CODEWITHSUNDEEP
cstructure
ajay bidari
ajay bidari

Reputation: 723

reading structure within a structure from a file

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?

Upvotes: 2

Views: 2095

Answers (2)

ckruse
ckruse

Reputation: 9740

I think you messed up something else, this works:

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

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

int main() {
    FILE *fd;
    B n, q;

    n.m.a = 12;
    n.m.b = 'a';
    n.c = 13;
    n.d = 'b';

    if((fd = fopen("test.dat", "wb")) == NULL) {
        fprintf(stderr, "Error opening file: %s\n", strerror(errno));
        exit(-1);
    }

    fwrite(&n, sizeof(n), 1, fd);
    fclose(fd);

    if((fd = fopen("test.dat", "rb")) == NULL) {
        fprintf(stderr, "Error opening file: %s\n", strerror(errno));
        exit(-1);
    }

    fread(&q, sizeof(q), 1, fd);
    fclose(fd);

    printf(
        "n.m.a: %d, q.m.a: %d; n.m.b: %c, q.m.b: %c; n.c: %d, q.c: %d; n.d: %c, q.d: %c\n",
        n.m.a, q.m.a, n.m.b, q.m.b, n.c, q.c, n.d, q.d
    );

    return 0;
}

Output:

n.m.a: 12, q.m.a: 12; n.m.b: a, q.m.b: a; n.c: 13, q.c: 13; n.d: b, q.d: b

Upvotes: 1

hmjd
hmjd

Reputation: 121961

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 }

Upvotes: 1

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