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In this noncompliant code example, a++ is not executed because the trigraph sequence ??/ is replaced by \, logically putting a++ on the same line as the comment.:

// whatWhat is the value of a now??/
a++;

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The following compliant solution eliminates the accidental introduction of the trigraph by separating the question marks.:

// whatWhat is the value of a now? ?/
a++;

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This noncompliant code example includes the trigraph sequence ??!, which is replaced by the character |.:

size_t i = /* some initial value */;
if (i > 9000) {
   if (puts("Over 9000!??!") == EOF) {
     /* Handle Errorerror */
   }
}

This example prints Over 9000!| if a C-compliant compiler is used.

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This compliant solution uses string concatenation to concatenate the two question marks; otherwise, they are interpreted as beginning a trigraph sequence.:

size_t i = /* some initial value */;
/* assignment of i */
if (i > 9000) {
   if (puts("Over 9000!?""?!") == EOF) {
     /* Handle Errorerror */
   }
}

This code prints Over 9000!??!, as intended.

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Bibliography

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