Question

Below is the structure defined.

typedef struct{
    int a;
    char b;
}X;
typedef struct{
    X m;
    int c;
    char d;
}B;
B n,q;
n.m.a = 12;
n.m.b = 'a';
n.c = 13;
n.d = 'b';

I do a fwrite of the following structure in a file. File is opened as below.

fp = fopen("D://tests//t11test.txt","wb");
fwrite(&n, sizeof(B), 1, fp);

The fwrite is successful and I checked the contents of the file corresponding to fp.
But when I do a fread on the same file after closing and reopening the file, I am not able to read the contents of the sub-structure m. The fread is

fp = fopen("D://tests//t11test.txt","rb");
fread(&q, sizeof(B), 1,fp);

Where am I going wrong?

Answer 1

I can’t see what the problem is, but, FWIW, this worked:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <assert.h>

typedef struct{ int a; char b; }X;
typedef struct{ X m; int c; char d; }B;

void print_B(B* a_b)
{
    printf("{ { %d, %c }, %d, %c }",
           a_b->m.a,
           a_b->m.b,
           a_b->c,
           a_b->d);
}

int main()
{
    /* Write structure. */
    {
        FILE* fp;
        B n;
        n.m.a = 12;
        n.m.b = 'a';
        n.c = 13;
        n.d = 'b';
        fp = fopen("x.dat", "wb");
        assert(0 != fp);
        if (1 != fwrite(&n, sizeof(B), 1, fp))
        {
            fprintf(stderr, "Failed to fwrite(): %s\n", strerror(errno));
            return 1;
        }
        fclose(fp);

        printf("wrote: ");
        print_B(&n);
        printf("\n");
    }

    /* Read structure. */
    {
        FILE* fp;
        B q;
        fp = fopen("x.dat", "rb");
        assert(0 != fp);
        if (1 != fread(&q, sizeof(B), 1, fp))
        {
            fprintf(stderr, "Failed to fread(): %s\n", strerror(errno));
            return 1;
        }
        fclose(fp);

        printf("read : ");
        print_B(&q);
        printf("\n");
    }

    return 0;
}

Output:

wrote: { { 12, a }, 13, b }
read : { { 12, a }, 13, b }