Question

I need to simulate the following bash commands using C under Linux (with fork, exec, kill, signal, wait, waitpid, dup2, open, sleep, pipe etc).

[0] echo 'tail-f $1' > /tmp/rtail
[1]/tmp/rtail ~/.bash_history >> /tmp/1.txt &
PID of process [1] should be saved.
[2] Expect termination of the command started on step [1]. After termination print on the screen: "Program 1 terminated."

So far I have this code:

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>  
#include <sys/wait.h> 

int main(int argc, char *argv[]) {
    pid_t pID = fork();
    if (pID == 0) // child
    {
        int file = open("/tmp/rtail", O_CREAT | O_WRONLY);

        //Now we redirect standard output to the file using dup2
        dup2(file, 1);

        puts("tail -f $1");
        close(file);
        system("chmod 777 /tmp/rtail");
        exit(0);
    } else if (pID < 0) // failed to fork
    {
        printf("Failed to fork");
        exit(1);
        // Throw exception
    } else // parent
    {
        pid_t pID2 = fork();
        if (pID2 == 0) {
            char tmp1[20];
            sprintf(tmp1, "echo %i > /tmp/pidprog1", getpid());
            system(tmp1);


            int file = open("/tmp/1.txt", O_APPEND | O_WRONLY);

            //Now we redirect standard output to the file using dup2
            dup2(file, 1);
            FILE* proc = popen("sh /tmp/rtail ~/.bash_history", "r");
            char tmp[20];
            while (fgets(tmp, 40, proc) != NULL) {
                printf(tmp);
            }
            fclose(proc);

            exit(0);
        }
        else if (pID2 < 0) // failed to fork
        {
            printf("Failed to fork");
            exit(1);
            // Throw exception
        } else {
            FILE* fl = fopen("/tmp/pidprog1", "r");
            char buff[10];
            fgets(buff, 10, fl);
            int pidc = atoi(buff);
            fclose(fl);
            int status;
            waitpid(pidc, &status, 0);
            printf("Program 1 terminated\n");
        }
    }

    // Code executed by both parent and child.  

    return 0;
}

The problem is that when I manually kill the process using PID saved into /tmp/pidprog1, parent process doesn’t stop waiting and doesn’t print “Program 1 terminated” line.

Answer 1

The parent is very likely reading a garbage value into pidc. You are doing nothing to ensure that the grandchild has actually written the pid before the parent tries to read it. You need to use wait to ensure that valid pids are in the file. (Or, just keep track of the pids from the return value of fork.)

You are not doing enough error checking: what happens if any open fails? (eg, when you try
to open /tmp/1.txt for appending but it doesn’t already exist?)

Why are you using fgets to read 40 characters into a buffer of size 20?

Why are you dup’ing and using fputs instead of just writing to the fd?

Why are you printing error messages to stdout instead of stderr ( use perror ).