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 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 mode argument is a pointer to a null-terminated string which must be either r for reading or w for writing.

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 waitpid.

RETURN VALUE
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 waitpid returns an error.

ERRORS
The popen function does not reliably set errno.

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). (Because of a lack of available sh(1), the current GNO implementation always calls gsh(1).)

This implementation makes use of waitpid(2). Because waitpid is not currently provided by the GNO kernel, pclose is affected by the same bug as is waitpid; other child processes' exit status may be caught (and discarded) while waiting for the process created by popen. Consequently, if the parent processes is doing any other waitpid calls (either directly or indirectly), pclose may never return.

Under GNO, the memory region pointed to by command must be able to be referenced up to the point that the child process does its execl(2). This implies that if command is allocated on the stack and the function which allocated it returns before the program calls pclose (or if the parent exits and makes its child into a zombie), there is a chance that the forked child of popen may try to execl an arbitrary command, the results of which are undefined.

gsh(1) appears to hang on occasion when executing popen's arguments. The symptom is that the pclose never returns, and the process table (see ps(1)) shows that the forked gsh's user time is forever increasing.

SEE ALSO
gsh(1), sh(1), fork(2), pipe(2), waitpid(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)

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