popen(3S)

popen, pclose -- initiate pipe to/from a process

Synopsis

   #include <stdio.h>
   
   FILE popen(const char command, const char type);
   

   int pclose(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 [see intro(3)]; and one can read from the standard output of the command, if the I/O mode is r, by reading from the file stream.

popen forks to create a child process that runs the command interpreter specified by the SHELL environment variable in the caller's environment. If SHELL is not set, then /bin/sh is used. This shell executes command.

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.

Errors

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

pclose returns -1 if stream is not associated with a popened command.

Usage

Here is an example of a typical call:

   #include <stdio.h>
   #include <stdlib.h>
   
   main()
   {
   	char *cmd = "/usr/bin/ls *.c";
   	char buf[BUFSIZ];
   	FILE *ptr;
   

   	if ((ptr = popen(cmd, "r")) != NULL)
   		while (fgets(buf, BUFSIZ, ptr) != NULL)
   			(void) printf("%s", buf);
   	return 0;
   }

This program will print on the standard output [see Intro(3S)] all the file names in the current directory that have a .c suffix.

References

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

Notices

If the original and popened processes concurrently read or write a common file, neither should use buffered I/O. Problems with an output filter may be forestalled by careful buffer flushing, for example, with fflush [see fclose(3S)].

A security hole exists through the IFS and PATH environment variables. Full pathnames should be used (or PATH reset) and IFS should be set to space and tab
(``" \t"'').

Compatibility

Previous versions of UnixWare and SCO OpenServer always used /bin/sh as the shell in which command is executed. Applications built for previous system versions may require that SHELL be set to /bin/sh in the caller's environment.