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Visual Studio Code shows red squiggles under #include <stdio.h> in the editor when the workspace is in ~/src but not when it is in /tmp. Both workspaces compile and run the code fine. It's only an error in the editor.

I can reproduce this with a very simple Hello World C workspace. I am using CMake along with CMake Tools. Versions:

• macOS Catalina 10.15.7
• CMake 3.19.5
• Visual Studio Code 1.53.2
• C/C++ Extension 1.2.1
• CMake Tools Extension 1.6.0

Here's the directory structure:

% tree hello_world
hello_world
├── CMakeLists.txt
└── main.c

Here's CMakeLists.txt:

cmake_minimum_required(VERSION 3.0.0)
project(hello_world VERSION 1.0.0)

add_executable(hello_world main.c)

Here's main.c:

#include <stdio.h>

int main(int argc, char** argv)
{
    printf("hello world\n");
    return 0;
}

Here's the screenshot for ~/src:

And here's the screenshot for /tmp:

Edit: Posted the output of "C/C++: Log Diagnostics" for both workspaces on a Gist. Here's part of the diff:

     Includes:
-        /usr/local/include
         /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/12.0.0/include
-        /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include
         /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
-    Frameworks:
-        /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks

So this explains why it cannot find it in one workspace, but not why the paths are different.

Answers

Edit: This was a bug in the VS Code C/C++ extension that was fixed in version 1.2.2.

This is apparently an FAQ:

Q: Why do I see red squiggles under Standard Library types?

A: The most common reason for this is missing include paths and defines. The easiest way to fix this on each platform is as follows:

Linux/Mac: Set "intelliSenseMode": "clang-x64" or "intelliSenseMode": "gcc-x64" and compilerPath in c_cpp_properties.json to the path to your compiler.

I changed only intelliSenseMode to clang-x64 (it was unset), leaving compilerPath alone, and that fixed the problem. I don't understand why the source directory causes "missing include paths" or why this mode is not the default, but it works now.

Edit: I created issue #7014 on the microsoft/vscode-cpptools GitHub project.

Edit: My issue was duped to issue #6992, which has been fixed in version 1.2.2.