time.c

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

#define NS_IN_S 1000000000L

void usage();
void display_results(char**, double);

int main(int argc, char **argv) {
    pid_t pid;

    if (argc < 2)
        usage();

    pid = fork();

    if (pid == -1) {
        perror("fork()");
        exit(1);
    }
    else if (!pid) {                    /* Child executes program */
        execvp(argv[1], argv + 1);
        perror("exec()");
        exit(1);
    }
    else {                              /* Parent waits and times */
        struct timespec start, end;
        double duration;                /* In seconds */
        int status;

        /* Record start time */
        if (clock_gettime(CLOCK_MONOTONIC, &start) == -1) {
            perror("clock_gettime()");
            exit(1);
        }

        /* Wait on our child */
        waitpid(pid, &status, 0);

        /* Only display results if program exits normally */
        if (WIFEXITED(status)) {
            if (clock_gettime(CLOCK_MONOTONIC, &end) == -1) {
                perror("clock_gettime()");
                exit(1);
            }
            duration = (end.tv_sec - start.tv_sec) + (double)(end.tv_nsec - start.tv_nsec) / NS_IN_S;
            display_results(argv, duration);
        }
        else
            fprintf(stderr, "Child process did not exit normally\n");
    }

    return 0;
}

void usage() {
    fprintf(stderr, "\n\t ./time <command> [args]\n\n");
    exit(2);
}

void display_results(char **argv, double duration) {
    char **ptr = argv + 1;
    while(*ptr){
        printf("%s ", *ptr);
        ptr++;
    }
    printf("took %lf seconds\n", duration);
}