8.5.2 Clean Use of the Comma Operator

Always write parentheses around a series of comma operators, except when it is at top level in an expression statement, or within the parentheses of an if, for, while, or switch statement (see Statements). For instance, in

for (i = 0, j = 10, k = 20; i < n; i++)

the commas between the assignments are clear because they are between a parenthesis and a semicolon.

The arguments in a function call are also separated by commas, but that is not an instance of the comma operator. Note the difference between

foo (4, 5, 6)

which passes three arguments to foo and

foo ((4, 5, 6))

which uses the comma operator and passes just one argument (with value 6).

Warning: don’t use the comma operator around an argument of a function unless it helps understand the code. When you do so, don’t put part of another argument on the same line. Instead, add a line break to make the parentheses around the comma operator easier to see, like this.

foo ((mumble (x, y), frob (z)),
     *p)