The C programming language is defined by a set of standards that dictate how code should behave across different systems. However, a significant gap exists between these formal specifications and the practical reality of using major compilers like GCC and Clang. This discrepancy arises because compilers often prioritize performance, legacy compatibility, or specific platform behaviors over strict adherence to the latest standards. For many developers, the 'real world' version of C is defined by the behavior of these tools rather than the text of the standard itself.
Several specific technical inconsistencies highlight this divide. For example, the `asm` keyword is a valid non-reserved identifier in standard C, yet GCC and Clang treat it as an extension by default. This means code that is technically standard-compliant might fail to compile unless specific flags are used. Similarly, the standard specifies five pragmas to control floating-point arithmetic (such as `STDC FP_CONTACT` and `STDC FENV_ACCESS`). Despite being part of the C99 standard over 27 years ago, GCC still treats these pragmas as no-ops, meaning they have no effect on the compiler's behavior. While Clang offers partial support for some C99 pragmas, it does not support the newer C23 pragmas.
Other discrepancies involve how compilers and libraries handle environment-specific details. In the case of `FLT_ROUNDS`, the standard requires the value to change dynamically based on the runtime floating-point environment. However, GCC defines `FLT_ROUNDS` as a constant (1), regardless of the runtime state. Furthermore, the `musl` C library provides its own version of `stdarg.h` which can conflict with compiler-provided headers; specifically, its definition of `va_start` is incompatible with the C23 standard, which allows the second argument to be omitted. These inconsistencies demonstrate that while the standard provides a theoretical framework, the practical execution of C is heavily influenced by the specific toolchain and library choices of the developer.
Ultimately, these deviations create a complex landscape for portability. While the standard aims to ensure that code behaves identically across different machines, the reality is that a program's behavior is often tied to the specific quirks of the compiler and the underlying system libraries. For many modern developers, the distinction between 'standard' and 'practical' C is a trade-off between theoretical consistency and the optimized, platform-specific features provided by the most widely used tools.