Simple stopwatch in C

A very simple stopwatch. Intended to be used for simple benchmarking when you want to do a task several times, but exclude some setup code from the benchmarking.

Some things I have already considered:

• Using the restrict keyword. I think it's overkill for this simple project.

• Adding error checking in case you start a non-initialized (reset) clock. I left this responsibility to the user.

• Error checking in general. Don't pass uninitialized pointers.

• Returning the return value of gettimeofday in start and stop. Very few people would use this.

• Returning sw in start, stop and reset. Chaining does not seem very useful here.

• Removing the chaining in stop. Basically just added it to motivate the return values for timeDiff and timeAdd, even if I think it looks ugly. :)

#include <sys/time.h>
#include <unistd.h>

struct timeval *timeDiff(struct timeval *out, 
    const struct timeval *a, 
    const struct timeval *b) 
{
    *out = (struct timeval) { .tv_sec  = a->tv_sec  - b->tv_sec,
                              .tv_usec = a->tv_usec - b->tv_usec };
    return out;
}

struct timeval *timeAdd(struct timeval *out, 
    const struct timeval *a, 
    const struct timeval *b) 
{
    *out = (struct timeval) { .tv_sec  = a->tv_sec  + b->tv_sec,
                              .tv_usec = a->tv_usec + b->tv_usec };
    return out;
}

struct stopwatch {
    struct timeval soFar;
    struct timeval start;
};

void reset(struct stopwatch *sw) {
    sw->soFar = sw->start = (struct timeval) {0, 0};
}

void start(struct stopwatch *sw) {
    gettimeofday(&sw->start, NULL);
}

void stop(struct stopwatch *sw) {
    struct timeval now;
    gettimeofday(&now, NULL);
    struct timeval diff;
    timeAdd(&sw->soFar, timeDiff(&diff, &now, &sw->start), &sw->soFar);
}

And here is some code if you want to try it out, but it's not intended to be a part of the review.

#include <stdio.h>

double sec(const struct stopwatch *sw) {
    const double M = 1000000;
    return (sw->soFar.tv_sec * M + sw->soFar.tv_usec)/M;
}

int main(void)
{
    struct stopwatch sw;
    reset(&sw);
    start(&sw);
    sleep(3.97);
    stop(&sw);
    printf("%f\n", sec(&sw));
    start(&sw);
    sleep(1);
    stop(&sw);
    printf("%f\n", sec(&sw));
    reset(&sw);
    start(&sw);
    sleep(1);
    stop(&sw);
    printf("%f\n", sec(&sw));
}