C Programming 2 pipes - c

Question

I want to set up 2 pipes in my program. I have 1 pipe working fine, but I don't know where to place the second pipe.
The pseudo code of my setup is shown below,
Here is it with curly braces sorry about that
//the first pipe:
pipe(pipe1)
//the second pipe:
pipe(pipe2)
pid = fork()
if(pid == 0) {
dup2(pipe1[1], 1)
close(pipe1[0])
execvp(beforepipe)
}
if(pid > 0) { //everything below is in here
pid2 = fork()
if(pid2 == 0){
//pipe1
dup2(pipe1[0],0)
dup2(out,1)
close(pipe1[1])
execvp(afterpipe)
//pipe2 does not work might need to be placed in different area
dup2(pipe1[1],1)
close(pipe1[0])
execvp(beforepipe1)
}
if(pid2 > 0){
close(pipe[0])
close(pipe[1])
wait() //this is an infinite for loop
pid3 = fork()
if(pid3 == 0){
dup2(pipe2[0],0)
dup2(out,1)
close(pipe2[1])
execvp(afterpipe2)
}
if(pid3 > 0) {
close(pipe2[0])
close(pipe2[1])
wait()
}
}
The position of the second pipe is in the wrong place or the code is altogether wrong.
Any suggestions?

Answer 1

Your main problem is that you are not closing anywhere near enough file descriptors. Given a file input1 in the current directory containing your string "eschew obfuscation\", this code works for me (but note how many file descriptors have to be closed!).
Rule of thumb: if a pipe is dup2()d or dup()d to standard input or output, close both file pipe file descriptors.
Example code (with debug tracing in place):
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
/* command pipeline: cat input1 | tr a-z A-Z | tr \\ q */
int main(void)
{
int pipe1[2];
int pipe2[2];
pid_t pid1;
char *cmd1[] = { "cat", "input1", 0 };
char *cmd2[] = { "tr", "a-z", "A-Z", 0 };
char *cmd3[] = { "tr", "\\", "q", 0 };
if (pipe(pipe1) != 0 || pipe(pipe2) != 0)
{
perror("pipe failed");
return 1;
}
pid1 = fork();
if (pid1 < 0)
{
perror("fork 1 failed");
return 1;
}
if (pid1 == 0)
{
/* Child 1 - cat */
dup2(pipe1[1], 1);
close(pipe1[0]);
close(pipe1[1]);
close(pipe2[0]);
close(pipe2[1]);
execvp(cmd1[0], cmd1);
perror("failed to execute cmd1");
return 1;
}
printf("pid 1 = %d\n", pid1);
fflush(stdout);
pid_t pid2 = fork();
if (pid2 < 0)
{
perror("fork 2 failed");
return 1;
}
if (pid2 == 0)
{
/* Child 2 - tr a-z A-Z */
dup2(pipe1[0], 0);
dup2(pipe2[1], 1);
close(pipe1[0]);
close(pipe1[1]);
close(pipe2[0]);
close(pipe2[1]);
execvp(cmd2[0], cmd2);
perror("failed to execute cmd2");
return 1;
}
printf("pid 2 = %d\n", pid2);
fflush(stdout);
pid_t pid3 = fork();
if (pid3 < 0)
{
perror("fork 3 failed");
return 1;
}
if (pid3 == 0)
{
/* Child 3 - tr \\ q */
dup2(pipe2[0], 0);
close(pipe1[0]);
close(pipe1[1]);
close(pipe2[0]);
close(pipe2[1]);
execvp(cmd3[0], cmd3);
perror("failed to execute cmd3");
return 1;
}
printf("pid 3 = %d\n", pid3);
fflush(stdout);
/* Parent - wait for the kids to all die */
close(pipe1[0]);
close(pipe1[1]);
close(pipe2[0]);
close(pipe2[1]);
pid_t corpse;
int status;
while ((corpse = wait(&status)) > 0)
printf("Child %d died status 0x%.4X\n", corpse, status);
return 0;
}

Answer 2

execvp(afterpipe)
//pipe2 does not work might need to be placed in different area
dup2(pipe1[1],1)
close(pipe1[0])
execvp(beforepipe1)
I think the execvp() didnot return. So the code below the execvp() is irrelevent.