sizeof(NULL) is 8 in C in a 64 bit system.
The difference is because NULL in C is a variable of a void pointer type with a value zero. In C++,NULL is a variable with a value zero that gets deduced to an int type.
That g++ compiler is using a gnu extension through the __GNUG__ code path. The "true" C++ code path is at line 11.
Edit:
Answering the person below,the file that defines NULL is "stddef.h"
An example of the file online that defines NULL is here: https://github.com/Alexpux/mingw-w64/blob/d0d7f784833bbb0b2d...
#include <iostream>
int main() { std::cout << "sizeof(NULL) = " << sizeof(NULL) << '\n'; }
Compiling with both g++ 6.2.1 and clang++ 3.9.1 with -std=c++11 in both cases gives: sizeof(NULL) = 8
AFAICT, NULL should not even be defined at all in C++ 11; certainly if you include neither <stddef.h> nor <stdlib.h> nor <cstddef> nor <cstdlib>.
I could never get it to compile on x86_64 linux without saying 8.
Bottom line: NULL works in the execlp call, whether from C or from C++. nullptr would be cleaner.