IPC between 2 programs using pipe

**cbgb** · 09-22-2006

Hi. I have been looking through the posts for fork, pipe, dup2, and IPC, but I have not found an answer to my question, so here goes:

I want to set up a pipe between 2 separate programs using fork. I have a parent process (pgm1) that will create the child via execve, and then read the child's response. The child will read an input file and send some calculated results back to the parent through the pipe. Again, they are two separate programs... So I am doing the following, and I would like to know if I can use dup2 to set up stdin and stdout between the processes. If so, can anyone point me at an example that involves 2 programs? I am also wondering, what has to be declared in program 2 to establish a link to the pipeFDs decriptors that are declared in my pgm1?

Thanks a lot!!

Code:

int pipeFDs[2];                                                               /* Pipe fds            */
char CHILD_PROC_TO_LOAD_PIPE[] = {"./pgm2.o"};   /* Load process   */
char *args[] = {NULL};
FILE *fdout;                                                  /* parent's output result file */
                                   
 ...

 pid = fork();

 switch(pid)          
 {
  case -1:                                    
   printf("ERROR");
  case 0:                                      
   dup2(0,pipeFDs[0]);                /* Don't know if this is correct. Want */
   dup2(1,pipeFDs[1]);                /* to link pipes to pgm2                     */
   execve(CHILD_PROC_TO_LOAD_PIPE,args,NULL);       
  default:                                     
   processLoadedPipe(pipeFDs,fdout,pid);        /* parent */
 }

**cbgb** · 09-22-2006

Sorry about that... this is my first post. I still can not find an example, so any help would be appreciated.

Thanks!

**whiteflags** · 09-22-2006

> I want to set up a pipe between 2 separate programs
Sounds like a great idea except that you got caught up in all the ways to set up a process. If you need to have communication with another process - and will constantly be exchanging data and control back and forth - you should consider piping as a simple solution.

Formatted piping: man popen
Low level piping: man pipe

Once you figure out how to open a stream then it's really easy to communicate via the standard library.

**Salem** · 09-22-2006

> ./pgm2.o
Is this really an executable file, or an object file?

Everything returns a status, you should check all of them. Maybe something is returning an error.

> char *args[] = {NULL};
I would use

Code:

char *argc[] = {
    "./pgm2.o",
    NULL,
};
....
execve(args[0],args,NULL);

Most exec functions work best with absolute paths. It's only the 'p' variants of exec which search the path for an executable.

Moved to linux board.

**cbgb** · 09-22-2006

Thanks for the reply... Actually, the execve seems to be working fine. My question is more about the use of pipe() between 2 separate programs (BTW ./pgm2.o is an executable). Basically, do I need to use dup2() to allow the two programs to communicate with each other? If so, what does the file descriptor declaration AND the dup2 calls look like in BOTH programs?

That would be a big help. Thanks a lot, everybody.

**Salem** · 09-22-2006

Well the child process just uses stdin and stdout as it would do anyway.
The parent would use the ends of the pipes (or it can dup2 them as well, if it wants to use stdio).

**Yasir_Malik** · 09-23-2006

The following man page on dup2(), I think, has the clearest description of dup2: http://docsun.cites.uiuc.edu/sun_doc...c/dup2.3c.html

Each process has a file descriptor table that is indexed by integers. fd[0] refers to stdin. When you do dup2(pipes[0], 0), you make fd[0] refer to the file descriptor pipes[0]. You are in effect tricking the child process thinking that it is reading out of stdin.

**fnoyan** · 09-28-2006

I recommend you to read IPC and threads sections of Advanced Linux programming. I am sure that you can find somethin useful.

**Dalren** · 10-04-2006

Since I had to do something like this just two days ago, I still have the code in my head and will share some of it with you.

Code:

...
pid=fork()
switch(pid)
{
case -1:
     //handle fork error here
     break;
case 0:
     //child
     dup2(pipeFDs[1],STDOUT_FILENO);
     close(pipeFDs[1]);
     //execute process here
     exit(0);
default:
     dup2(pipeFDs[0],STDIN_FILENO);
     close(pipeFDs[0]);
     //do whatever you want with the input from child proc
}
...

That will set the child to replace stdout with the write end of the pipe, and the parent will replace stdin with the read end of the pipe.

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IPC between 2 programs using pipe

My apologies

Thanks for replying

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