How to pipe different processes? (C)

Question

I'm trying to pipe two different processes to implement the terminal's functionality "|".

I need the new child to execute a command itself, and fork another process to execute a different command using the first process's output, then output the final process's results on the terminal.

Here's my code, I keep the original parent untouched because I need to continue executing the other parts of my program afterwards.

int exec_pipe(char **args, int index, int *i, char**** jobs){

int fd1[2];
pid_t pid, wpid;
int status;
int savedStdOut = dup(1);
int savedStdIn = dup(fileno(stdin));

if (pipe(fd1)==-1)
{
    fprintf(stderr, "Pipe Failed" );
    return 1;
}

pid = fork();

if (pid < 0)
{
    fprintf(stderr, "fork Failed" );
    return 1;
}else if (pid == 0)
{
    // Child process
    close(fd1[0]);
    dup2(fd1[1], 1);

    printf("%s\n", args[index - 1]);

    if (execvp(args[0], args) == -1)
    {
        printf("command not found\n");
    }

    int childId = fork();

    if(childId < 0){
        fprintf(stderr, "fork Failed" );
        return 1;
    }else if(childId == 0){
        // child of child of parent

        fdopen(fd1[1], "r");
        dup2(fd1[1], savedStdOut);


        if (execvp(args[index + 1], args) == -1)
        {
            printf("command not found\n");
        }
        exit(EXIT_FAILURE);

    }else {
        // parent of child of child of parent
        do
        {
            wpid = waitpid(pid, &status, WUNTRACED);
        } while (!WIFEXITED(status) && !WIFSIGNALED(status)); // wait for child until it's exited or been killed
        fdopen(savedStdIn, "r");
        close(fd1[0]);
        close(fd1[1]);
    }
}else{
    // Parent process
    do
    {
        wpid = waitpid(pid, &status, WUNTRACED);
    } while (!WIFEXITED(status) && !WIFSIGNALED(status)); // wait for child until it's exited or been killed
}
return 1;

}

I'm getting a "No such file or directory" error with the program ending with exit code 9.

At the risk of sounding self-serving, [you may find this helpful](https://stackoverflow.com/a/19357317/1322972). — WhozCraig, Nov 23 '19 at 17:16
You might want to read: [three star programmer](https://stackoverflow.com/questions/21488544/triple-pointers-in-c-is-it-a-matter-of-style) There are also many online resources about this kind of construct. Overall, being called a 'three star programmer' is NOT a compliment. — user3629249, Nov 23 '19 at 17:53
OT: regarding: `if (execvp(args[0], args) == -1) { printf("command not found\n"); }` the `exec*()` functions ONLY returned if an error occurred. Therefore, a much better code would be: `execvp(args[0], args); perror("execvp failed"); exit( EXIT_FAILURE );` — user3629249, Nov 23 '19 at 17:58
OT: regarding: `}else if(childId == 0){` for ease of readability and understanding: please follow the axiom: *only one statement per line and (at most) one variable declaration per statement.* — user3629249, Nov 23 '19 at 18:01
OT: when calling C library functions, like `fdopen()` and `dup()`, always check the returned value to assure the operation was successful. If not successful, then call `perror()`, cleanup, and call `exit( EXIT_FAILURE );` — user3629249, Nov 23 '19 at 18:03
OT: regarding: `fprintf(stderr, "fork Failed" );` This does not tell the user why the failure occurred. Suggest: `perror( "fork failed" );` as that will output to `stderr`, both the error message AND the text reason the system thinks the error occurred. — user3629249, Nov 23 '19 at 18:05
OT: regarding: `int childId = fork();` this should be written as: `pid_t childId = fork();` — user3629249, Nov 23 '19 at 18:08
regarding: `if (execvp(args[0], args) == -1) { printf("command not found\n"); } int childId = fork();` the call to `fork()` and the following code will 'never' be executed if the prior call to `execvp()` was successful — user3629249, Nov 23 '19 at 18:11

score 0 · Accepted Answer · answered Dec 13 '19 at 13:35

Solved it based on some comments, thanks all!

int fd1[2];
pid_t pid;

// allocate two pipe sets
if (pipe(fd1) < 0)
{
    perror("Failed to create pipe.");
    return EXIT_FAILURE;
}

// launch first child process.
if ((pid = fork()) < 0)
{
    perror("Failed to fork child(1)");
    return EXIT_FAILURE;
}

if (pid == 0)
{
    // child
    //  stdout  = fd1(write)
    if(strcmp(args[0], args[index - 1]) == 0)
    {
        dup2(fd1[WRITE], STDOUT_FILENO);
        close(fd1[READ]);
        close(fd1[WRITE]);
        execlp(args[0], args[0], (char*) NULL);
        fprintf(stderr, "failed to execute '%s'\n", args[0]);
        exit(1);
    }else{
        dup2(fd1[WRITE], STDOUT_FILENO);
        close(fd1[READ]);
        close(fd1[WRITE]);
        execlp(args[0], args[0], args[index - 1], (char*) NULL);
        fprintf(stderr, "failed to execute '%s'\n", args[0]);
        exit(1);
    }
}else
{
    // parent
    //  fork once more.
    if ((pid = fork()) < 0)
    {
        perror("Failed to fork child(2)");
        return EXIT_FAILURE;
    }

    if (pid == 0)
    {
        // child of child
        //  stdin = fd1(read)
        dup2(fd1[READ], STDIN_FILENO);
        close(fd1[WRITE]);
        close(fd1[READ]);
        execlp(args[index + 1], args[index + 1], (char*) NULL);
        fprintf(stderr, "failed to execute '%s'\n", args[index + 1]);
        exit(1);
    }else{
        int status;
        close(fd1[READ]);
        close(fd1[WRITE]);
        waitpid(pid, &status, 0);
    }
}

Please note there are some functions used in the code that aren't a part of the piping process, I just posted my entire code, you just need to focus on the piping parts of it. — Bassam, Dec 13 '19 at 13:37

How to pipe different processes? (C)

1 Answers1