly following the last
non-null wide character which was copied last. Note that in this case
there is no null terminator written into wto.
If the length of wfrom is less than size, then wcpncpy
copies all of wfrom, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is implemented to be useful in contexts where this behavior of the
wcsncpy is used. wcpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
Its behavior is undefined if the strings overlap.
wcpncpy is a GNU extension and is declared in ‘wchar.h’.
This macro is similar to strdup but allocates the new string
using alloca instead of malloc (see section Automatic Storage with Variable Size). This means of course the returned string has the same
limitations as any block of memory allocated using alloca.
For obvious reasons strdupa is implemented only as a macro;
you cannot get the address of this function. Despite this limitation
it is a useful function. The following code shows a situation where
using malloc would be a lot more expensive.
#include <paths.h>
#include <string.h>
#include <stdio.h>
const char path[] = _PATH_STDPATH;
int
main (void)
{
char *wr_path = strdupa (path);
char *cp = strtok (wr_path, ":");
while (cp != NULL)
{
puts (cp);
cp = strtok (NULL, ":");
}
return 0;
}
|
Please note that calling strtok using path directly is
invalid. It is also not allowed to call strdupa in the argument
list of strtok since strdupa uses alloca
(see section Automatic Storage with Variable Size) can interfere with the parameter
passing.
This function is only available if GNU CC is used.
This function is similar to strndup but like strdupa it
allocates the new string using alloca
see section Automatic Storage with Variable Size. The same advantages and limitations
of strdupa are valid for strndupa, too.
This function is implemented only as a macro, just like strdupa.
Just as strdupa this macro also must not be used inside the
parameter list in a function call.
strndupa is only available if GNU CC is used.
The strcat function is similar to strcpy, except that the
characters from from are concatenated or appended to the end of
to, instead of overwriting it. That is, the first character from
from overwrites the null character marking the end of to.
An equivalent definition for strcat would be:
char *
strcat (char *restrict to, const char *restrict from)
{
strcpy (to + strlen (to), from);
return to;
}
|
This function has undefined results if the strings overlap.
The wcscat function is similar to wcscpy, except that the
characters from wfrom are concatenated or appended to the end of
wto, instead of overwriting it. That is, the first character from
wfrom overwrites the null character marking the end of wto.
An equivalent definition for wcscat would be:
wchar_t *
wcscat (wchar_t *wto, const wchar_t *wfrom)
{
wcscpy (wto + wcslen (wto), wfrom);
return wto;
}
|
This function has undefined results if the strings overlap.
Programmers using the strcat or wcscat function (or the
following strncat or wcsncar functions for that matter)
can easily be recognized as lazy and reckless. In almost all situations
the lengths of the participating strings are known (it better should be
since how can one otherwise ensure the allocated size of the buffer is
sufficient?) Or at least, one could know them if one keeps track of the
results of the various function calls. But then it is very inefficient
to use strcat/wcscat. A lot of time is wasted finding the
end of the destination string so that the actual copying can start.
This is a common example:
/* This function concatenates arbitrarily many strings. The last parameter must be |
This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient:
char *
concat (const char *str, …)
{
va_list ap;
size_t allocated = 100;
char *result = (char *) malloc (allocated);
if (result != NULL)
{
char *newp;
char *wp;
va_start (ap, str);
wp = result;
for (s = str; s != NULL; s = va_arg (ap, const char *))
{
size_t len = strlen (s);
/* Resize the allocated memory if necessary. */
if (wp + len + 1 > result + allocated)
{
allocated = (allocated + len) * 2;
newp = (char *) realloc (result, allocated);
if (newp == NULL)
{
free (result);
return NULL;
}
wp = newp + (wp - result);
result = newp;
}
wp = mempcpy (wp, s, len);
}
/* Terminate the result string. */
*wp++ = '\0';
/* Resize memory to the optimal size. */
newp = realloc (result, wp - result);
if (newp != NULL)
result = newp;
va_end (ap);
}
return result;
}
|
With a bit more knowledge about the input strings one could fine-tune
the memory allocation. The difference we are pointing to here is that
we don't use strcat anymore. We always keep track of the length
of the current intermediate result so we can safe us the search for the
end of the string and use mempcpy. Please note that we also
don't use stpcpy which might seem more natural since we handle
with strings. But this is not necessary since we already know the
length of the string and therefore can use the faster memory copying
function. The example would work for wide characters the same way.
