bsd man page for popen

POPEN(3)						     Library Functions Manual							  POPEN(3)

NAME
       popen, pclose - initiate I/O to/from a process

SYNOPSIS
       #include <stdio.h>

       FILE *popen(command, type)
       char *command, *type;

       pclose(stream)
       FILE *stream;

DESCRIPTION
       The arguments to popen are pointers to null-terminated strings containing respectively a shell command line and an I/O mode, either "r" for
       reading or "w" for writing.  It creates a pipe between the calling process and the command to be executed.  The value returned is a  stream
       pointer that can be used (as appropriate) to write to the standard input of the command or read from its standard output.

       A stream opened by popen should be closed by pclose, which waits for the associated process to terminate and returns the exit status of the
       command.

       Because open files are shared, a type "r" command may be used as an input filter, and a type "w" as an output filter.

SEE ALSO
       pipe(2), fopen(3S), fclose(3S), system(3), wait(2), sh(1)

DIAGNOSTICS
       Popen returns a null pointer if files or processes cannot be created, or the shell cannot be accessed.

       Pclose returns -1 if stream is not associated with a `popened' command.

BUGS
       Buffered reading before opening an input filter may leave the standard input of that filter mispositioned.  Similar problems with an output
       filter may be forestalled by careful buffer flushing, for instance, with fflush, see fclose(3S).

       Popen always calls sh, never calls csh.

7th Edition							   May 15, 1985 							  POPEN(3)