#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

#define MEGABYTE 1048576

int main (void)
{
    int fd;
    unsigned long int r;
    void *m;
    void *p;
    void *b;
    struct timeval ts;
    struct timeval te;
    float e;
    
    fd = open("/home/jwb/bigfile", O_RDWR);
    if (-1 == fd)
        exit(EXIT_FAILURE);
    
    m = mmap(0, 1024 * MEGABYTE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);    
    if (NULL == m)
        exit(EXIT_FAILURE);
    
    b = malloc(16 * MEGABYTE);    
    if (NULL == b)
        exit(EXIT_FAILURE);
        
    srand(1);
    
    for (;;) {
        r = (unsigned long int)((1024.0 - 16.0) * MEGABYTE * rand() / RAND_MAX + 0.0);
        p = m + r;

        gettimeofday(&ts, NULL);
                
        memcpy(b, p, 16 * MEGABYTE);
        
        gettimeofday(&te, NULL);

        e = te.tv_sec - ts.tv_sec + (te.tv_usec - ts.tv_usec)/1000000.0;
        
        printf("Read: %.2f MB/sec\n", 16/e);
        
        r = (unsigned long int)((1024.0 - 16.0) * MEGABYTE * rand() / RAND_MAX + 0.0);
        p = m + r;

        gettimeofday(&ts, NULL);
                
        memset(p, '1', 16 * MEGABYTE);
        fsync(fd);
        
        gettimeofday(&te, NULL);
        
        e = te.tv_sec - ts.tv_sec + (te.tv_usec - ts.tv_usec)/1000000.0;
        printf("Write: %.2f MB/sec\n", 16/e);
        
        fflush(stdout);
    }
    
    exit(EXIT_SUCCESS);
}