Skip to content

cppreference 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 f1() or f2() at run time.

NOTE : expression中还嵌套有多个expression，比如f1() + f2() + f3()，f1() 是+的**operands**，同时它自己也是也expression，它也需要evaluation；

Definitions

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.

Ordering

"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
  • If evaluation A is sequenced before evaluation B, then evaluation of A will be complete before evaluation of B begins.
  • If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.
  • If A is not sequenced before B and B is not sequenced before A, then two possibilities exist:
  • evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
  • evaluations of A and B are indeterminably-sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

Rules

1) There is a sequence point ** after the evaluation of all **function arguments and of the function designator, and before the actual function call.

NOTE : 在函数调用之前，先evaluate它的argument，这是非常容易理解的；但是多个argument的order并没有被规定；上面这段话所描述的evaluation order是：function arguments，function designator，function call；

2) There is a sequence point after evaluation of the first (left) operand and before evaluation of the second (right) operand of the following binary operators: && (logical AND), || (logical OR), and , (comma).

NOTE : 对于&&、||而言，这种安排便于实现逻辑短路；

3) There is a sequence point after evaluation of the first (left) operand and before evaluation of the second or third operand (whichever is evaluated) of the conditional operator ?:

NOTE : 这种安排也是符合流程的；

4) There is a sequence point after the evaluation of a full expression (an expression that is not a subexpression: typically something that ends with a semicolon（分号） or a controlling statement（条件判断部分） of if/switch/while/do) and before the next full expression.

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.

NOTE : 这是什么意思？

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)

8) There are sequence points before and immediately after each call to a comparison function made by the library functions qsort and bsearch, as well as between any call to the comparison function and the movement of the associated objects made by qsort