Each task spawned through PVM has its stdout and stderr
files connected to a pipe that is read by the pvmd managing the task.
Anything printed by the task is packed into a PVM message by the
pvmd and sent to the task's stdout sink.
The implementation of
this mechanism is described in §
.
Each spawned task has /dev/null opened as stdin.
Output from a task running on any host in a virtual machine (unless redirected by the console, or a parent task) is written in the log file of the master pvmd by default.
You can use the console spawn command with flag -> to collect output from an application (the spawned tasks and any others they in turn spawn). Use function pvm_catchout() to collect output within an application.
The C stdio library (fgets(), printf(), etc.) buffers input and output whenever possible, to reduce the frequency of actual read() or write() system calls. It decides whether to buffer by looking at the underlying file descriptor of a stream. If the file is a tty, it buffers only a line at a time, that is, the buffer is flushed whenever the newline character is encountered. If the descriptor is a file, pipe, or socket, however, stdio buffers up much more, typically 4k bytes.
A task spawned by PVM writes output through a pipe back to its pvmd, so the stdout buffer isn't flushed after every line (stderr probably is). The pvm_exit() function closes the stdio streams, causing them to be flushed so you should eventually see all your output. You can flush stdout by calling fflush(stdout) anywhere in your program. You can change the buffering mode of stdout to line-oriented for the entire program by calling setlinebuf(stdout) near the top of the program.
Fortran systems handle output buffering in many different ways. Sometimes there is a FLUSH subroutine, sometimes not.
In a PVM task, you can open a file to read or write, but remember that spawned components inherit the working directory (by default $HOME) from the pvmd so the file path you open must be relative to your home directory (or an absolute path). You can change the pvmd (and therefore task) working directory (per-host) by using the host file option wd=.