How can I benchmark C code easily?

Is there a simple library to benchmark the time it takes to execute a portion of C code? What I want is something like:

int main(){
    benchmarkBegin(0);
    //Do work
    double elapsedMS = benchmarkEnd(0);

    benchmarkBegin(1)
    //Do some more work
    double elapsedMS2 = benchmarkEnd(1);

    double speedup = benchmarkSpeedup(elapsedMS, elapsedMS2); //Calculates relative speedup
}

It would also be great if the library let you do many runs, averaging them and calculating the variance in timing!