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Answered
Ghulam Nabi
Asked: 2023-02-08T12:58:02+00:00

how sizeof works on unimplemented function?

I have two functions without any implementation.

I expect that the linker returns an undefined reference to hello and world error.

But surprisingly, the code compiles and runs without any error.

#include <stdio.h>

int hello();
char world();

int main() {
  printf("sizeof hello = %zu, sizeof world = %zu\n", sizeof(hello()), sizeof(world()));
}

sizeof hello = 4, sizeof world = 1
c++
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2 Answers

  1. Best Answer

    The sizeof operator in C returns the size in bytes of its operand. For function operands, it returns the size of a function pointer. In most implementations, a function pointer is the same size as a pointer to any other type, which is usually 4 bytes on a 32-bit system and 8 bytes on a 64-bit system. That’s why the sizeof operator returns 4 for hello() and 1 for world().

    It’s important to note that the actual values of sizeof(hello()) and sizeof(world()) are not related to the return type of the functions, but rather to the size of a function pointer in the implementation.

    Also, it’s important to note that the lack of implementation for the functions hello and world is not a compile-time error, but a link-time error. The code will compile without any issues, but when it is linked the linker will complain about the undefined reference to hello and world as they are not implemented.

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  2. The sizeof operator in C returns the size in bytes of its operand. For function operands, it returns the size of a function pointer. In most implementations, a function pointer is the same size as a pointer to any other type, which is usually 4 bytes on a 32-bit system and 8 bytes on a 64-bit system. That’s why the sizeof operator returns 4 for hello() and 1 for world().

    It’s important to note that the actual values of sizeof(hello()) and sizeof(world()) are not related to the return type of the functions, but rather to the size of a function pointer in the implementation.

    Also, it’s important to note that the lack of implementation for the functions hello and world is not a compile-time error, but a link-time error. The code will compile without any issues, but when it is linked the linker will complain about the undefined reference to hello and world as they are not implemented.

    • 0

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