how to write a struct to a file directly?

Using write and read system call. Following is an example:

blk.h:

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

struct data{
    char s[];
    int d1;
    char c;
};

#define BLK_SIZE 25

writeb.c:

#include "blk.h"

int main(){
    struct data *d= malloc(BLK_SIZE);
    d->d1= ;
    d->c= 'a';
    strcpy(d->s, "first try..");

    int fd= open("blk.dat", O_RDWR | O_CREAT | O_EXCL, S_IREAD | S_IWRITE);
    write(fd, d, BLK_SIZE);
    printf("writed!\n");
    close(fd);
    return ;
}

readb.c:

include "blk.h"

int main(){
    int fd;
    struct data *d;
    fd= open("blk.dat", O_RDWR);
    read(fd, d, BLK_SIZE);
    printf("%d, %c, %s\n", d->d1, d->c, d->s);
    close(fd);
    return ;
}