NAME
popen, popenve,
pclose —
process I/O
LIBRARY
library “libc”
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
Thepopen()
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.