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POPEN(3) Library Functions Manual POPEN(3)

popen, pcloseprocess I/O

#include <stdio.h>

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

pclose(FILE *stream);

The () 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 () 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 () 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 () function waits for the associated process to terminate and returns the exit status of the command as returned by wait4(2).

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.

The popen() function does not reliably set errno.

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

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

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).

August 31, 2014 OpenBSD-5.7