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.