Editing C syntax (section)

===Comments===
Text starting with the [[Lexical analysis#Token|token]] {{code|/*}} is treated as a [[comment (computer programming)|comment]] and ignored. The comment ends at the next {{code|*/}}; it can occur within expressions, and can span multiple lines. Accidental omission of the comment terminator is problematic in that the next comment's properly constructed comment terminator will be used to terminate the initial comment, and all code in between the comments will be considered as a comment. C-style comments do not nest; that is, accidentally placing a comment within a comment has unintended results:

<syntaxhighlight lang=C line="GESHI_FANCY_LINE_NUMBERS">
/*
This line will be ignored.
/*
A compiler warning may be produced here. These lines will also be ignored.
The comment opening token above did not start a new comment,
and the comment closing token below will close the comment begun on line 1.
*/
This line and the line below it will not be ignored. Both will likely produce compile errors.
*/
</syntaxhighlight>

[[C++]] style line comments start with {{code|//}} and extend to the end of the line. This style of comment originated in [[BCPL]] and became valid C syntax in [[C99]]; it is not available in the original K&R C nor in [[ANSI C]]:

<syntaxhighlight lang=C>
// this line will be ignored by the compiler

/* these lines
   will be ignored
   by the compiler */

x = *p/*q;  /* this comment starts after the 'p' */
</syntaxhighlight>