Bad conversion of an array contained in a C struct

[Jan Igger]

Reproduction steps:

1. Create

-------------- guid.h --------------
typedef unsigned char uint8_t;

typedef struct {
  uint8_t data[16];
} GUID;

GUID getGUID();
----------- end of guid.h ----------

-------------- guid.c --------------
#include <string.h>
#include "guid.h"

GUID getGUID()
{
  GUID emptyGUID;
  memset(&emptyGUID, 0, sizeof(emptyGUID));
  return emptyGUID;
}
----------- end of guid.c ----------

2. compile

$ gcc -c -fPIC guid.c -o guid.o
$ gcc guid.o -shared -o libguid.so

3. on lisp side (I am using SBCL 2.1.1 on Arch Linux):

(cffi:define-foreign-library guid-lib
  (:unix "./libguid.so"))

(cffi:load-foreign-library 'guid-lib)

(cffi:defctype uint8 :unsigned-char)

(cffi:defcstruct (anon-struct-39 :size 16)
  (data (:array uint8 16) :offset 0))

(cffi:defctype guid (:struct anon-struct-39))

(cffi:defcfun ("getGUID" get-guid)
    guid)

Now,

(get-guid)

yields

=> (DATA #(0 0 0 0 0 0 0 0 132 36 194 2 0 0 0 0))

(except numbers and their places change)

instead of all zeros.

4. (optional) as a sanity check, add this to guid.c:

-------------- addition --------------
#include <stdio.h>

int main()
{
  GUID guid = getGUID();
  for (unsigned int i = 0; i < sizeof(guid); i++)
  {
    printf("%i", guid.data[i]);
  }
}
----------- end of guid.c ----------

$ gcc guid.c -o guid
$ ./guid
0000000000000000%

(as expected)