Whenever a programmer feels the need to use strcat she or he
should think twice and look through the program whether the code cannot
be rewritten to take advantage of already calculated results. Again: it
is almost always unnecessary to use strcat.
This function is like strcat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The strncat function could be implemented like this:
char *
strncat (char *to, const char *from, size_t size)
{
to[strlen (to) + size] = '\0';
strncpy (to + strlen (to), from, size);
return to;
}
|
The behavior of strncat is undefined if the strings overlap.
This function is like wcscat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The wcsncat function could be implemented like this:
wchar_t *
wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
wto[wcslen (to) + size] = L'\0';
wcsncpy (wto + wcslen (wto), wfrom, size);
return wto;
}
|
The behavior of wcsncat is undefined if the strings overlap.
Here is an example showing the use of strncpy and strncat
(the wide character version is equivalent). Notice how, in the call to
strncat, the size parameter is computed to avoid
overflowing the character array buffer.
#include <string.h>
#include <stdio.h>
#define SIZE 10
static char buffer[SIZE];
main ()
{
strncpy (buffer, "hello", SIZE);
puts (buffer);
strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
puts (buffer);
}
|
The output produced by this program looks like:
hello hello, wo |
This is a partially obsolete alternative for memmove, derived from
BSD. Note that it is not quite equivalent to memmove, because the
arguments are not in the same order and there is no return value.
This is a partially obsolete alternative for memset, derived from
BSD. Note that it is not as general as memset, because the only
value it can store is zero.
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You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.
Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is “less” than the second, while a positive value indicates that the first string is “greater”.
The most common use of these functions is to check only for equality. This is canonically done with an expression like ‘! strcmp (s1, s2)’.
All of these functions are declared in the header file ‘string.h’.
The function memcmp compares the size bytes of memory
beginning at a1 against the size bytes of memory beginning
at a2. The value returned has the same sign as the difference
between the first differing pair of bytes (interpreted as unsigned
char objects, then promoted to int).
If the contents of the two blocks are equal, memcmp returns
0.
The function wmemcmp compares the size wide characters
beginning at a1 against the size wide characters beginning
at a2. The value returned is smaller than or larger than zero
depending on whether the first differing wide character is a1 is
smaller or larger than the corresponding character in a2.
If the contents of the two blocks are equal, wmemcmp returns
0.
On arbitrary arrays, the memcmp function is mostly useful for
testing equality. It usually isn't meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes. For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn't likely to tell you anything about the relationship between the
values of the floating-point numbers.
wmemcmp is really only useful to compare arrays of type
wchar_t since the function looks at sizeof (wchar_t) bytes
at a time and this number of bytes is system dependent.
You should also be careful about using memcmp to compare objects
that can contain “holes”, such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size. The contents of these “holes” are
indeterminate and may cause strange behavior when performing byte-wise
comparisons. For more predictable results, perform an explicit
component-wise comparison.
For example, given a structure type definition like:
struct foo
{
unsigned char tag;
union
{
double f;
long i;
char *p;
} value;
};
|
you are better off writing a specialized comparison function to compare
struct foo objects instead of comparing them with memcmp.
The strcmp function compares the string s1 against
s2, returning a value that has the same sign as the difference
between the first differing pair of characters (interpreted as
unsigned char objects, then promoted to int).
If the two strings are equal, strcmp returns 0.
A consequence of the ordering used by strcmp is that if s1
is an initial substring of s2, then s1 is considered to be
“less than” s2.
strcmp does not take sorting conventions of the language the
strings are written in into account. To get that one has to use
strcoll.
The wcscmp function compares the wide character string ws1
against ws2. The value returned is smaller than or larger than zero
depending on whether the first differing wide character is ws1 is
smaller or larger than the corresponding character in ws2.
If the two strings are equal, wcscmp returns 0.
A consequence of the ordering used by wcscmp is that if ws1
is an initial substring of ws2, then ws1 is considered to be
“less than” ws2.
wcscmp does not take sorting conventions of the language the
strings are written in into account. To get that one has to use
wcscoll.
This function is like strcmp, except that differences in case are
ignored. How uppercase and lowercase characters are related is
determined by the currently selected locale. In the standard "C"
locale the characters Ä and ä do not match but in a locale which
regards these characters as parts of the alphabet they do match.
strcasecmp is derived from BSD.
