Order of evaluation
Order of evaluation of the operands of any C operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the same expression is evaluated again.
There is no concept of left-to-right or right-to-left evaluation in C, which is not to be confused with left-to-right and right-to-left associativity of operators: the expression
f1() + f2() + f3() is parsed as
(f1() + f2()) + f3() due to left-to-right associativity of operator+, but the function call to
f3 may be evaluated first, last, or between
f2() at run time.
There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):
- value computation: calculation of the value that is returned by the expression. This may involve determination of the identity of the object (lvalue evaluation) or reading the value previously assigned to an object (rvalue evaluation)
- side effect: access (read or write) to an object designated by a volatile lvalue, modification (writing) to an object, atomic synchronization (since C11), modifying a file, modifying the floating-point environment (if supported), or calling a function that does any of those operations.
If no side effects are produced by an expression and the compiler can determine that the value is not used, the expression may not be evaluated.
"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread (it may extend across threads if atomic types and memory barriers are involved).
- If a sequence point is present between the subexpressions E1 and E2, then both value computation and side effects of E1 are sequenced-before every value computation and side effect of E2
||(logical OR), and
5) There is a sequence point at the end of a full declarator.
6) There is a sequence point immediately before the return of a library function.
7) There is a sequence point after the action associated with each conversion specifier in formatted I/O (in particular, it is well-formed for scanf to write different fields into the same variable and for printf to read and modify or modify the same variable more than once using %n)
9) The value computations (but not the side-effects) of the operands to any operator are sequenced before the value computation of the result of the operator (but not its side-effects).
10) The side effect (modification of the left argument) of the direct assignment operator and of all compound assignment operators is sequenced after the value computation (but not the side effects) of both left and right arguments.
11) The value computation of the postincrement and postdecrement operators is sequenced before its side-effect.
12) A function call that is not sequenced before or sequenced after another function call is indeterminately sequenced (CPU instructions that constitute different function calls cannot be interleaved, even if the functions are inlined)
13) In initialization list expressions, all evaluations are indeterminately sequenced
14) With respect to an indeterminately-sequenced function call, the operation of compound assignment operators, and both prefix and postfix forms of increment and decrement operators are single evaluations.
 Undefined behavior
i = ++i + i++; // undefined behavior i = i++ + 1; // undefined behavior f(++i, ++i); // undefined behavior f(i = -1, i = -1); // undefined behavior
f(i, i++); // undefined behavior a[i] = i++; // undefined bevahior
 See also
Operator precedence which defines how expressions are built from their source code representation.
C++ documentation for Order of evaluation