mmap_sync.c

#include <stdio.h>
#include <errno.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>

int main(int argc, char **argv)
{
    int ret;
    int fd;
    void *addr;
    struct stat s;
    int file_size = 0;

    if (argc != 3) {
        printf("%s [filename] [filesize in MB]\n", argv[0]);
        return -1;
    }

    file_size = atoi(argv[2]);

    fd = open(argv[1], O_RDWR | O_CREAT, S_IRWXU);
    if (fd < 0) {
        printf("failed to open %s\n", argv[1]);
        return -1;
    }

    ret = ftruncate(fd, file_size * 1024 * 1024);
    if (ret < 0) {
        printf("failed to trucate file %s to %d MB\n", argv[1], file_size);
        return -1;
    }

    ret = stat(argv[1], &s);
    if (ret < 0) {
        printf("failed to stat %s\n", argv[1]);
        return -1;
    }

    addr = mmap(NULL, s.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (!addr) {
        printf("Failed to mmap %s\n", strerror(errno));
        return -1;
    }

    printf("data at the address %p is %s\n", addr, addr);

    memset(addr, 0, s.st_size);
    strcpy(addr, "Hello Mmap");

    // sync to the disk sychrnously
    msync(addr, s.st_size, MS_SYNC);
    perror("msync");

    munmap(addr, s.st_size);

    close(fd);
    return 0;
}