C extensions, portability, and alternative compilers

Real-world C code rarely adheres strictly to ISO C standards, instead relying on non-standard extensions and compiler-specific features to work around bugs and compatibility issues. The author documents how standard library headers like glibc's create portability barriers for alternative C compilers by including hardcoded checks that only recognize GCC, Clang, and TinyC, effectively blocking other compliant compilers from functioning properly. Critical headers required by both the C standard and POSIX specifications often have conflicting definitions across compilers and platforms, making it nearly impossible for new compilers to achieve true portability without extensive workarounds.