I’ve been having trouble for hours now while trying to compare 2 seemingly identical structs. I’ve looked into alignment and padding as possible issues but found no issues. When I played around with different combinations of designated initialisation and zero initialisation combined with strcpy() I found that memcpy() returned either 1 or 0. I hope I’ve finally narrowed down the problem into a single C file.

name     = my-lib
version  = 1.2.3.4.5
name     = my-lib
version  = 1.2.3.4.5

FAILED (res1) memcmp() = 1
FAILED (res2) memcmp() = 1
PASSED (res3)

is the output of;

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

#define DRIVERS_NAME_BUFF_SIZE          64
#define DRIVERS_VERSION_BUFF_SIZE       64

typedef const struct drivers {
    char name[DRIVERS_NAME_BUFF_SIZE];
    char version[DRIVERS_VERSION_BUFF_SIZE];
} const drivers;

void print_drivers(drivers drvers) {
    printf("name     = %sn", drvers.name);
    printf("version  = %sn", drvers.version);
}

int main() {
    drivers before = {
        .name    = "my-lib",
        .version = "1.2.3.4.5"
    };
    drivers after = {0};
    strcpy(after.name, "my-lib");
    strcpy(after.version, "1.2.3.4.5");
    
    print_drivers(before);
    print_drivers(after);
    printf("n");

    /****************
     * res1
     ***************/
    int res1 = memcmp((uint8_t *)(&before), (uint8_t *)(&after), sizeof(drivers));
    if (res1 != 0) {
        printf("FAILED (res1) memcmp() = %dn", res1);
    } else {
        printf("PASSED (res1)n");
    }

    /****************
     * res2
     ***************/
    int res2 = memcmp(&before, &after, sizeof(drivers));
    if (res2 != 0) {
        printf("FAILED (res2) memcmp() = %dn", res2);
    } else {
        printf("PASSED (res2)n");
    }

    /****************
     * res3
     ***************/
    uint8_t * base_before = &before; 
    uint8_t * base_after  = &after;
    int res3 = memcmp(base_before, base_after, sizeof(drivers));
    if (res3 != 0) {
        printf("FAILED (res3) memcmp() = %dn", res3);
    } else {
        printf("PASSED (res3)n");
    }
}

I’ll quickly note that removing both consts from the typedef “fixes” the problem in that all memcmp()s return 0. I don’t understand this, can someone please explain it to me. Thank you