UNIX for Dummies Questions & Answers

Hi,
Program A: uses pipe()

I am able to read the stdout of PROGAM B (stdout got through system() command) into PROGRAM A using:

* child
-> dup2(fd[1], STDOUT_FILENO);
-> execl("/path/PROGRAM B", "PROGRAM B", NULL);

* parent
-> char line[100];
-> read(fd[0], line, 100);


Question:
---------
How to write to stdin of PROGRAM B from PROGRAM A?
* should I use a different pipe?
* how to I read stdin in PROGRAM B? using cin?

Thanks in advance,
Vivek

/*
* here's how you can do it...
* using popen()
*
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define READ 0
#define WRITE 1

pid_t
popen2(const char *command, int *infp, int *outfp)
{
int p_stdin[2], p_stdout[2];
pid_t pid;

if (pipe(p_stdin) != 0 || pipe(p_stdout) != 0)
return -1;

pid = fork();
if (pid < 0)
return pid;
else if (pid == 0)
{
close(p_stdin[WRITE]);
dup2(p_stdin[READ], READ);
close(p_stdout[READ]);
dup2(p_stdout[WRITE], WRITE);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
exit(1);
}

if (infp == NULL)
close(p_stdin[WRITE]);
else
*infp = p_stdin[WRITE];
if (outfp == NULL)
close(p_stdout[READ]);
else
*outfp = p_stdout[READ];
return pid;
}

/*
* now in main... infp will be the stdin (in file descriptor)
* and outfp will be the stdout (out file descriptor)
* have fun
*/

int
main(int argc, char **argv)
{
int infp, outfp;
char buf[128];

if (popen2("your-program-B", &infp, &outfp) <= 0)
{
printf("Unable to exec your-program-B\n");
exit(1);
}

memset (buf, 0x0, sizeof(buf));

write(infp, "Z\n", 2);
write(infp, "D\n", 2);
write(infp, "A\n", 2);
write(infp, "C\n", 2);
close(infp);
read(outfp, buf, 128);
printf("buf = '%s'\n", buf);
return 0;
}

Also note: popen is a one way deal - you choose either to write to or to read from a child process, popen will not let you do both at the same time. Otherwise, you get into more interesting and advanced interprocess communication (IPC) programming like maybe pipes.

That's right! The POSIX.1-2001 function popen() only allows to read or write, not both. But if you look closely to my implementation, I don't actually use popen(). Instead, I created an popen2() that opens two pipes for the parent process (infp, outfp). Try that with "cat" for instance.

Code:

if (popen2("your-program-B", &infp, &outfp) <= 0)
        {
        printf("Unable to exec your-program-B\n");
        exit(1);
        }

        memset (buf, 0x0, sizeof(buf));

/* 
 * writing to stdin here
 */
        write(infp, "Z\n", 2);
        write(infp, "D\n", 2);
        write(infp, "A\n", 2);
        write(infp, "C\n", 2);
        close(infp);
/*
 * reading stdout here
 */
        read(outfp, buf, 128);
        printf("buf = '%s'\n", buf);

I wrote and then I read from the child process "your-program-B"