popen(3C) Standard C Library Functions popen(3C)NAME
popen, pclose - initiate a pipe to or from a process
SYNOPSIS
#include <stdio.h>
FILE *popen(const char *command, const char *mode);
int pclose(FILE *stream);
DESCRIPTION
The popen() function creates a pipe between the calling program and the
command to be executed. The arguments to popen() are pointers to null-
terminated strings. The command argument consists of a shell command
line. The mode argument 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. A trailing F charac‐
ter can also be included in the mode argument as described in fopen(3C)
to enable extended FILE facility.
The environment of the executed command will be as if a child process
were created within the popen() call using fork(2). If the application
is standard-conforming (see standards(5)), the child is created as if
invoked with the call:
execl("/usr/xpg4/bin/sh", "sh", "-c",command, (char *)0);
otherwise, the child is created as if invoked with the call:
execl("/usr/bin/sh", "sh", "-c",command, (char *)0);
The pclose() function closes a stream opened by popen() by closing the
pipe. It waits for the associated process to terminate and returns the
termination status of the process running the command language inter‐
preter. This is the value returned by waitpid(3C). See wait.h(3HEAD)
for more information on termination status. If, however, a call to
waitpid() with a pid argument equal to the process ID of the command
line interpreter causes the termination status to be unavailable to
pclose(), then pclose() returns −1 with errno set to ECHILD to report
this condition.
RETURN VALUES
Upon successful completion, popen() returns a pointer to an open stream
that can be used to read or write to the pipe. Otherwise, it returns a
null pointer and may set errno to indicate the error.
Upon successful completion, pclose() returns the termination status of
the command language interpreter as returned by waitpid(). Otherwise,
it returns −1 and sets errno to indicate the error.
ERRORS
The pclose() function will fail if:
ECHILD The status of the child process could not be obtained, as
described in the DESCRIPTION.
The popen() function may fail if:
EMFILE There are currently FOPEN_MAX or STREAM_MAX streams open in
the calling process.
EINVAL The mode argument is invalid.
The popen() function may also set errno values as described by fork(2)
or pipe(2).
USAGE
If the original and popen() processes concurrently read or write a com‐
mon file, neither should use buffered I/O. Problems with an output fil‐
ter may be forestalled by careful buffer flushing, for example, with
fflush() (see fclose(3C)). 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").
Even if the process has established a signal handler for SIGCHLD, it
will be called when the command terminates. Even if another thread in
the same process issues a wait(3C) call, it will interfere with the
return value of pclose(). Even if the process's signal handler for
SIGCHLD has been set to ignore the signal, there will be no effect on
pclose().
EXAMPLES
Example 1 popen() example
The following program will print on the standard output (see stdio(3C))
the names of files in the current directory with a .c suffix.
#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);
(void) pclose(ptr);
}
return 0;
}
Example 2 system() replacement
The following function can be used in a multithreaded process in place
of the most common usage of the Unsafe system(3C) function:
int my_system(const char *cmd)
{
FILE *p;
if ((p = popen(cmd, "w")) == NULL)
return (-1);
return (pclose(p));
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Committed │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │Safe │
├─────────────────────────────┼─────────────────────────────┤
│Standard │See below. │
└─────────────────────────────┴─────────────────────────────┘
For pclose() and all aspects of popen() except the F character in the
mode argument, see standards(5).
SEE ALSOksh(1), pipe(2), fclose(3C), fopen(3C), posix_spawn(3C), stdio(3C),
system(3C), wait(3C), waitpid(3C), wait.h(3HEAD), attributes(5), stan‐
dards(5)SunOS 5.11 14 Dec 2006 popen(3C)
