How can I get/set a struct member by offset

Question

Ignoring padding/alignment issues and given the following struct, what is best way to get and set the value of member_b without using the member name.

struct mystruct {     int member_a;     int member_b; } struct mystruct *s = malloc(sizeof(struct mystruct)); 

Put another way; How would you express the following in terms of pointers/offsets:

s->member_b = 3; printf("%i",s->member_b); 

My guess is to

• calculate the offset by finding the sizeof the member_a (int)
• cast the struct to a single word pointer type (char?)
• create an int pointer and set the address (to *charpointer + offset?)
• use my int pointer to set the memory contents

but I get a bit confused about casting to a char type or if something like memset is more apropriate or if generally i'm aproching this totally wrong.

Cheers for any help

Answer 1

The approach you've outlined is roughly correct, although you should use offsetof instead of attempting to figure out the offset on your own. I'm not sure why you mention memset -- it sets the contents of a block to a specified value, which seems quite unrelated to the question at hand.

Here's some code to demonstrate how it works:

#include <stdio.h> #include <stdlib.h> #include <stddef.h>  typedef struct x {     int member_a;     int member_b; } x;  int main() {      x *s = malloc(sizeof(x));     char *base;     size_t offset;     int *b;      // initialize both members to known values     s->member_a = 1;     s->member_b = 2;      // get base address     base = (char *)s;      // and the offset to member_b     offset = offsetof(x, member_b);      // Compute address of member_b     b = (int *)(base+offset);      // write to member_b via our pointer     *b = 10;      // print out via name, to show it was changed to new value.     printf("%d\n", s->member_b);     return 0; } 

Answer 2

The full technique:

1. Get the offset using offsetof:

   b_offset = offsetof(struct mystruct, member_b);

2. Get the address of your structure as a char * pointer.

   char *sc = (char *)s;

3. Add the add the offset to the structure address, cast the value to a pointer to the appropriate type and dereference:

   *(int *)(sc + b_offset)