This function is like wcscmp, except that differences in case are
ignored. How uppercase and lowercase characters are related is
determined by the currently selected locale. In the standard "C"
locale the characters Ä and ä do not match but in a locale which
regards these characters as parts of the alphabet they do match.
wcscasecmp is a GNU extension.
This function is the similar to strcmp, except that no more than
size characters are compared. In other words, if the two
strings are the same in their first size characters, the
return value is zero.
This function is the similar to wcscmp, except that no more than
size wide characters are compared. In other words, if the two
strings are the same in their first size wide characters, the
return value is zero.
This function is like strncmp, except that differences in case
are ignored. Like strcasecmp, it is locale dependent how
uppercase and lowercase characters are related.
strncasecmp is a GNU extension.
This function is like wcsncmp, except that differences in case
are ignored. Like wcscasecmp, it is locale dependent how
uppercase and lowercase characters are related.
wcsncasecmp is a GNU extension.
Here are some examples showing the use of strcmp and
strncmp (equivalent examples can be constructed for the wide
character functions). These examples assume the use of the ASCII
character set. (If some other character set—say, EBCDIC—is used
instead, then the glyphs are associated with different numeric codes,
and the return values and ordering may differ.)
strcmp ("hello", "hello")
⇒ 0 /* These two strings are the same. */
strcmp ("hello", "Hello")
⇒ 32 /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
⇒ -15 /* The character |
The strverscmp function compares the string s1 against
s2, considering them as holding indices/version numbers. Return
value follows the same conventions as found in the strverscmp
function. In fact, if s1 and s2 contain no digits,
strverscmp behaves like strcmp.
Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them:
strverscmp ("no digit", "no digit")
⇒ 0 /* same behavior as strcmp. */
strverscmp ("item#99", "item#100")
⇒ <0 /* same prefix, but 99 < 100. */
strverscmp ("alpha1", "alpha001")
⇒ >0 /* fractional part inferior to integral one. */
strverscmp ("part1_f012", "part1_f01")
⇒ >0 /* two fractional parts. */
strverscmp ("foo.009", "foo.0")
⇒ <0 /* idem, but with leading zeroes only. */
|
This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers.
strverscmp is a GNU extension.
This is an obsolete alias for memcmp, derived from BSD.
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In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence ‘ll’ is treated as a single letter that is collated immediately after ‘l’.
You can use the functions strcoll and strxfrm (declared in
the headers file ‘string.h’) and wcscoll and wcsxfrm
(declared in the headers file ‘wchar’) to compare strings using a
collation ordering appropriate for the current locale. The locale used
by these functions in particular can be specified by setting the locale
for the LC_COLLATE category; see Locales and Internationalization.
In the standard C locale, the collation sequence for strcoll is
the same as that for strcmp. Similarly, wcscoll and
wcscmp are the same in this situation.
Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.
The functions strcoll and wcscoll perform this translation
implicitly, in order to do one comparison. By contrast, strxfrm
and wcsxfrm perform the mapping explicitly. If you are making
multiple comparisons using the same string or set of strings, it is
likely to be more efficient to use strxfrm or wcsxfrm to
transform all the strings just once, and subsequently compare the
transformed strings with strcmp or wcscmp.
The strcoll function is similar to strcmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
The wcscoll function is similar to wcscmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
Here is an example of sorting an array of strings, using strcoll
to compare them. The actual sort algorithm is not written here; it
comes from qsort (see section Array Sort Function). The job of the
code shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using strxfrm.)
/* This is the comparison function used with |
The function strxfrm transforms the string from using the
collation transformation determined by the locale currently selected for
collation, and stores the transformed string in the array to. Up
to size characters (including a terminating null character) are
stored.
The behavior is undefined if the strings to and from overlap; see Copying and Concatenation.
The return value is the length of the entire transformed string. This
value is not affected by the value of size, but if it is greater
or equal than size, it means that the transformed string did not
entirely fit in the array to. In this case, only as much of the
string as actually fits was stored. To get the whole transformed
string, call strxfrm again with a bigger output array.
The transformed string may be longer than the original string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, strxfrm simply returns the number of characters that would
be the length of the transformed string. This is useful for determining
what size the allocated array should be. It does not matter what
to is if size is zero; to may even be a null pointer.
The function wcsxfrm transforms wide character string wfrom
using the collation transformation determined by the locale currently
selected for collation, and stores the transformed string in the array
wto. Up to size wide characters (including a terminating null
character) are stored.
The behavior is undefined if the strings wto and wfrom overlap; see Copying and Concatenation.
