I am working on RTOS project and I'm trying to pass struct type typecasted to void pointer to thread function and derefernce that void pointer using typecast to same struct type. I was getting error when trying to do it this way (eUartDriver_t*) *args. Then found on the internet to use *(eUartDriver_t*) args , but it didn't explained the difference and why does it work

CodePudding user response：

Presumably args is declared to be a void *. The expression *args means “the thing args points to,” so *args would be a void, but void is not a usable type. So *args is bad code, and the compiler complains.

(eUartDriver_t *) args says “Convert the value of args to eUartDriver_t *”. That type is a pointer to an eUartDriver_t. The result of this conversion is a pointer to an eUartDriver_t, so applying *, as in * (eUartDriver_t *) args, refers to a eUartDriver_t, which is a usable type.

CodePudding user response：

@UnholySheep

Order of operations matters a lot. *args dereferences the pointer, so casting that to another pointer type afterwards may not be possible (since the result is possibly not a pointer type)

#include <stdio.h>

int main()
{
  //Consider integer:
  int x;
  //And pointer to it.
  int *p= &x;
  
  //These expressions
  p; &x;
  //are of type int*, which means the dereferencing operator can be used on these.
  //This is legal:
  *p; *(&x); p[0];
  
  //Pointer casting <==> reinterpretation of pointer (address)
  //These expressions are almost the same (have the same values and storage space)
  (int*)p; (char*)p; p; (void*)p;
  
  /*The only difference is in dereferencing or artmetics, which must happen 
  AFTER the pointer is known as (new) type pointer.
  
  So once you've got this expression:*/
  (char*)p;
  //or new variable:
  char *newP= (char*)p;
  //or macro:
  #define newP (char*)p
  
  //THAN you can do derefrencing with proper use of operators *ptr or ptr[offset]
  *newP; *((char*)p); newP[0]; ((char*)p)[0];
}