Multiple pipes in C, program waiting for input

Question

I try to edit my post, since there were some issues with it.

I am still lost, trying to multipipe my program. When I run the program, goes into a state where it just takes some input - a like, perhaps its because I am not getting input to the second program in my piping process.

I have tried to follow the code from this post: Does this multiple pipes code in C makes sense?

My code looks like this:

int status;
int newpipe[2];
int oldpipe[2];

pid_t pid;
int countcmds = 0;
while (firstCmd != NULL) {
    printf("En iteration \n");
    if (firstCmd -> next != NULL) {
        pipe(newpipe);
    }
    pid = fork();

    if(pid == 0){
        if (firstCmd -> prev != NULL) {
            dup2(oldpipe[0],0);
            close(oldpipe[0]);
            close(oldpipe[1]);
        }
        if (firstCmd -> next != NULL) {
            close(newpipe[0]);
            dup2(newpipe[1],1);
            close(newpipe[1]);
        }
        char** file = firstCmd -> cmd;
        char* specfile = *file;
        execvp(specfile, file);
    }
    else{
        waitpid(pid, &status, 0);
        if (firstCmd -> prev != NULL) {
            close(oldpipe[0]);
            close(oldpipe[1]);
        }
        if(firstCmd -> next != NULL){
            oldpipe[0] = newpipe[0];
            oldpipe[1] = newpipe[1];
        }
        countcmds++;
        firstCmd = firstCmd -> next;
    }
}
if(countcmds){
    close(oldpipe[0]);
    close(oldpipe[1]);
}

Do the answers to [this question](http://stackoverflow.com/questions/948221/does-this-multiple-pipes-code-in-c-makes-sense) help? — legoscia, Oct 01 '12 at 17:16
I see you accepted answers, thanks. Now, can you explain exactly what "won't work" means? What do you get and what do you expect? — netcoder, Oct 01 '12 at 18:21

cdarke · Answer 1 · 2012-10-01T18:51:24.700

Your execvp(cmd,numberFile); arguments are way off, the prototype is:

int execvp(const char *file, char *const argv[]);

No idea what your cmd looks like, but you are supplying an int for the second parameter.

The execvp within execute_piping() also look suspicious, in that you appear to be supplying the complete argument list to the first parameter.

Note: char *argv[] means that argv is an array of pointers to char. I don't see that anywhere, but you should show what structs Cmd and ShellCmd are.

I got these warnings from your code:

gash.c:33: warning: passing argument 1 of ‘execvp’ from incompatible pointer type
gash.c:33: warning: passing argument 2 of ‘execvp’ makes pointer from integer without a cast

it is a good idea to fix warnings, warnings are your friends.

Multiple pipes in C, program waiting for input

1 Answers1