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#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
#warning is like
#error, but it reports a
warning instead of an error. The tokens following
used as the warning message.
You might use
#warning in obsolete header files, with a message
saying which header file to use instead.
#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.