26.7 Diagnostics

The directive #error reports a fatal error. The tokens forming the rest of the line following #error are used as the error message.

The usual place to use #error is inside a conditional that detects a combination of parameters that you know the program does not properly support. For example,

#if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
#error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
#endif

The directive #warning is like #error, but it reports a warning instead of an error. The tokens following #warning are used as the warning message.

You might use #warning in obsolete header files, with a message saying which header file to use instead.

Neither #error nor #warning macro-expands its argument. Internal whitespace sequences are each replaced with a single space. The line must consist of complete tokens. It is wisest to make the argument of these directives be a single string constant; this avoids problems with apostrophes and the like.