Question

I got a segmentation fault when invoked a function immediately following a pointer assignment.

typedef struct
{
   DMINT field1;
   DMINT field2;
   DMINT field3;
} MSG1;

typedef struct
{
....
} MSG;

/* MSG is size of 1040 byte, bigger than MSG1 in size */

int main()
{
    MSG     Msg;
    MSG1   *pMsg1;
    int     mid;
    pthread_t  tid;
    ...

    Recv_msg( mid, &Msg);   /* this function does a memcpy to &Msg */
    pMsg1 = (MSG1 *)&Msg;

    //ret = pthread_join(pMsg1->..... );    /* Got Segmentation fault here by GDB*/
    /* even the first argument has nothing to do with pMsg1, SEGV is still received */
     ret = pthread_creat(&tid, NULL, thread_function, NULL); /* Got Segmentation fault here by GDB*/

It works fine if I remove pMsg1 = (MSG1 *)&Msg.
Is it because the two pointers have different sizes?

Answer 1

You can safely convert one struct pointer to another only if one struct lays in the beginning of another (no matter sizes they are, see C std.):

typedef struct {
  int a;
} S1;
typedef struct {
  S1 s1; // <- s1 it the FIRST structure field
  int b;
} S2;
S2 s2;
S1 *s1;
s1= (S1*)&s2; // <- safe

Otherwise you could get into aligning issues and undefined behavior.