The UNIX and Linux Forums  
Hello and Welcome from United States to the UNIX and Linux Forums! Thank You for Visiting and Joining Our Global Community.

Go Back   The UNIX and Linux Forums > Top Forums > UNIX for Dummies Questions & Answers
.
google unix.com



UNIX for Dummies Questions & Answers If you're not sure where to post a UNIX or Linux question, post it here. All UNIX and Linux newbies welcome !!

More UNIX and Linux Forum Topics You Might Find Helpful
Thread Thread Starter Forum Replies Last Post
A program to trace execution of another program jiten_hegde High Level Programming 3 08-19-2008 05:26 AM
C++ How to use pipe() & fork() with stdin and stdout to another program vvaidyan High Level Programming 2 05-16-2008 07:30 PM
How to clear the content of a pipe (STDIN) after it is written to another program? vvaidyan High Level Programming 10 05-15-2008 07:08 PM
How to write to stdin of another program (program A -> [stdin]program B) vvaidyan High Level Programming 1 04-30-2008 01:44 PM
how can i write process_pool program? hit High Level Programming 4 06-17-2002 03:36 AM

Closed Thread
English Japanese Spanish French German Portuguese Italian Dutch Swedish Russian Norwegian Hungarian Hebrew Danish
 
LinkBack Thread Tools Search this Thread Rate Thread Display Modes
  #1 (permalink)  
Old 04-30-2008
vvaidyan vvaidyan is offline
Registered User
  
 

Join Date: Mar 2008
Posts: 15
How to write to stdin of another program (program A -> [stdin]program B)

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
  #2 (permalink)  
Old 08-02-2008
redoubtable redoubtable is offline
Registered User
  
 

Join Date: Aug 2008
Location: Portugal
Posts: 242
/*
* 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;
}
  #3 (permalink)  
Old 08-02-2008
jim mcnamara jim mcnamara is offline Forum Staff  
...@...
  
 

Join Date: Feb 2004
Location: NM
Posts: 5,643
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.
  #4 (permalink)  
Old 08-02-2008
redoubtable redoubtable is offline
Registered User
  
 

Join Date: Aug 2008
Location: Portugal
Posts: 242
Post

Quote:
Originally Posted by jim mcnamara View Post
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"
Sponsored Links
Closed Thread

Bookmarks

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -4. The time now is 06:46 PM.


Powered by: vBulletin, Copyright ©2000 - 2006, Jelsoft Enterprises Limited. Language translation by Google.
vBCredits v1.4 Copyright ©2007 - 2008, PixelFX Studios
The UNIX and Linux Forums Content Copyright ©1993-2009. All Rights Reserved.Ad Management by RedTyger

Content Relevant URLs by vBSEO 3.2.0