Question

I’d really love your help with understanding why doesn’t the process reach the “son process” after using fork() command. I’m trying to write a program that runs another program, but It seems that the program dosen’t even reach the son process. I can tell that since “son process” is not being printed to the screen, and I really wonder why.

Here’s a sketch of the code- I can’t even check if it is alright since as I said, it doesn’t even reaching the son process, I always get “son exited with error”.

#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <assert.h>
#include <signal.h>
#include <string.h>
#include <stdio.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/time.h>
#include <time.h>



#define MAXARGV 5;

int main() {
    char* cmd;

    int child_status;

    char* s;
    char** argv;
    int counter;

    cmd= (char*) calloc( 5, sizeof(char)*20);
    s=(char*) calloc(1,sizeof(char)*20);
    argv=(char**) calloc(5, sizeof(char*)*20);




    printf("Please write a command\n");

    gets(cmd);

    counter = 0;

    while (strcmp(cmd, "exit") != 0) {

        int pid = fork();


        if (pid == 0) {
            printf("son process");

            while (sscanf(cmd, "%s", s) == 1) {

                strcpy(argv[counter], s);
                counter++;
            }

            execv(argv[0], argv);

            printf("the command is not legal");
            assert(0);

        }

        else {

            if (wait(&child_status) == -1) {
                printf("error waiting for pid=%d\n", pid);
                exit(-1);
            }

              if(WIFEXITED(child_status)!=0)
                    printf("son status=%d\n", WEXITSTATUS(child_status));
                else
                    printf("son exited with error\n");

        }

        printf("Please write a command");

        gets(cmd);

    }

    free(s);
    free(cmd);
    free(argv);
    printf("here as well");
    return 1;
}

Answer 1

1. The program reaches the printf("son process") just fine, but that just puts the string in a buffer inside the process and since you didn’t fflush() it, it doesn’t make it to the screen and is discarded with the rest of the process’ memory in the exec call. Note, that stdout is normally line-buffered, so if you had newline there, it would auto-flush. Also stderr is by default unbuffered and more suitable for debug prints (fprintf(stderr, "child process")).
2. You are trying to assemble the command read from standard input in argv, but it only has memory for the actual arguments given to you, so you overrun this memory and get segmentation fault.
3. if WIFEXITED gives zero, you should use WIFSIGNALED and WTERMSIG to confirm that the error is indeed SIGSEGV.
4. assert(0) is not a good way to terminate process after error. exit(1) is. Assertions are only for conditions that indicate bug in the code itself if they happen and are often eliminated (by defining NDEBUG) from production code.