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1 @c Copyright (C) 1999-2020 Free Software Foundation, Inc.
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2 @c This is part of the CPP and GCC manuals.
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3 @c For copying conditions, see the file gcc.texi.
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4
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5 @c ---------------------------------------------------------------------
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6 @c Options affecting preprocessor warnings
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7 @c ---------------------------------------------------------------------
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8
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9 @c If this file is included with the flag ``cppmanual'' set, it is
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10 @c formatted for inclusion in the CPP manual; otherwise the main GCC manual.
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11
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12 @item -Wcomment
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13 @itemx -Wcomments
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14 @opindex Wcomment
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15 @opindex Wcomments
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16 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
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17 comment, or whenever a backslash-newline appears in a @samp{//} comment.
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18 This warning is enabled by @option{-Wall}.
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19
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20 @item -Wtrigraphs
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21 @opindex Wtrigraphs
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22 @anchor{Wtrigraphs}
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23 Warn if any trigraphs are encountered that might change the meaning of
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24 the program. Trigraphs within comments are not warned about,
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25 except those that would form escaped newlines.
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26
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27 This option is implied by @option{-Wall}. If @option{-Wall} is not
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28 given, this option is still enabled unless trigraphs are enabled. To
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29 get trigraph conversion without warnings, but get the other
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30 @option{-Wall} warnings, use @samp{-trigraphs -Wall -Wno-trigraphs}.
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31
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32 @item -Wundef
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33 @opindex Wundef
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34 @opindex Wno-undef
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35 Warn if an undefined identifier is evaluated in an @code{#if} directive.
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36 Such identifiers are replaced with zero.
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37
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38 @item -Wexpansion-to-defined
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39 @opindex Wexpansion-to-defined
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40 Warn whenever @samp{defined} is encountered in the expansion of a macro
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41 (including the case where the macro is expanded by an @samp{#if} directive).
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42 Such usage is not portable.
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43 This warning is also enabled by @option{-Wpedantic} and @option{-Wextra}.
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44
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45 @item -Wunused-macros
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46 @opindex Wunused-macros
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47 Warn about macros defined in the main file that are unused. A macro
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48 is @dfn{used} if it is expanded or tested for existence at least once.
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49 The preprocessor also warns if the macro has not been used at the
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50 time it is redefined or undefined.
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51
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52 Built-in macros, macros defined on the command line, and macros
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53 defined in include files are not warned about.
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54
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55 @emph{Note:} If a macro is actually used, but only used in skipped
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56 conditional blocks, then the preprocessor reports it as unused. To avoid the
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57 warning in such a case, you might improve the scope of the macro's
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58 definition by, for example, moving it into the first skipped block.
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59 Alternatively, you could provide a dummy use with something like:
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60
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61 @smallexample
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62 #if defined the_macro_causing_the_warning
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63 #endif
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64 @end smallexample
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65
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66 @item -Wno-endif-labels
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67 @opindex Wno-endif-labels
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68 @opindex Wendif-labels
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69 Do not warn whenever an @code{#else} or an @code{#endif} are followed by text.
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70 This sometimes happens in older programs with code of the form
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71
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72 @smallexample
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73 #if FOO
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74 @dots{}
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75 #else FOO
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76 @dots{}
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77 #endif FOO
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78 @end smallexample
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79
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80 @noindent
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81 The second and third @code{FOO} should be in comments.
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82 This warning is on by default.
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