CODEWITHSUNDEEP
c++cpreprocessor
Kiseong Yoo
Kiseong Yoo

Reputation: 97

in contrast to C code, why C++ code doesn't need "#define _POSIX_C_SOURCE 200809L"?

When I use "POSIX interval timer" or do signal handling, I have to insert

#define _POSIX_C_SOURCE 200809L

on line 1 inside any of my files. But I figured out that only C code needs it, but not in C++ code.

How does the g++ compiler work differently from the gcc compiler on this issue?

below is my system environment (gcc : same version)

 user@~ $ g++ --version
 g++ (Ubuntu 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609
 Copyright (C) 2015 Free Software Foundation, Inc.
 This is free software; see the source for copying conditions.
 There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

and I build the project adding this line in CMakeLists.txt

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -std=c++14 -Wall")

Upvotes: 1

Views: 535

Answers (2)

lod
lod

Reputation: 1100

g++ defines _GNU_SOURCE, which defines _POSIX_C_SOURCE.

This is done because it is required by the standard c++ template library.

Fuller discussion at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=2082

Description of the macros at https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html

Upvotes: 5

user2486888
user2486888

Reputation:

Let's do a simple experiment. Try to compile this short source file:

auto f()
{
    return _POSIX_C_SOURCE;
}

Nope. It doesn't compile since _POSIX_C_SOURCE is not defined.

But what if we include a C++ header?

#include <iostream>

auto f()
{
    return _POSIX_C_SOURCE;
}

It compiles well.

(Try it live: https://godbolt.org/z/C6_a6P)

That means it has nothing to do with g++. Somewhere in some of the C++ standard library header files, this macro is defined.

Upvotes: 2

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