memset not filling array

Question

 u32 iterations = 5;
 u32* ecx = (u32*)malloc(sizeof(u32) * iterations);

 memset(ecx, 0xBAADF00D, sizeof(u32) * iterations);
 printf("%.8X\n", ecx[0]);

 ecx[0] = 0xBAADF00D;
 printf("%.8X\n", ecx[0]);

 free(ecx);

Very simply put, why is my output the following?

0D0D0D0D
BAADF00D

ps: u32 is a simple typedef of unsigned int

edit:

• Compiling with gcc 4.3.4
• string.h is included

Answer 1

The second parameter to memset is typed as an int but is really an unsigned char. 0xBAADF00D converted to an unsigned char (least significant byte) is 0x0D, so memset fills memory with 0x0D.

Answer 2

The second argument to memset() is a char not a int or u32. C automatically truncates the 0xBAADF00D int into a 0x0D char and sets each character in memory as requested.

Answer 3

I tried it with wmemset(). It seems to work:

#include <stdlib.h>
#include <stdio.h>
#include <inttypes.h>
#include <wchar.h>

int main(void){
  uint32_t iterations = 5;
  uint32_t *ecx = (uint32_t*)malloc(sizeof(uint32_t) * iterations);

  wmemset( (wchar_t*)ecx, 0xBAADF00D, sizeof(uint32_t) * iterations);
  printf("%.8X\n", ecx[0]);

  ecx[0] = 0xBAADF00D;
  printf("%.8X\n", ecx[0]);

  /* Update: filling the array with memcpy() */
  ecx[0] = 0x11223344;
  memcpy( ecx+1, ecx, sizeof(*ecx) * (iterations-1) );
  printf("memcpy:   %.8X %.8X %.8X %.8X %.8X\n",
             ecx[0], ecx[1], ecx[2], ecx[3], ecx[4] );
}