ARM: unaligned memory access (EXC_ARM_DA_ALIGN)

align.c

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

#define DEREFENCE_MODE 1

#define IS_ALIGNED(P_tr) \
  (((uintptr_t) (P_tr) & 3) == 0 ? "YES" : "NO")

static struct {
    const char *buf;
    int len;
    double x;
} foo[] = {
    {"abcd", 4, 3.14159} /* OK */,
    {"abc",  3, 6.28318} /* KO */
};

int main() {
  int ntests = (sizeof(foo)/sizeof(foo[0]));
    for (int i = 0; i < ntests; i++) {
      /* Serialize */
      int siz = sizeof(foo[i].len) + foo[i].len + sizeof(foo[i].x);
      char *ptr = malloc(siz);
      char *wp = ptr;
      memcpy(wp, (char *)&foo[i].len, sizeof(int));
      wp += sizeof(int);
      memcpy(wp, foo[i].buf, foo[i].len);
      wp += foo[i].len;
      memcpy(wp, (char *)&foo[i].x, sizeof(double));
      wp += sizeof(double);

      /* Unserialize */
      const char *rp = ptr;
      printf("(1) %p (4-byte aligned: %s)\n", rp, IS_ALIGNED(rp));
      int len = *((int *) rp);
      rp += sizeof(int);
      printf("(2) %p (4-byte aligned: %s)\n", rp, IS_ALIGNED(rp));
      char *buf = malloc(len+1);
      memcpy(buf, (const char *) rp, len);
      buf[len] = '\0';
      rp += len;
      /* On ARM-based systems, When `rp' is not aligned on a word boundary
         this causes an "unaligned memory accesses" (EXC_ARM_DA_ALIGN). See:
         http://www.galloway.me.uk/2010/10/arm-hacking-exc_arm_da_align-exception/
         https://brewx.qualcomm.com/bws/content/gi/common/appseng/en/knowledgebase/docs/kb95.html
      */
      printf("(3) %p (4-byte aligned: %s)\n", rp, IS_ALIGNED(rp));
#if DEREFENCE_MODE
      double x = *((double *) rp);
#else
      double x;
      memcpy(&x, rp, sizeof(x));
#endif

      printf(" -> len = %d\n", len);
      printf(" -> buf = %s\n", buf);
      printf(" -> x   = %f\n",   x);

      free(buf);
      printf(" -- end of test %d\n", i);
    }

  return 0;
}