POPEN(3) BSD Library Functions Manual POPEN(3)NAME
popen, pclose — process I/O
LIBRARY
Standard C Library (libc, -lc)
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 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 bidirec‐
tional 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 read‐
ing 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 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, read‐
ing 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().
RETURN VALUES
The popen() function returns NULL if the fork(2), pipe(2), or
socketpair(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 has already been “pclosed”, or if wait4(2)
returns an error.
ERRORS
The popen() function does not reliably set errno.
SEE ALSOsh(1), fork(2), pipe(2), socketpair(2), wait4(2), fclose(3), fflush(3),
fopen(3), shquote(3), stdio(3), system(3)STANDARDS
The popen() and pclose() functions conform to IEEE Std 1003.2-1992
(“POSIX.2”).
HISTORY
A popen() and a pclose() function appeared in Version 7 AT&T UNIX.
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 fail‐
ure 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).
BSD June 24, 2011 BSD
