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). If  the  applica-
       tion 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  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().   Other-
       wise, 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 (" 	").

       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),  stan-
       dards(5)

SunOS 5.11							    14 Dec 2006 							 popen(3C)