[go-nuts] Is this a bug of cgo?

On Tue, Jun 9, 2015 at 11:18 PM, Hoping White wrote:

go version go1.4.2 darwin/amd64

I am using shmid_ds struct in <sys/shm.h>

#if (defined(_POSIX_C_SOURCE) && !defined(_DARWIN_C_SOURCE))
struct shmid_ds
#else
#define shmid_ds __shmid_ds_new
struct __shmid_ds_new
#endif
{
struct __ipc_perm_new shm_perm; /* [XSI] Operation permission value */
size_t shm_segsz; /* [XSI] Size of segment in bytes */
pid_t shm_lpid; /* [XSI] PID of last shared memory op */
pid_t shm_cpid; /* [XSI] PID of creator */
shmatt_t shm_nattch; /* [XSI] Number of current attaches */
time_t shm_atime; /* [XSI] Time of last shmat() */
time_t shm_dtime; /* [XSI] Time of last shmdt() */
time_t shm_ctime; /* [XSI] Time of last shmctl() change */
void *shm_internal; /* reserved for kernel use */
};
#else /* !__DARWIN_UNIX03 */
#define shmid_ds __shmid_ds_old
#endif /* !__DARWIN_UNIX03 */

On Wed, Jun 10, 2015 at 9:26 PM, minux wrote:

Apparently shm_atime is not correctly aligned in the struct,
and as Go struct require 8-byte alignment for the C.time_t
(which is typedef to be long), those unaligned fields can't
be translated to equivalent fields in a Go type.

$ cc -x c - <<EOF
#include <sys/shm.h>
#include <stdio.h>
#include <stddef.h>

int main() {
printf("%d\n", (int)offsetof(struct shmid_ds, shm_atime));
printf("%d\n", (int)__alignof__(time_t));
}
EOF
$ ./a.out
44
8