如何为另一个程序运行命令usig sys调用“ execvp”

Question

i've written a comp.out which accept 2 arguments to the main argv and compare between the files. the main must return the values 1,2,3 where 3 means identical, 2 almost identical and 1 not identical. exmp : ./comp.out /home/demo/code/1.txt /home/demo/code/2.txt in another program i'm trying to apply this "comp.out" i've written. the thing is that i need to know what is its return status and to make it work. I've noticed i had recieved "-1" in the execvp command. this is the code i've written so far.

so how do I execute the "comp.out" command?

Thanks for the helpers!

void compareOutputFiles(char *path, char *arg2) {
    pid_t   runner;
    char *cmd [] = {"./comp.out",path, arg2, NULL};
    int status;
    int savedFD     = dup(0);
    int dirchange   = chdir(path);
    int fdin, fdout;

    strcat(path, "/output.txt");
    cmd[1] = path;
    fdin    = open(path,O_RDONLY);


    dup2(fdin,  0);


    if ((runner = fork()) < 0) {perror("could not make fork");}
    else if (runner == 0) {
                execvp(cmd[0],cmd); ->>>>>>>>>>>>returns -1
                exit(0);
    } else if (runner != 0) {
        waitpid(runner,&status,0);
        printf("return val :%d\n", (status));

    }
    dup2(savedFD, 0);
    close(fdin);
} 

Answer 1

According to man waitpid , the parameter you pass for status is not set exactly to the return code of the child process but the value that can be checked by various macros including

   WIFEXITED(status)
          returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().

   WEXITSTATUS(status)
          returns  the  exit status of the child.  This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as
          the argument for a return statement in main().  This macro should be employed only if WIFEXITED returned true.

so your code should probably look like this:

waitpid(runner,&status,0);
printf("status: %d\n", (status));
//probably exit here or return error if child died
printf("finished normally: %d\n", WIFEXITED(status));
printf("return code: %d\n", WEXITSTATUS(status));