popen(3C) (man pages section 3: Basic Library Functions)

man pages section 3: Basic Library Functions

popen(3C)

NAME

popen, pclose– initiate a pipe to or from a process

SYNOPSIS

#include <stdio.h>

command

mode

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.

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 invoked with the call:

execl("/usr/xpg4/bin/sh", "sh", "-c", command, (char *)0);

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(2). See wstat(3HEAD) for more information on termination status.

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 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 pclose() function will fail if:

ECHILD: The status of the child process could not be obtained, as described above.

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").

The signal handler for SIGCHLD should be set to default when using popen(). If the process has established a signal handler for SIGCHLD, it will be called when the command terminates. If the signal handler or another thread in the same process issues a wait(2) call, it will interfere with the return value of pclose(). If the process's signal handler for SIGCHLD has been set to ignore the signal, pclose() will fail and errno will be set to ECHILD.

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 code fragment can be used in a multithreaded process in place of the MT-Unsafe system(3C) function:

pclose(popen(cmd, "w"));

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Standard
MT-Level	Safe