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
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")
转载于:https://stackoverflow.com/questions/53037824/in-contrast-to-c-code-why-c-code-doesnt-need-define-posix-c-source-200809
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.
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