What is the use of the #error directive in C?

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It’s a preprocessor directive that is used (for example) when you expect one of several possible -D symbols to be defined, but none is.

#if defined(BUILD_TYPE_NORMAL)
# define DEBUG(x) do {;} while (0) /* paranoid-style null code */
#elif defined(BUILD_TYPE_DEBUG)
# define DEBUG(x) _debug_trace x /* e.g. DEBUG((_debug_trace args)) */
#else
# error "Please specify build type in the Makefile"
#endif

When the preprocessor hits the #error directive, it will report the string as an error message and halt compilation; what exactly the error message looks like depends on the compiler.

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