The return value is the length of the entire transformed wide character
string. This value is not affected by the value of size, but if
it is greater or equal than size, it means that the transformed
wide character string did not entirely fit in the array wto. In
this case, only as much of the wide character string as actually fits
was stored. To get the whole transformed wide character string, call
wcsxfrm again with a bigger output array.
The transformed wide character string may be longer than the original wide character string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, wcsxfrm simply returns the number of wide characters that
would be the length of the transformed wide character string. This is
useful for determining what size the allocated array should be (remember
to multiply with sizeof (wchar_t)). It does not matter what
wto is if size is zero; wto may even be a null pointer.
Here is an example of how you can use strxfrm when
you plan to do many comparisons. It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings. Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
struct sorter { char *input; char *transformed; };
/* This is the comparison function used with |
The interesting part of this code for the wide character version would look like this:
void
sort_strings_fast (wchar_t **array, int nstrings)
{
…
/* Transform |
Note the additional multiplication with sizeof (wchar_t) in the
realloc call.
Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.
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This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file ‘string.h’.
This function finds the first occurrence of the byte c (converted
to an unsigned char) in the initial size bytes of the
object beginning at block. The return value is a pointer to the
located byte, or a null pointer if no match was found.
This function finds the first occurrence of the wide character wc in the initial size wide characters of the object beginning at block. The return value is a pointer to the located wide character, or a null pointer if no match was found.
Often the memchr function is used with the knowledge that the
byte c is available in the memory block specified by the
parameters. But this means that the size parameter is not really
needed and that the tests performed with it at runtime (to check whether
the end of the block is reached) are not needed.
The rawmemchr function exists for just this situation which is
surprisingly frequent. The interface is similar to memchr except
that the size parameter is missing. The function will look beyond
the end of the block pointed to by block in case the programmer
made an error in assuming that the byte c is present in the block.
In this case the result is unspecified. Otherwise the return value is a
pointer to the located byte.
This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like
rawmemchr (str, '\0') |
will never go beyond the end of the string.
This function is a GNU extension.
The function memrchr is like memchr, except that it searches
backwards from the end of the block defined by block and size
(instead of forwards from the front).
This function is a GNU extension.
The strchr function finds the first occurrence of the character
c (converted to a char) in the null-terminated string
beginning at string. The return value is a pointer to the located
character, or a null pointer if no match was found.
For example,
strchr ("hello, world", 'l')
⇒ "llo, world"
strchr ("hello, world", '?')
⇒ NULL
|
The terminating null character is considered to be part of the string,
so you can use this function get a pointer to the end of a string by
specifying a null character as the value of the c argument. It
would be better (but less portable) to use strchrnul in this
case, though.
The wcschr function finds the first occurrence of the wide
character wc in the null-terminated wide character string
beginning at wstring. The return value is a pointer to the
located wide character, or a null pointer if no match was found.
The terminating null character is considered to be part of the wide
character string, so you can use this function get a pointer to the end
of a wide character string by specifying a null wude character as the
value of the wc argument. It would be better (but less portable)
to use wcschrnul in this case, though.
strchrnul is the same as strchr except that if it does
not find the character, it returns a pointer to string's terminating
null character rather than a null pointer.
This function is a GNU extension.
wcschrnul is the same as wcschr except that if it does not
find the wide character, it returns a pointer to wide character string's
terminating null wide character rather than a null pointer.
This function is a GNU extension.
One useful, but unusual, use of the strchr
function is when one wants to have a pointer pointing to the NUL byte
terminating a string. This is often written in this way:
s += strlen (s); |
This is almost optimal but the addition operation duplicated a bit of
the work already done in the strlen function. A better solution
is this:
s = strchr (s, '\0'); |
There is no restriction on the second parameter of strchr so it
could very well also be the NUL character. Those readers thinking very
hard about this might now point out that the strchr function is
more expensive than the strlen function since we have two abort
criteria. This is right. But in the GNU C library the implementation of
strchr is optimized in a special way so that strchr
actually is faster.
The function strrchr is like strchr, except that it searches
backwards from the end of the string string (instead of forwards
from the front).
For example,
strrchr ("hello, world", 'l')
⇒ "ld"
|
The function wcsrchr is like wcschr, except that it searches
backwards from the end of the string wstring (instead of forwards
from the front).
