Including stdatomic.h causes size_t to report the wrong type in Intellisense

Environment

OS and Version: Windows 11 Pro, 26H2
VS Code Version: 1.141.0
C/C++ Extension Version: 1.34.4
If using SSH remote, specify OS of remote machine: N/A

Bug Summary and Steps to Reproduce

Bug Summary:

Including stdatomic.h causes havoc with the size_t type, causing Intellisense failures.

Steps to reproduce:

Create a c source file that includes stdatomic.h

Make a variable of size_t (e.g size_t capacity = 0) in the code.
Hover over size_t
It will report that size_t is defined as typedef int size_t(bool)
This is incorrect.

If you remove or comment out the stdatomic.h header and hover over size_t
It will report that size_t is defined as unsigned long long size_t
This is correct.

Apparently, this is an issue in Visual Studio as well.

Configuration and Logs

{
    "configurations": [
        {
            "name": "Win32",
            "includePath": [
                "${workspaceFolder}/**",
                "${env:INCLUDE}"
            ],
            "defines": [
                "_DEBUG",
                "UNICODE",
                "_UNICODE"
            ],
            "windowsSdkVersion": "10.0.28000.0",
            "compilerPath": "cl.exe",
            "cStandard": "c11",
            "cppStandard": "c++23",
            "intelliSenseMode": "windows-msvc-x64",
            "compilerArgs": [
                "/std:c17",
                "/experimental:c11atomics"
            ]
        }
    ],
    "version": 4
}

Other Extensions

N/A

Additional context

Intellisense is completely confused about how to parse the stdatomic.h header, or more precisely, the platform specific of the header that stdatomic.h includes.

Nathan Dykman (@ndykman)

Nathan Dykman (@ndykman) Fixed with https://github.com/microsoft/vscode-cpptools/releases/tag/v1.35.5

Installed the newest version of the plugins. No change in the bug. size_t is still not recognized correctly and atomic_size_t still reports as undefined/unknown. The plugin seems to think that stdatomic.h is not used, which is incorrect.

ETA: I manually removed the Intellisense database and reset it. This seems to have fixed the incorrect types issue. However, it still thinks that stdatomic.h isn't a needed include file, but it is.

Will research and file a separate issue if needed.

Nathan Dykman (@ndykman)

Hi Nathan Dykman (@ndykman) . I'm not able to reproduce the issue with the latest version of the C/C++ Extension.

Correct behavior requires that IntelliSense is properly configured. The language standard must be C11 or greater, and must include the compiler argument (/experimental:c11atomics) that is also required when invoking cl.exe, to enable support for the contents of that header:

            "compilerPath": "cl.exe",
            "compilerArgs": ["/experimental:c11atomics"],
            "cStandard": "c11",

Hi Nathan Dykman (@ndykman) . I'm not able to reproduce the issue with the latest version of the C/C++ Extension.

Correct behavior requires that IntelliSense is properly configured. The language standard must be C11 or greater, and must include the compiler argument (/experimental:c11atomics) that is also required when invoking cl.exe, to enable support for the contents of that header:
            "compilerPath": "cl.exe",
            "compilerArgs": ["/experimental:c11atomics"],
            "cStandard": "c11",

I confirmed that my configuration is setup correctly. Again, all the types are found and are correct, it just seems that Intellisense doesn't think certain headers are required. I filed a new issue as I am seeing this with a very simple program using stdlib.h.

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Including `stdatomic.h` causes `size_t` to report the wrong type in Intellisense #14825

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Including stdatomic.h causes size_t to report the wrong type in Intellisense #14825

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Bug Summary and Steps to Reproduce

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Other Extensions

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sean-mcmanus commented on Oct 28, 2026

ndykman commented on Nov 6, 2026

Colengms commented on Nov 6, 2026

ndykman commented on Nov 6, 2026

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Including `stdatomic.h` causes `size_t` to report the wrong type in Intellisense #14825