NAME
pclose, popen
— process I/O
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include
<stdio.h>
FILE *
popen(const char *command,
const char *mode);
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 mode
argument to specify reading or writing, not both. Because
popen() is now implemented using a bidirectional
pipe, the mode argument may request a bidirectional
data flow. The mode 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 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.
The
pclose()
function waits for the associated process to terminate; it 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)
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).
HISTORY
A popen() and a
pclose() function appeared in
Version 7 AT&T UNIX.
Bidirectional functionality was added in FreeBSD 2.2.6.