This is like strchr, except that it searches haystack for a
substring needle rather than just a single character. It
returns a pointer into the string haystack that is the first
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
For example,
strstr ("hello, world", "l")
⇒ "llo, world"
strstr ("hello, world", "wo")
⇒ "world"
|
This is like wcschr, except that it searches haystack for a
substring needle rather than just a single wide character. It
returns a pointer into the string haystack that is the first wide
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
wcswcs is an deprecated alias for wcsstr. This is the
name originally used in the X/Open Portability Guide before the
Amendment 1 to ISO C90 was published.
This is like strstr, except that it ignores case in searching for
the substring. Like strcasecmp, it is locale dependent how
uppercase and lowercase characters are related.
For example,
strcasestr ("hello, world", "L")
⇒ "llo, world"
strcasestr ("hello, World", "wo")
⇒ "World"
|
This is like strstr, but needle and haystack are byte
arrays rather than null-terminated strings. needle-len is the
length of needle and haystack-len is the length of
haystack.
This function is a GNU extension.
The strspn (“string span”) function returns the length of the
initial substring of string that consists entirely of characters that
are members of the set specified by the string skipset. The order
of the characters in skipset is not important.
For example,
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
⇒ 5
|
Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
The wcsspn (“wide character string span”) function returns the
length of the initial substring of wstring that consists entirely
of wide characters that are members of the set specified by the string
skipset. The order of the wide characters in skipset is not
important.
The strcspn (“string complement span”) function returns the length
of the initial substring of string that consists entirely of characters
that are not members of the set specified by the string stopset.
(In other words, it returns the offset of the first character in string
that is a member of the set stopset.)
For example,
strcspn ("hello, world", " \t\n,.;!?")
⇒ 5
|
Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
The wcscspn (“wide character string complement span”) function
returns the length of the initial substring of wstring that
consists entirely of wide characters that are not members of the
set specified by the string stopset. (In other words, it returns
the offset of the first character in string that is a member of
the set stopset.)
The strpbrk (“string pointer break”) function is related to
strcspn, except that it returns a pointer to the first character
in string that is a member of the set stopset instead of the
length of the initial substring. It returns a null pointer if no such
character from stopset is found.
For example,
strpbrk ("hello, world", " \t\n,.;!?")
⇒ ", world"
|
Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
The wcspbrk (“wide character string pointer break”) function is
related to wcscspn, except that it returns a pointer to the first
wide character in wstring that is a member of the set
stopset instead of the length of the initial substring. It
returns a null pointer if no such character from stopset is found.
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index is another name for strchr; they are exactly the same.
New code should always use strchr since this name is defined in
ISO C while index is a BSD invention which never was available
on System V derived systems.
rindex is another name for strrchr; they are exactly the same.
New code should always use strrchr since this name is defined in
ISO C while rindex is a BSD invention which never was available
on System V derived systems.
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It's fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens. You can do this with the strtok function, declared
in the header file ‘string.h’.
A string can be split into tokens by making a series of calls to the
function strtok.
The string to be split up is passed as the newstring argument on
the first call only. The strtok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same string are indicated by passing a null pointer as
the newstring argument. Calling strtok with another
non-null newstring argument reinitializes the state information.
It is guaranteed that no other library function ever calls strtok
behind your back (which would mess up this internal state information).
The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.
On the next call to strtok, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to strtok.
If the end of the string newstring is reached, or if the remainder of
string consists only of delimiter characters, strtok returns
a null pointer.
Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
A string can be split into tokens by making a series of calls to the
function wcstok.
The string to be split up is passed as the newstring argument on
the first call only. The wcstok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same wide character string are indicated by passing a
null pointer as the newstring argument. Calling wcstok
with another non-null newstring argument reinitializes the state
information. It is guaranteed that no other library function ever calls
wcstok behind your back (which would mess up this internal state
information).
The delimiters argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string newstring is overwritten by a null wide character, and the pointer to the beginning of the token in newstring is returned.
On the next call to wcstok, the searching begins at the next
wide character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to wcstok.
If the end of the wide character string newstring is reached, or
if the remainder of string consists only of delimiter wide characters,
wcstok returns a null pointer.
Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
Warning: Since strtok and wcstok alter the string
they is parsing, you should always copy the string to a temporary buffer
before parsing it with strtok/wcstok (see section Copying and Concatenation). If you allow strtok or wcstok to modify
a string that came from another part of your program, you are asking for
trouble; that string might be used for other purposes after
strtok or wcstok has modified it, and it would not have
the expected value.
