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All occurrences in a source file of the following sequences of three characters (that is, trigraph sequences) are replaced with the corresponding single character.
??=
#
??)
]
??!
|
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="0998e90b74411d22-401ba658-447648f4-baa29e66-53b828087c2ec841c2a4b043"><ac:plain-text-body><![CDATA[
??(
[
??'
^
??>
}
]]></ac:plain-text-body></ac:structured-macro>
??/
\
??<
{
??-
~
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In this non-compliant code example, a++ is not executed, as because the trigraph sequence ??/ is replaced by \, logically putting a++ on the same line as the comment.
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Non-Compliant Code Example
This non-compliant code has example includes the trigraph sequence of ??! included, which is replaced by the character |.
| Code Block | ||
|---|---|---|
| ||
size_t i;
/* assignment of i */
if (i > 9000) {
puts("Over 9000!??!");
}
|
The above code This example prints out Over 9000!| if a C99-compliant compiler is used.
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The compliant solution uses string concatenation to place concatenate the two question marks together, because they will be ; otherwise they are interpreted as beginning a trigraph sequence otherwise.
| Code Block | ||
|---|---|---|
| ||
size_t i;
/* assignment of i */
if (i > 9000) {
puts("Over 9000!?""?!");
}
|
The above code will print out prints Over 9000!??!, as intended.
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Some compilers provide options to warn when trigraphs are encountered, or to disable trigraph expansion. Use the warning options and ensure your code compiles cleanly (MSC00-A. Compile cleanly at high warning levels)
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