NAME
popen,
popenve,
pclose
—
process I/O
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdio.h>
FILE *
popen(
const char
*command,
const char
*type);
FILE *
popenve(
const
char *path,
char * const
*argv,
char * const
*envp,
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 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. 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
popenve() function is similar to
popen()
but the first three arguments are passed to
execve(2) and there is no shell
involved in the command invocation.
The return value from
popen() and
popenve()
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().
RETURN VALUES
The
popen() function returns
NULL
if
the
vfork(2),
pipe(2), or
socketpair(2) calls fail, or
if it cannot allocate memory, preserving the errno from those functions.
The
pclose() function returns -1 if
stream is not associated with a “popened”
command, if
stream has already been
“pclosed”, setting errno to
ESRCH
or if
wait4(2) returns an error,
preserving the errno returned by
wait4(2).
SEE ALSO
sh(1),
execve(2),
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 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).
The
popenve() function first appeared in
NetBSD 8.