gcc/doc/cpp/diagnostics.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: diagnostic, reporting errors, reporting warnings, #error
   7
   8 .. _diagnostics:
   9
  10 Diagnostics
  11 -----------
  12
  13 The directive :samp:`#error` causes the preprocessor to report a fatal
  14 error.  The tokens forming the rest of the line following :samp:`#error`
  15 are used as the error message.
  16
  17 You would use :samp:`#error` inside of a conditional that detects a
  18 combination of parameters which you know the program does not properly
  19 support.  For example, if you know that the program will not run
  20 properly on a VAX, you might write
  21
  22 .. code-block:: c++
  23
  24   #ifdef __vax__
  25   #error "Won't work on VAXen.  See comments at get_last_object."
  26   #endif
  27
  28 If you have several configuration parameters that must be set up by
  29 the installation in a consistent way, you can use conditionals to detect
  30 an inconsistency and report it with :samp:`#error`.  For example,
  31
  32 .. code-block:: c++
  33
  34   #if !defined(FOO) && defined(BAR)
  35   #error "BAR requires FOO."
  36   #endif
  37
  38 .. index:: #warning
  39
  40 The directive :samp:`#warning` is like :samp:`#error`, but causes the
  41 preprocessor to issue a warning and continue preprocessing.  The tokens
  42 following :samp:`#warning` are used as the warning message.
  43
  44 You might use :samp:`#warning` in obsolete header files, with a message
  45 directing the user to the header file which should be used instead.
  46
  47 Neither :samp:`#error` nor :samp:`#warning` macro-expands its argument.
  48 Internal whitespace sequences are each replaced with a single space.
  49 The line must consist of complete tokens.  It is wisest to make the
  50 argument of these directives be a single string constant; this avoids
  51 problems with apostrophes and the like.