sayonara memcpy

overlap.c

/*
 * The purpose of this program is to show you should always use memmove
 * instead of memcpy if there is no particular reason.
 *
 * This program intentionally copies overlapped memories, though.
 *
 * Try to compile with different optimizations, compilers, OSs.
 *
 * For example,
 *
 * $ gcc -O0 overlap.c && ./a.out
 * $ gcc -O2 overlap.c && ./a.out
 * $ gcc -O3 overlap.c && ./a.out
 *
 * $ clang -O0 overlap.c && ./a.out
 * $ clang -O3 overlap.c && ./a.out
 *
 * On OS X (10.8), Apple's LLVM 4.2 (clang-425.0.28)
 * gave different results of memcpy between -O0 and -O3.
 *
 *
 */

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

static void copy_byte(void* (func)(void *dst, const void* src, size_t len)) {
  unsigned char* ptr;
  unsigned char* block;

  block = (unsigned char *)malloc(64);
  if (block == NULL) {
    fprintf(stderr, "out of memory.\n");
    exit(1);
  }
  memset(block, 0, 64);

  block[17] = 0xffU;
  printf("block[%d] = 0x%x\n", 17, block[17]);

  ptr = block + 16;
  func(block, ptr, 32);
  printf("block[%d] = 0x%x\n", 17, block[17]);
  printf("block[%d] = 0x%x\n", 1, block[1]);

  free(block);
}

int main() {
  printf("memmove:\n");
  copy_byte(memmove);

  printf("\nmemcpy:\n");
  copy_byte(memcpy);

  return 0;
}