c - 为什么在指针赋值后会出现段错误？

Question

在指针赋值后立即调用函数时，我遇到了段错误。

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*/

如果我删除pMsg1 = (MSG1 *)&Msg，它就可以正常工作。是因为两个指针的大小不同吗？

Answer 1

只有当一个结构体位于另一个结构体的开头时，您才可以安全地将一个结构体指针转换为另一个结构体指针(无论它们的大小如何，请参阅 C 标准。):

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

否则您可能会陷入协调问题和未定义行为的境地。