Manual page for popen(3S)
popen, pclose - initiate pipe to/from a process
SYNOPSIS
#include <stdio.h>
FILE *popen(const char *command,
const char *type);
int pclose (FILE *stream);
MT-LEVEL
Unsafe
DESCRIPTION
popen()
creates a pipe between the calling program and the command to be executed.
The arguments to
popen()
are pointers to null-terminated strings.
command
consists of a shell command line.
type
is an I/O mode, either
r
for reading or
w
for writing.
The value returned is a stream pointer such that
one can write to the standard input of the command,
if the I/O mode is
w,
by writing to the file
stream
(see
intro.3
and one can read from the standard output of the command,
if the I/O mode is
r,
by reading from the file
stream.
Because open files are shared, a type
r
command
may be used as an input filter and a type
w
as an output filter.
The environment of the executed command will be as if a child process were
created within the
popen()
call using
fork.2
and the child invoked the shell using the call:
- Solaris
-
execl("/usr/bin/sh", "sh", "-c", command, (char *)0);
- XPG4
-
execl("/usr/bin/ksh", "ksh", "-c", command, (char *)0);
A stream opened by
popen()
should be closed by
pclose(),
which closes the pipe, and waits for the associated process to terminate
and returns the termination status of the
process running the command language interpreter.
This is the value returned by
waitpid.2
See
wstat.5
for more details on termination status.
RETURN VALUES
popen()
returns a null pointer
if files or processes cannot be created.
pclose()
returns the termination status of the command.
pclose()
returns
-1
if
stream
is not associated with a
popen()
command and sets
errno
to indicate the error.
EXAMPLES
The following is an example of a typical call:
-
#include <stdio.h>
#include <stdlib.h>
main()
{
char *cmd = "/usr/bin/ls *.c";
char buf[BUFSIZ];
FILE *ptr;
if ((ptr = popen(cmd, "r")) != NULL)
while (fgets(buf, BUFSIZ, ptr) != NULL)
(void) printf("%s", buf);
return 0;
}
This program will print on the standard output
(see
stdio.3s
all the file names in the current directory that have a .c suffix.
SEE ALSO
ksh.1
pipe.2
wait.2
waitpid.2
fclose.3s
fopen.3s
stdio.3s
system.3s
wstat.5
xpg4.5
NOTES
If the original and
popen()
processes concurrently read or write a common file,
neither should use buffered I/O.
Problems with an output filter may be
forestalled by careful buffer flushing, for example, with
fflush()
(see
fclose.3s
A security hole exists through the
IFS
and
PATH
environment
variables. Full pathnames should be used (or
PATH
reset) and
IFS
should be set to space and tab (" \t").
Created by unroff & hp-tools.
© by Hans-Peter Bischof. All Rights Reserved (1997).
Last modified 21/April/97
