POPEN(3)

NAME
popen, pclose - process I/O

LIBRARY
libc

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 a bidirectional pipe forking, and invoking the shell. Any streams opened by previous popen() calls in the parent process are closed in the new child process. 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 a bidirectional pipe, the type argument 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.

The type argument may be augmented by appending an ‘e’ to set the descriptor's close-on-exec flag. For example, ‘re’ for reading, ‘we’ for writing, or ‘r+e’ for reading and writing. Use of this flag is important when operating in threaded environments.

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.

popen() automatically interlocks and closes descriptors associated with other active popen() files in any sub-process it creates, preventing file descriptor leakage between popen() calls in a thread-safe manner. However, popen() has no control over fork or fork/exec sequences run by other threads which do not use the popen mechanism and in this situation it is likely that popen descriptors will leak into those sub-processes. It is recommended that the ‘e’ flag be used to prevent descriptor leakages into miscellaneous fork/exec sequences that might be executed by other threads in a multi-threaded program.

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) or pipe(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 already “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), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)

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

Bidirectional functionality was added in FreeBSD 2.2.6.

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() function always calls sh(1), never calls csh(1).