The string that you are operating on might even be a constant. Then
when strtok or wcstok tries to modify it, your program
will get a fatal signal for writing in read-only memory. See section Program Error Signals. Even if the operation of strtok or wcstok
would not require a modification of the string (e.g., if there is
exactly one token) the string can (and in the GNU libc case will) be
modified.
This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.
The functions strtok and wcstok are not reentrant.
See section Signal Handling and Nonreentrant Functions, for a discussion of where and why reentrancy is
important.
Here is a simple example showing the use of strtok.
#include <string.h> #include <stddef.h> … const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token, *cp; … cp = strdupa (string); /* Make writable copy. */ token = strtok (cp, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */ |
The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings.
Just like strtok, this function splits the string into several
tokens which can be accessed by successive calls to strtok_r.
The difference is that the information about the next token is stored in
the space pointed to by the third argument, save_ptr, which is a
pointer to a string pointer. Calling strtok_r with a null
pointer for newstring and leaving save_ptr between the calls
unchanged does the job without hindering reentrancy.
This function is defined in POSIX.1 and can be found on many systems which support multi-threading.
This function has a similar functionality as strtok_r with the
newstring argument replaced by the save_ptr argument. The
initialization of the moving pointer has to be done by the user.
Successive calls to strsep move the pointer along the tokens
separated by delimiter, returning the address of the next token
and updating string_ptr to point to the beginning of the next
token.
One difference between strsep and strtok_r is that if the
input string contains more than one character from delimiter in a
row strsep returns an empty string for each pair of characters
from delimiter. This means that a program normally should test
for strsep returning an empty string before processing it.
This function was introduced in 4.3BSD and therefore is widely available.
Here is how the above example looks like when strsep is used.
#include <string.h> #include <stddef.h> … const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; … running = strdupa (string); token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => NULL */ |
The GNU version of the basename function returns the last
component of the path in filename. This function is the preferred
usage, since it does not modify the argument, filename, and
respects trailing slashes. The prototype for basename can be
found in ‘string.h’. Note, this function is overriden by the XPG
version, if ‘libgen.h’ is included.
Example of using GNU basename:
#include <string.h>
int
main (int argc, char *argv[])
{
char *prog = basename (argv[0]);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
…
}
|
Portability Note: This function may produce different results on different systems.
This is the standard XPG defined basename. It is similar in
spirit to the GNU version, but may modify the path by removing
trailing '/' characters. If the path is made up entirely of '/'
characters, then "/" will be returned. Also, if path is
NULL or an empty string, then "." is returned. The prototype for
the XPG version can be found in ‘libgen.h’.
Example of using XPG basename:
#include <libgen.h>
int
main (int argc, char *argv[])
{
char *prog;
char *path = strdupa (argv[0]);
prog = basename (path);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
…
}
|
The dirname function is the compliment to the XPG version of
basename. It returns the parent directory of the file specified
by path. If path is NULL, an empty string, or
contains no '/' characters, then "." is returned. The prototype for this
function can be found in ‘libgen.h’.
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The function below addresses the perennial programming quandary: “How do
I take good data in string form and painlessly turn it into garbage?”
This is actually a fairly simple task for C programmers who do not use
the GNU C library string functions, but for programs based on the GNU C
library, the strfry function is the preferred method for
destroying string data.
The prototype for this function is in ‘string.h’.
strfry creates a pseudorandom anagram of a string, replacing the
input with the anagram in place. For each position in the string,
strfry swaps it with a position in the string selected at random
(from a uniform distribution). The two positions may be the same.
The return value of strfry is always string.
Portability Note: This function is unique to the GNU C library.
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The memfrob function converts an array of data to something
unrecognizable and back again. It is not encryption in its usual sense
since it is easy for someone to convert the encrypted data back to clear
text. The transformation is analogous to Usenet's “Rot13” encryption
method for obscuring offensive jokes from sensitive eyes and such.
Unlike Rot13, memfrob works on arbitrary binary data, not just
text.
For true encryption, See section DES Encryption and Password Handling.
This function is declared in ‘string.h’.
memfrob transforms (frobnicates) each byte of the data structure
at mem, which is length bytes long, by bitwise exclusive
oring it with binary 00101010. It does the transformation in place and
its return value is always mem.
Note that memfrob a second time on the same data structure
returns it to its original state.
This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in See section DES Encryption and Password Handling.
