Does anyone know why this compiles successfully?

int main()
{
   // forward declaration
   struct myStruct_st *mS; // Note that this will expand as "struct struct myStruct_st *mS wh开发者_运维问答ich does not make any sense to me"

   return 0;
}

// definition of myStruct_s
typedef struct myStruct_s
{
   int x;
   int y;
} myStruct_st;

I understand that myStruct_s is the structure that needs to be forward declared. I had this typo in my code which seemed to compile. I wonder how though. Does anyone know?

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The local struct has nothing to do with the struct defined outside of main(). In main() you (forward-)declare a struct, define a pointer to that struct and never define the struct. That's perfectly OK. It so happens that you define a struct with the same name outside main().

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I think you misunderstand how the typedef works -- it is not macro substitution.

In particular, using struct myStruct_s after the typedef is not the same as "struct struct myStruct_s" -- it's simply struct myStruct_s, as it reads on the face. The typedef introduces a token which can be used rather than struct ..., but it doesn't expand like a macro and it doesn't "wipe out" the struct ... declaration, which can still be used.