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BSD 2.11 - man page for pclose (bsd section 3)

POPEN(3)						     Library Functions Manual							  POPEN(3)

popen, pclose - initiate I/O to/from a process
#include <stdio.h> FILE *popen(command, type) char *command, *type; pclose(stream) FILE *stream;
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.
pipe(2), fopen(3S), fclose(3S), system(3), wait(2), sh(1)
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.
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)