Can __STDC_VERSION__ be trusted to guarantee full C standard support?

No, __STDC_VERSION__ macro cannot be trusted to guarantee full support of a C standard.

While it indicates the version of the standard the compiler claims to target, it is a declaration of intent rather than a verification of implementation completeness.

The Gap Between Declaration and Implementation

The__STDC_VERSION__ macro expands to a long integer representing the year of the standard (e.g., 201112L for C11, 201710L for C17). However, relying on it alone for feature-detection is risky for several reasons:

• Partial Implementations: Compilers often define the macro as soon as they support core features of a standard, even if specific niche features or edge-case behaviors are missing.
• Library vs. Language: The macro refers to the C language standard. It does not reflect the availability of the C Standard Library. For example, a compiler might claim C11 support but lack a fully compliant <stdatomic.h> or <threads.h>.
• Flag Overrides: Command-line flags like -std=c99 can force the compiler to report an older version value for a specific translation unit, regardless of the compiler's actual capabilities in newer standards.

How to Implement Robust Feature Detection

To ensure your code safely falls back when a compiler upgrade fails, you should supplement the version check with feature-test macros and probes:

1. Feature-Test Macros: Use the specific macros defined by the standard itself. For instance, check for __STDC_LIB_ATOMICS__ or __STDC_THREADS__ instead of just checking the global version number.
2. Header Availability: Use __has_include("header.h") in C11 to verify that required headers exist before attempting to include them, preventing compilation errors on older environments.
3. Functional Probes: For critical features (like specific alignment types or atomic function signatures), use your build system to attempt to compile and link a small snippet of code. This is the only way to guarantee the library actually provides the feature.

#if ____STDC_VERSION__ >= 201112L && defined(__STDC_LIB_ATOMICS__)
    // Use C11 atomics
#else
    // Fallback to older standard or custom implementation
#endif

Diagnostic request: Are you targeting a specific toolchain (e.g., embedded glibc vs. musl) where the library version lags significantly behind the compiler version?