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 character 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). 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
interpreter. 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 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(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 | See below. |
+--------------------+-----------------+
|MT-Level | Safe |
+--------------------+-----------------+


The F character in the mode argument of popen() is Evolving. In all
other respects this function is Standard. The pclose() function is
Standard.

SEE ALSO


ksh(1), pipe(2), fclose(3C), fopen(3C), posix_spawn(3C), stdio(3C),
system(3C), wait(3C), waitpid(3C), wait.h(3HEAD), attributes(5),
standards(5)


December 14, 2006 POPEN(3C)