NAME

popen, pclose − initiate pipe to/from a process

SYNOPSIS

#include <stdio.h>

FILE ∗popen(const char ∗command, const char ∗type);

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.  command consists of a shell command line.  type 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/bin/ksh", "ksh", "−c", command, (char ∗)0);

otherwise, the child is invoked with the call:

execl("/usr/bin/sh", "sh", "−c", command, (char ∗)0);

A stream opened by popen() should be closed by pclose(), which closes the pipe, and 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(5) for more details on termination status. 

RETURN VALUES

The popen() function returns a null pointer if files or processes cannot be created. 

The pclose() function returns the termination status of the command.  The pclose() function returns −1 if stream is not associated with a popen() command and sets errno to indicate the error. 

EXAMPLES

The following is an example of a typical call:

#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);
return 0;
}

This program will print on the standard output (see stdio(3S)) all the file names in the current directory that have a .c suffix. 

ATTRIBUTES

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

SEE ALSO

ksh(1), pipe(2), wait(2), waitpid(2), fclose(3S), fopen(3S), stdio(3S), system(3S), attributes(5), wstat(5), standards(5)

NOTES

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(3S)).  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"). 

SunOS 5.6  —  Last change: 26 Feb 1997