NAME

popen, pclose — process I/O

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdio.h>

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

int
pclose(FILE *stream);

DESCRIPTION

The popen() function “opens” a process by creating an IPC connection, forking, and invoking the shell. Historically, popen was implemented with a unidirectional pipe; hence many implementations of popen only allow the type argument to specify reading or writing, not both. Since popen is now implemented using sockets, the type may request a bidirectional data flow. The type argument is a pointer to a null-terminated string which must be ‘r’ for reading, ‘w’ for writing, or ‘r+’ for reading and writing. In addition if the character ‘e’ is present in the type string, the file descriptor used internally is set to be closed on exec(3).

The command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to /bin/sh using the -c flag; interpretation, if any, is performed by the shell.

The return value from popen() is a normal standard I/O stream in all respects save that it must be closed with pclose() rather than fclose(). Writing to such a stream writes to the standard input of the command; the command's standard output is the same as that of the process that called popen(), unless this is altered by the command itself. Conversely, reading from a “popened” stream reads the command's standard output, and the command's standard input is the same as that of the process that called popen().

Note that output popen() streams are fully buffered by default.

The pclose() function waits for the associated process to terminate and returns the exit status of the command as returned by wait4().

RETURN VALUES

The popen() function returns NULL if the fork(2), pipe(2), or socketpair(2) calls fail, or if it cannot allocate memory.

The pclose() function returns -1 if stream is not associated with a “popened” command, if stream has already been “pclosed”, or if wait4(2) returns an error.

ERRORS

The popen() function does not reliably set errno.

SEE ALSO

sh(1), fork(2), pipe(2), socketpair(2), wait4(2), fclose(3), fflush(3), fopen(3), shquote(3), stdio(3), system(3)

STANDARDS

The popen() and pclose() functions conform to IEEE Std 1003.2-1992 (“POSIX.2”).

HISTORY

A popen() and a pclose() function appeared in Version 7 AT&T UNIX.

BUGS

Since the standard input of a command opened for reading shares its seek offset with the process that called popen(), if the original process has done a buffered read, the command's input position may not be as expected. Similarly, the output from a command opened for writing may become intermingled with that of the original process. The latter can be avoided by calling fflush(3) before popen().

Failure to execute the shell is indistinguishable from the shell's failure to execute command, or an immediate exit of the command. The only hint is an exit status of 127.

The popen() argument always calls sh(1), never calls csh(1).