/Zc:__STDC__ (Enable __STDC__ macro)
The /Zc:__STDC__ compiler option defines the built-in __STDC__ preprocessor macro as 1 in C code.
Syntax
/Zc:__STDC__
Remarks
The /Zc:__STDC__ compiler option implements Standard C conforming behavior for the __STDC__ preprocessor macro, setting it to 1 when compiling C11 and C17 code.
The /Zc:__STDC__ option is new in Visual Studio 2022 version 17.2. This option is off by default, but can be enabled explicitly when /std:c11 or /std:c17 is specified. There's no negative version of the option.
This option is a source breaking change. Due to the behavior of the UCRT, which doesn't expose POSIX functions when __STDC__ is 1, it isn't possible to define this macro for C by default without introducing breaking changes to the stable language versions.
Example
// test__STDC__.c
#include <io.h>
#include <fcntl.h>
#include <stdio.h>
int main() {
#if __STDC__
int f = _open("file.txt", _O_RDONLY);
_close(f);
#else
int f = open("file.txt", O_RDONLY);
close(f);
#endif
}
/* Command line behavior
C:\Temp>cl /EHsc /W4 /Zc:__STDC__ test__STDC__.c && test__STDC__
*/
To set this compiler option in Visual Studio
Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.
Select the Configuration Properties > C/C++ > Command Line property page.
In Additional options, add
/Zc:__STDC__. Choose OK or Apply to save your changes.