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10.5 Ordering of Operands

Operands and arguments can be computed in any order, but there are limits to this intermixing in GNU C:

• The operands of a binary arithmetic operator can be computed in either order, but they can’t be intermixed: one of them has to come first, followed by the other. Any side effects in the operand that’s computed first are executed before the other operand is computed.
• That applies to assignment operators too, except that, in simple assignment, the previous value of the left operand is unused.
• The arguments in a function call can be computed in any order, but they can’t be intermixed. Thus, one argument is fully computed, then another, and so on until they have all been done. Any side effects in one argument are executed before computation of another argument begins.

These rules don’t cover side effects caused by postincrement and postdecrement operators—those can be deferred up to the next sequence point.

If you want to get pedantic, the fact is that GCC can reorder the computations in many other ways provided that it doesn’t alter the result of running the program. However, because it doesn’t alter the result of running the program, it is negligible, unless you are concerned with the values in certain variables at various times as seen by other processes. In those cases, you should use volatile to prevent optimizations that would make them behave strangely. See volatile Variables and Fields.