Linux System Programming: Creating pipes in C using “popen()” and “pclose()” system call in Linux

In this chapter we shall learn about:

1. Introduction

2. Function prototype for popen()

3. Function prototype for pclose()

4. Simple example for popen()

Introduction:

Using popen() and pclose() system calls, it is fairy easy to create pipes.

Function prototype for popen()

FILE *popen ( char *command, char *type);

It will return new file stream on success and NULL on unsuccessful fork() or pipe() call.

This will call pipe() system call internally. It will executes the “command” argument within the shell.

“type” can be “r” or “w”, for “read” or “write”.

Pipe created with popen() needs to be closed using pclose().

Function prototype for pclose()

int pclose( FILE *stream );

pclose will wait on the pipe process to terminate, then closes the stream.

Simple example for popen()

#include <stdio.h>
#include <stdlib.h>

#define MAXSTRS 5

int main(void)
{
        int  cntr;
        FILE *pipe_fp;
        char *strings[MAXSTRS] = { "a", "d", "b","c", "e"};

        /* Create one way pipe line with call to popen() */
        if (( pipe_fp = popen("sort", "w")) == NULL)
        {
                perror("popen");
                exit(1);
        }

        /* Processing loop */
        for(cntr=0; cntr<MAXSTRS; cntr++) 
        {
                fputs(strings[cntr], pipe_fp);
                fputc('\n', pipe_fp);
        }

        /* Close the pipe */
        pclose(pipe_fp);
        
        return(0);
}

Notes on half-duplex pipes:

* You can create a 2 way pipes by opening 2 pipes and properly reassigning the fd’s.

* pipe() call should be made before fork() call.

* For “pipe()” there should be a ancestry process. But that is not the case in named pipes.

List Of Tutorials available in this website: