c - How are struct members allocated in memory?

Question

While attempting to create a memory manager for future C programs, I've come across this question:

"when structs are allocated, are their member fields stored in the order specified?"

For instance, consider the following struct.

typedef struct {
    int field1;
    int field2;
    char field3;
} SomeType;

When allocated, will the memory addresses of the fields be in the order field1, field2, field3? Or is this not guaranteed?

Answer 1

Short answer: they are allocated with the order as they declared in the struct.

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Example:

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

struct student 
{
    int id1;
    int id2;
    char a;
    char b;
    float percentage;
};

int main() 
{
    int i;
    struct student record1 = {1, 2, 'A', 'B', 90.5};

    printf("size of structure in bytes : %d
", 
        sizeof(record1));

    printf("
Address of id1        = %u", &record1.id1 );
    printf("
Address of id2        = %u", &record1.id2 );
    printf("
Address of a          = %u", &record1.a );
    printf("
Address of b          = %u", &record1.b );
    printf("
Address of percentage = %u",&record1.percentage);

    return 0;
}

Output:

size of structure in bytes : 16 
Address of id1 = 675376768
Address of id2 = 675376772
Address of a = 675376776
Address of b = 675376777
Address of percentage = 675376780

The pictorial representation of above structure memory allocation is given below. This diagram will help you to understand the memory allocation concept in C very easily.

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Further reading: check out here (also the source for the above example) for C – Structure Padding and Structure dynamic memory allocation in C.