问题
I have heard that pointers should first be cast to void to ensure consistency of values across different platforms and should use %p format specifier. Why is it and what exactly are the problems?
int x=100;
int *pi=&x;
printf("value of pi is: %p",(void*)pi);
回答1:
printf is a variadic function and must be passed arguments of the right types. The standard says %p takes void *.
Implicit cast doesn't take place for variadic functions.
Quoting from N1570 7.21.6.1 The fprintf function
p : The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing characters, in an implementation-defined manner.
回答2:
Internal presentation or size of different pointer types is not necessarily same.
For example on one system sizeof(void*) may be 2, but sizeof(int*) is 1.
Since printf is variable argument function, it cannot check the types of incoming parameters. If you passed int* to it, it would read wrong number of bytes, because it expects void*.
回答3:
p conversion specification in printf requires an argument of type void *. C says if you pass an argument of an other type the call invokes undefined behavior.
Besides that, pointer objects of different types are not required to have the same representation: C does not guarantee that sizeof (void *) == sizeof (int *) for example. C only guarantees that void * has the same representation as pointers to character types.
来源：https://stackoverflow.com/questions/26751722/usage-of-void-pointers-across-different-platforms