POPEN(3)

NAME
popen, popenve, pclose - process I/O

LIBRARY
libc

SYNOPSIS
#include <stdio.h>
FILE *
popen(const char *command, const char *type);
FILE *
popenve(const char *path, char * const *argv, char * const *envp, 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 popenve() function is similar to popen() but the first three arguments are passed to execve(2) and there is no shell involved in the command invocation.

The return value from popen() and popenve() 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(2).

RETURN VALUES
The popen() function returns NULL if the vfork(2), pipe(2), or socketpair(2) calls fail, or if it cannot allocate memory, preserving the errno from those functions.

The pclose() function returns -1 if stream is not associated with a “popened” command, if stream has already been “pclosed”, setting errno to ESRCH, or if wait4(2) returns an error, preserving the errno returned by wait4(2).

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

STANDARDS
The popen() and pclose() functions conform to p1003.2-92.

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

The popenve() function first appeared in NetBSD 8.

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 the 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).