Question

I am trying to mimic the bash feature of running process in the background if “&” is found at the end of the command. I have the following function…and I don’t think it’s doing what I want it to do

int execute(char* args[],int background,int *cstatus){
    pid_t   child;
    pid_t   ch;                         /*Pid of child returned by wait*/
    if ((child = fork()) == 0){                 /*Child Process*/
        execvp(args[0],args);       
        fprintf(stderr, "RSI: %s: command not found\n",args[0]); /*If execvp failes*/
        exit(1);

    }else{          /*Parent process*/
        if (child== (pid_t)(-1)) {
            fprintf(stderr,"Fork failed\n"); exit(1);
        }else{
            if (background==0){             /*If not running in background..wait for process to finish*/
                ch = wait(cstatus);
            }else{
                printf("%ld Started\n",(long)getpid());
        /*  printf("Parent: Child %ld exited with status = %ld\n", (long) ch, (long)cstatus);
    */  }}
    }
return 0;
}
int wait_and_poll(int *cstatus){
    pid_t status;
    status = waitpid(-1,cstatus,WNOHANG);
    if (status>0){
        fprintf(stdout,"%ld Terminated.\n",(long) status);
    }
return 0;
}

If i just run “ls -l ” it works as expected..but if I want to run ls in the background..and have the program keep accepting new commands I call the function with background flag set as 1 and I want it to run the process in the background, tell me that it has created the process..and then prompt to accept next command.

Answer 1

I don’t think waitpid(-1, &cstatus, WNOHANG); does what you think it does. You need to check its return value. If it is > 0, that’s the PID of the child process that has exited. If it’s 0 or -1, no child process has changed state.

You can just call waitpid(-1, &cstatus, WNOHANG); before and/or after each command you run. Call it in a loop to catch more than one child exit.

You can also handle SIGCHILD. Your process will receive this signal immediately after a child exit, which is good if you want to report child process termination immediately, without waiting for user’s input.