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  About the comma operator
  source: comp.lang.c
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ISO C Standard 9899:1999 (E)

6.5.17 Comma operator

Syntax:
expression:
assignment-expression
expression , assignment-expression

Semantics:
The left operand of a comma operator is evaluated as a void expression;
there is a sequence point after its evaluation. Then the right operand
is evaluated; the result has its type and value. If an attempt is
made to modify the result of a comma operator or to access it after the
next sequence point, the behavior is undefined.