Test struct layout by GCC

test5.c

#include <stdio.h>

struct s0 {
    char c1;
};

struct s1 {
    char c1;
    char c2;
};

struct s2 {
    void *ptr;
    char c2;
};

struct s3 {
    char c1;
    void *ptr;
    char c2;
};

struct s4 {
    char c1;
    long double ld;
    char c2;
};

int main (int argc, char **argv) {
    printf("size of struct s0 = %lu\n", sizeof(struct s0));
    printf("size of struct s1 = %lu\n", sizeof(struct s1));
    printf("size of struct s2 = %lu\n", sizeof(struct s2));
    printf("size of struct s3 = %lu\n", sizeof(struct s3));
    printf("size of struct s4 = %lu\n", sizeof(struct s4));
}

8
8
16
24

@asandroq Nice try! But the first two are wrong.

(At least with gcc's default options.)

Woah, struct s4 is 48 bytes on x86_64 with GCC 4.6.3. That's the only one that threw me off.

Also, Clang 3.1 gives the same output.

@komiga Yup! Apparently, gcc starts aligning on double-words whenever you have long double in the structure. So 3 * 16 = 48, which explains it.

Unfortunately, that means our ooc varargs bug is very hard to solve...

(And yes, actually all compilers should give the same output, except if you use -m96-bit-long-double, because otherwise it'd break the ABI.

@nddrylliog

On my Pi (32bit ARM):

size of struct s0 = 1
size of struct s1 = 2
size of struct s2 = 8
size of struct s3 = 12
size of struct s4 = 24

On my desktop (64bit x86):

size of struct s0 = 1
size of struct s1 = 2
size of struct s2 = 16
size of struct s3 = 24
size of struct s4 = 48