putchar_unlocked doesn't work in C++ 14 standard - c++

Question

I am using g++ (GCC) 4.9.3 on Cygwin. I am not able to use getchar_unlocked or putchar_unlocked with C++ 14 standard.
Consider this sample code
#include <cstdio>
int main() {
putchar_unlocked('1');
return 0;
}
When I compile and run with
g++ foo.cpp && a.exe && rm ./a.exe
I am getting expected output 1.
But when I do
g++ -std=c++14 foo.cpp && a.exe && rm ./a.exe
I am getting error saying putchar_unlocked was not declared.
foo.cpp: In function 'int main()':
foo.cpp:4:22: error: 'putchar_unlocked' was not declared in this scope
putchar_unlocked('1');
^

Answer 1

putchar_unlocked isn't part of any version of the C or C++ standards, and Cygwin doesn't implement any other standard that does provide putchar_unlocked.
Cygwin does provide putchar_unlocked as a non-standard extension, but you need to actually leave non-standard extensions enabled.
The default -std= version is -std=gnu++03 (or one of its synonyms). This is C++03 plus extensions. You changed it to -std=c++14. This is C++14 without extensions. Use -std=gnu++14 to leave extensions enabled.

Answer 2

putchar_unlocked is not part of any C++ standard. It is part of POSIX standard, but defining -std=c++14 causes gcc to define __STRICT_ANSI__ macro. Cygwin uses Newlib for C standard library, and from its sources we can see that this prevents putchar_unlocked from being declared, and also that there isn't any other macro to enable it anyway.
Therefore, we need to get rid of __STRICT_ANSI__. Using -std=gnu++14 should do that:
g++ -std=gnu++14 foo.cpp && a.exe && rm ./a.exe
A comment under the question points out that the code works with Ideone. This is probably, because Ideone runs on different platform (such as Linux), which probably has glibc, which provides putchar_unlocked with different conditions (from this manual page):
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
getc_unlocked(), getchar_unlocked(), putc_unlocked(),
putchar_unlocked():
_POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE || _BSD_SOURCE || _SVID_SOURCE