Portability Note: This function is unique to the GNU C library.
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To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task.
This function encodes a 32-bit input value using characters from the
basic character set. It returns a pointer to a 7 character buffer which
contains an encoded version of n. To encode a series of bytes the
user must copy the returned string to a destination buffer. It returns
the empty string if n is zero, which is somewhat bizarre but
mandated by the standard.
Warning: Since a static buffer is used this function should not
be used in multi-threaded programs. There is no thread-safe alternative
to this function in the C library.
Compatibility Note: The XPG standard states that the return
value of l64a is undefined if n is negative. In the GNU
implementation, l64a treats its argument as unsigned, so it will
return a sensible encoding for any nonzero n; however, portable
programs should not rely on this.
To encode a large buffer l64a must be called in a loop, once for
each 32-bit word of the buffer. For example, one could do something
like this:
char *
encode (const void *buf, size_t len)
{
/* We know in advance how long the buffer has to be. */
unsigned char *in = (unsigned char *) buf;
char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
char *cp = out, *p;
/* Encode the length. */
/* Using `htonl' is necessary so that the data can be
decoded even on machines with different byte order.
`l64a' can return a string shorter than 6 bytes, so
we pad it with encoding of 0 ('.') at the end by
hand. */
p = stpcpy (cp, l64a (htonl (len)));
cp = mempcpy (p, "......", 6 - (p - cp));
while (len > 3)
{
unsigned long int n = *in++;
n = (n << 8) | *in++;
n = (n << 8) | *in++;
n = (n << 8) | *in++;
len -= 4;
p = stpcpy (cp, l64a (htonl (n)));
cp = mempcpy (p, "......", 6 - (p - cp));
}
if (len > 0)
{
unsigned long int n = *in++;
if (--len > 0)
{
n = (n << 8) | *in++;
if (--len > 0)
n = (n << 8) | *in;
}
cp = stpcpy (cp, l64a (htonl (n)));
}
*cp = '\0';
return out;
}
|
It is strange that the library does not provide the complete functionality needed but so be it.
To decode data produced with l64a the following function should be
used.
The parameter string should contain a string which was produced by
a call to l64a. The function processes at least 6 characters of
this string, and decodes the characters it finds according to the table
below. It stops decoding when it finds a character not in the table,
rather like atoi; if you have a buffer which has been broken into
lines, you must be careful to skip over the end-of-line characters.
The decoded number is returned as a long int value.
The l64a and a64l functions use a base 64 encoding, in
which each character of an encoded string represents six bits of an
input word. These symbols are used for the base 64 digits:
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | |
0 | | | | | | | | |
8 | | | | | | | | |
16 | | | | | | | | |
24 | | | | | | | | |
32 | | | | | | | | |
40 | | | | | | | | |
48 | | | | | | | | |
56 | | | | | | | | |
This encoding scheme is not standard. There are some other encoding methods which are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings.
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argz vectors are vectors of strings in a contiguous block of
memory, each element separated from its neighbors by null-characters
('\0').
Envz vectors are an extension of argz vectors where each element is a
name-value pair, separated by a '=' character (as in a Unix
environment).
| 5.12.1 Argz Functions | Operations on argz vectors. | |
| 5.12.2 Envz Functions | Additional operations on environment vectors. |
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Each argz vector is represented by a pointer to the first element, of
type char *, and a size, of type size_t, both of which can
be initialized to 0 to represent an empty argz vector. All argz
functions accept either a pointer and a size argument, or pointers to
them, if they will be modified.
The argz functions use malloc/realloc to allocate/grow
argz vectors, and so any argz vector creating using these functions may
be freed by using free; conversely, any argz function that may
grow a string expects that string to have been allocated using
malloc (those argz functions that only examine their arguments or
modify them in place will work on any sort of memory).
See section Unconstrained Allocation.
All argz functions that do memory allocation have a return type of
error_t, and return 0 for success, and ENOMEM if an
allocation error occurs.
These functions are declared in the standard include file ‘argz.h’.
The argz_create function converts the Unix-style argument vector
argv (a vector of pointers to normal C strings, terminated by
(char *)0; see section Program Arguments) into an argz vector with
the same elements, which is returned in argz and argz_len.
The argz_create_sep function converts the null-terminated string
string into an argz vector (returned in argz and
argz_len) by splitting it into elements at every occurrence of the
character sep.
Returns the number of elements in the argz vector argz and argz_len.
