NAME

popen, pclose − initiate pipe to/from a process

SYNOPSIS

#include <stdio.h>

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

int pclose (stream)
FILE ∗stream;

DESCRIPTION

popen creates a pipe between the calling program and the command to be executed.  The arguments to popen are pointers to null-terminated strings.  command consists of a shell command line.  type is an I/O mode, either r for reading or w for writing.  The value returned is a stream pointer such that one can write to the standard input of the command, if the I/O mode is w, by writing to the file stream; and one can read from the standard output of the command, if the I/O mode is r, by reading from the file stream.

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. 

EXAMPLE

A typical call may be:

char ∗cmd = "ls ∗.c";
FILE ∗ptr;
if ((ptr = popen(cmd, "r")) != NULL)
      while (fgets(buf, n, ptr) != NULL)
              (void) printf("%s ",buf);

This prints in stdout [see stdio (3S)] all the file names in the current directory that have a “.c” suffix. 

SEE ALSO

fclose(3S), fopen(3S), pipe(2), stdio(3S), system(3S), wait(2)

DIAGNOSTICS

popen returns a NULL pointer if files or processes cannot be created. 

pclose returns −1 if stream is not associated with a “popened” command.

WARNING

If the original and “popened” processes concurrently read or write a common file, neither should use buffered I/O, because the buffering gets all mixed up. Problems with an output filter may be forestalled by careful buffer flushing, e.g. with fflush [see fclose(3S)].

September 02, 1992