The -d2.9 debugging switch tells sendmail to display the properties of each open file descriptor. That output is produced by the dumpfd( ) routine, and each line of output is for a single file descriptor:

number: fl=flags mode=mode type stats

Here, the number is the count of the open file descriptor. Note that descriptors 0, 1, and 2 are usually tied to the standard input, output, and error output, respectively.

The flags is a hexadecimal representation of the state flags associated with a file descriptor. F_GETFL is used with ioctl(2) to fetch each, and all are described in <sys/fcntl.h> on most systems.

The mode is printed in octal and is the st_mode associated with an fstat(2) of the file descriptor.

The type examines the file type portion of the st_mode and prints SOCK for a socket, CHR: for a character special device, BLK: for a block special device, FIFO: for a named pipe, DIR: for a directory, LNK: for a symbolic link, and nothing otherwise (e.g., nothing if it is a file).

The stats are printed for all but the socket. They look like this:

dev=major/minor ino=inum nlink=nlink u/gid=user-id/group-id size=bytes

Here the dev= shows the major and minor device numbers for the device that the file descriptor is associated with. The inum is the inode number on the disk (if there is one) and nlink is the number of hard links to the file on disk. The u/gid shows the user and group ownership associated with the file descriptor. The bytes is the number of bytes in a file, and zero for almost everything else.

For a socket, the stats part of each line looks like this:

[addr]/port-> host

Here, addr is the IP address (surrounded in square braces) of the local end of the socket. If the connection is of type AF_INET or AF_INET6, the port number of the connection is also shown as /port. The host is the hostname, as returned by getpeername(3), of the connecting host. If any of these cannot be found, the error string associated with errno is printed parenthetically in its place.

The -d7.9, -d40.9, and -d46.9 debugging switches also print a line such as this for specific file descriptors. Also, if sendmail is run with the -d10.100 switch, or if sendmail fails to open a tf queue file (Section 11.2.6), or if sendmail exited because of too many open files, it will syslog all its open file descriptors within this format.