The argz_extract function converts the argz vector argz and
argz_len into a Unix-style argument vector stored in argv,
by putting pointers to every element in argz into successive
positions in argv, followed by a terminator of 0.
Argv must be pre-allocated with enough space to hold all the
elements in argz plus the terminating (char *)0
((argz_count (argz, argz_len) + 1) * sizeof (char *)
bytes should be enough). Note that the string pointers stored into
argv point into argz—they are not copies—and so
argz must be copied if it will be changed while argv is
still active. This function is useful for passing the elements in
argz to an exec function (see section Executing a File).
The argz_stringify converts argz into a normal string with
the elements separated by the character sep, by replacing each
'\0' inside argz (except the last one, which terminates the
string) with sep. This is handy for printing argz in a
readable manner.
The argz_add function adds the string str to the end of the
argz vector *argz, and updates *argz and
*argz_len accordingly.
The argz_add_sep function is similar to argz_add, but
str is split into separate elements in the result at occurrences of
the character delim. This is useful, for instance, for
adding the components of a Unix search path to an argz vector, by using
a value of ':' for delim.
The argz_append function appends buf_len bytes starting at
buf to the argz vector *argz, reallocating
*argz to accommodate it, and adding buf_len to
*argz_len.
If entry points to the beginning of one of the elements in the
argz vector *argz, the argz_delete function will
remove this entry and reallocate *argz, modifying
*argz and *argz_len accordingly. Note that as
destructive argz functions usually reallocate their argz argument,
pointers into argz vectors such as entry will then become invalid.
The argz_insert function inserts the string entry into the
argz vector *argz at a point just before the existing
element pointed to by before, reallocating *argz and
updating *argz and *argz_len. If before
is 0, entry is added to the end instead (as if by
argz_add). Since the first element is in fact the same as
*argz, passing in *argz as the value of
before will result in entry being inserted at the beginning.
The argz_next function provides a convenient way of iterating
over the elements in the argz vector argz. It returns a pointer
to the next element in argz after the element entry, or
0 if there are no elements following entry. If entry
is 0, the first element of argz is returned.
This behavior suggests two styles of iteration:
char *entry = 0;
while ((entry = argz_next (argz, argz_len, entry)))
action;
|
(the double parentheses are necessary to make some C compilers shut up
about what they consider a questionable while-test) and:
char *entry;
for (entry = argz;
entry;
entry = argz_next (argz, argz_len, entry))
action;
|
Note that the latter depends on argz having a value of 0 if
it is empty (rather than a pointer to an empty block of memory); this
invariant is maintained for argz vectors created by the functions here.
Replace any occurrences of the string str in argz with
with, reallocating argz as necessary. If
replace_count is non-zero, *replace_count will be
incremented by number of replacements performed.
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Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.
Each element in an envz vector is a name-value pair, separated by a '='
character; if multiple '=' characters are present in an element, those
after the first are considered part of the value, and treated like all other
non-'\0' characters.
If no '=' characters are present in an element, that element is
considered the name of a “null” entry, as distinct from an entry with an
empty value: envz_get will return 0 if given the name of null
entry, whereas an entry with an empty value would result in a value of
""; envz_entry will still find such entries, however. Null
entries can be removed with envz_strip function.
As with argz functions, envz functions that may allocate memory (and thus
fail) have a return type of error_t, and return either 0 or
ENOMEM.
These functions are declared in the standard include file ‘envz.h’.
The envz_entry function finds the entry in envz with the name
name, and returns a pointer to the whole entry—that is, the argz
element which begins with name followed by a '=' character. If
there is no entry with that name, 0 is returned.
The envz_get function finds the entry in envz with the name
name (like envz_entry), and returns a pointer to the value
portion of that entry (following the '='). If there is no entry with
that name (or only a null entry), 0 is returned.
The envz_add function adds an entry to *envz
(updating *envz and *envz_len) with the name
name, and value value. If an entry with the same name
already exists in envz, it is removed first. If value is
0, then the new entry will the special null type of entry
(mentioned above).
The envz_merge function adds each entry in envz2 to envz,
as if with envz_add, updating *envz and
*envz_len. If override is true, then values in envz2
will supersede those with the same name in envz, otherwise not.
Null entries are treated just like other entries in this respect, so a null entry in envz can prevent an entry of the same name in envz2 from being added to envz, if override is false.
The envz_strip function removes any null entries from envz,
updating *envz and *envz_len.
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