/*  This code, useupper.c, accepts a file name as an argument    and will respond with an error if called incorrectly.  */#include <unistd.h>#include <stdio.h>int main(int argc, char *argv[]){    char *filename;    if(argc != 2) {        fprintf(stderr, "usage: useupper file\n");        exit(1);    }    filename = argv[1];/*  That done, we reopen the standard input, again checking for any errors as we do so,    and then use execl to call upper.  */    if(!freopen(filename, "r", stdin)) {        fprintf(stderr, "could not redirect stdin to file %s\n", filename);        exit(2);    }    execl("./upper", "upper", 0);/*  Don't forget that execl replaces the current process;    provided there is no error, the remaining lines are not executed.  */    fprintf(stderr, "could not exec upper!\n");    exit(3);}