Memcpy vs pointer dereference

test.c

/**
 * Memcpy demonstration. Try compiling with:
 *
 * $ gcc -Wall -Werror -pedantic -std=c99 -S test.c -o test-deref.s
 * $ gcc -Wall -Werror -pedantic -std=c99 -S test.c -DMEMCPY -o test-memcpy.s
 * $ diff -u test-deref.s test-memcpy.s
 *
 * And play with -O0 to -O3 flags to see the effect of optimizations.
 */

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

/*
 * Char must point to at least sizeof(unsigned int) bytes of memory
 */
static unsigned int
test_cpy(const char *n) {
    unsigned int res = 0;
#ifdef MEMCPY
    memcpy(&res, n, sizeof(unsigned int));
#else
    res = *((unsigned int *)n);
#endif
    return res;
}

int
main(int argc, const char *argv[])
{
    if (argc <= 1) {
        printf("Requires at least one arg\n");
        return 1;
    }
    if (strlen(argv[1]) < sizeof(unsigned int)) {
        printf("Length of first arg must be at least %llu\n", (unsigned long long) sizeof(unsigned int));
        return 1;
    }
    unsigned int res = test_cpy(argv[1]);
    printf("Interpreting %s as unsigned int: %u\n", argv[1], res);
    return 0;
}

Hi sebastian, you're right! #ifdef is cleaner. Good catch. GCC has me covered, because -D defines something as 1 by default, so this will still work. You can test it by putting a #error "Using memcpy" in the memcpy block. This will break only -DMEMCPY.

Thanks for the feedback, I'll change it to #ifdef as you suggested to prevent further confusion.