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.

       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 invoked with the call:

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

       otherwise, the child is 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.

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 (" 	").

       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(3C) 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 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	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

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

SunOS 5.10							    17 Mar 2004 							 popen(3C)