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Question

I wanna implement a program that reads from stdin using a shell command and then I have to store the input and do other things with it.

        int a[2];
        if(pipe(a)==-1){
                err(1, "error while creating a pipe");
        }

        int fd=fork();
        if(fd<0){
                err(1, "error while forking");
        }

        if(fd==0){
                close(a[0]);
                dup2(a[1], 1);
                execlp("read", "read", NULL);
        }

        close(a[1]);
        dup2(a[0], 0);

It does not take an input, how I can fix it?

Answer 1

read is a shell built-in. It can be run only as a shell command, not launched directly via any of the exec -family functions. If you check, you will very likely find that your execlp() call is failing (returning a nonzero value), which is a twofold problem for you:

1. The failure itself means that the work you wanted done does not, in fact, get done. But also

2. After execlp() returns, which it does only if it fails, the child process goes on to perform work that was intended only for the parent process.

You should always check your system calls for errors, but more so for the exec -family functions than for most. Generally speaking, if execlp or one of its siblings fails, you want that process to perform an _exit(1) soon after.

The correct way to run a shell command via execlp is to launch a shell, specifying the command(s) to run via a -c argument. That would look something like this:

    execlp("/bin/sh", "sh", "-c", "read", (char *) NULL);

You can use a different shell ( bash , zsh , etc .) if you prefer.

But also , read consumes input and stores it in one or more shell variables. It does not produce output, so I'm not sure how you expect to determine whether it read anything or not.