POPEN(3)

NAME
popen, pclose - process I/O

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 pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.

The command argument is a pointer to a NUL-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 type argument is a pointer to a NUL-terminated string which must be either "r" or "re" for reading or "w" or "we" for writing. If the letter "e" is present in the string then the close-on-exec flag shall be set on the file descriptor underlying the FILE that is returned.

The return value from popen() is a normal standard I/O stream in all respects except that it must be closed with pclose() rather than fclose(3). 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 popen() output streams are fully buffered by default. In addition, fork handlers established using pthread_atfork(3) are not called when a multithreaded program calls popen().

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 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.

CAVEATS
Never supply the popen() function with a command containing any part of an unsanitized user-supplied string. Shell meta-characters present will be honored by the sh(1) command interpreter.

It is often simpler to bypass the shell entirely and use pipe(2), fork(2), dup2(2), execlp(3), and fdopen(3) directly instead of having to sanitize a string for shell consumption.

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.