popen, pclose -- process I/O
Standard C Library (libc, -lc)
#include <stdio.h>
FILE *
popen(const char *command, const char *type);
int
pclose(FILE *stream);
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 type argument to specify reading
or writing, not both. Since popen() is now implemented using a bidirectional
pipe, the type argument may request a bidirectional 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 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 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)
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).
A popen() and a pclose() function appeared in Version 7 AT&T UNIX.
Bidirectional functionality was added in FreeBSD 2.2.6.
FreeBSD 5.2.1 May 3, 1995 FreeBSD 5.2.1 [ Back ] |
