C/C++ structs for wall-clock timing code execution.

cputimer.h

#ifndef __CPU_TIMER_H__
#define __CPU_TIMER_H__

#include <time.h>

struct CpuTimer {
    struct timespec start;
    struct timespec stop;
};

typedef struct CpuTimer CpuTimer;

void start(CpuTimer *timer) { clock_gettime(CLOCK_REALTIME, &timer->start); }

void stop(CpuTimer *timer) { clock_gettime(CLOCK_REALTIME, &timer->stop); }

float elapsed(CpuTimer *timer)
{
    struct timespec *start = &timer->start;
    struct timespec *stop = &timer->stop;
    float elapsed = (stop->tv_sec - start->tv_sec) +
                    1e-9 * (stop->tv_nsec - start->tv_nsec);
    return elapsed;
}

#endif

cputimer.hpp

#ifndef __CPU_TIMER_H__
#define __CPU_TIMER_H__

#include <ctime>

class CpuTimer {
public:
    void start() { clock_gettime(CLOCK_REALTIME, &start_); }
    void stop() { clock_gettime(CLOCK_REALTIME, &stop_); }
    float elapsed()
    {
        float elapsed = (stop_.tv_sec - start_.tv_sec) +
                        1e-9 * (stop_.tv_nsec - start_.tv_nsec);
        return elapsed;
    }
private:
    timespec start_;
    timespec stop_;
};

#endif

cputimer2.hpp

// Portable C++11 timer.
#pragma once

#include <chrono>

using namespace std::chrono;

class CpuTimer {
public:
    void start() { t1 = high_resolution_clock::now(); }
    void stop() { t2 = high_resolution_clock::now(); }
    double elapsed() 
    { 
        return duration_cast<duration<double>>(t2 - t1).count(); 
    }
private:
    high_resolution_clock::time_point t1, t2;
};

Create an instance of the struct/class, invoke start before a section to be timed, stop after, and then print the elapsed time.
C++:

#include "cputimer.hpp"
// or
#include "cputimer2.hpp"

CpuTimer timer;
timer.start();
...
timer.stop();
std::cout << "Wall time: " << timer.elapsed() << "\n";

C:

#include "cputimer.h"

CpuTimer timer;
start(&timer);
...
stop(&timer);
printf("Wall time: %f \n", elapsed(&timer));