c function parameter evaluation and passing [duplicate]

Answer 1

The accepted answer in the marked duplicate is Incorrect.

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f(a,a++,a++); 

Causes Undefined behavior. It tries to modify the argument a without an intervening sequence point. Also, it is important to note that order of evaluation of function arguments is Unspecified. It can be:

• Left to Right or
• Right to Left or
• Any magical order the compiler chooses.

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If you are using GCC you can use the warning flag -wsequence-point to warn you of sequence point related undefined behaviors.

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In GCC If you compile your program at strictest warning levels, the compiler will give you this diagnostic:

prog.c:10:18: error: operation on ‘a’ may be undefined [-Werror=sequence-point]
prog.c:10:18: error: operation on ‘a’ may be undefined [-Werror=sequence-point]

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Reference:

C99 Standard §6.5.2.2:
Para 10:

The order of evaluation of the function designator, the actual arguments, and subexpressions within the actual arguments is unspecified, but there is a sequence point before the actual call.

Note that the quote only says that there is a sequence point before the actual function call, it does not imply there is a sequence point between evaluation of subexpression arguments.

Answer 2

f(a,a++,a++); seems like undefined behaviour because:

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. If there are multiple allowable orderings of the subexpressions of an expression, the behavior is undefined if such an unsequenced side effect occurs in any of the orderings.

When you use undefined behaviour, there are no requirements from the C standard.