frame_counter.c

#include <dlfcn.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "GL/glx.h"

// compile: "gcc frame_counter.c -ldl -D_GNU_SOURCE -shared -fPIC -o frame_counter.so"
// run: LD_PRELOAD=./frame_counter.so glxgears

void (*_glXSwapBuffers)(Display *dpy, GLXDrawable drawable);
struct timespec then,now;
static unsigned frame_counter, accumulator, interval;
FILE *fd;
char name[128] = {0}; 

__attribute__((constructor))
void foo(void) 
{
    clock_gettime(CLOCK_REALTIME,&then);
    char *executable_path = getenv("_");
    char *base_name = executable_path;
    while(*++executable_path)
    {
        if(*executable_path == '/')
        {
            base_name = executable_path+1;
        }
    }

    strncpy(name, base_name, 120);
    int len = strlen(base_name);
    strcpy((char*)(name+len), ".output");
    fd = fopen(name, "w");
    if(!fd)
    {
        fprintf(stderr, "FML, I cant write to %s\n", name);
        exit(1);
    }
    else
        printf("Saving FPS measurements into %s\n", name);
}

__attribute__((destructor))
void foo2(void)
{
    fprintf(fd, "%d\n", accumulator / (interval ? interval : 1));
    fclose(fd);
}

void glXSwapBuffers(Display *dpy, GLXDrawable drawable)
{
    _glXSwapBuffers = (typeof(_glXSwapBuffers)) dlsym(RTLD_NEXT, "glXSwapBuffers");
    _glXSwapBuffers(dpy, drawable);

    clock_gettime(CLOCK_REALTIME,&now);

    frame_counter++;
    
    long diff = ( now.tv_nsec - then.tv_nsec )  + ( now.tv_sec - then.tv_sec ) * 1E9;
    // one sec passed between then and current time?
    if(diff >= 1E9)
    {
        then = now;
        fprintf(fd, "%d\n", frame_counter);
        accumulator += frame_counter;
        interval++;
        frame_counter = 0;
    }
}