10.3 Sequence Points

There are some points in the code where C makes limited guarantees about the order of operations. These are called sequence points. Here is where they occur:

• At the end of a full expression; that is to say, an expression that is not part of a larger expression. All side effects specified by that expression are carried out before execution moves on to subsequent code.
• At the end of the first operand of certain operators: ‘,’, ‘&&’, ‘||’, and ‘?:’. All side effects specified by that expression are carried out before any execution of the next operand.

  The commas that separate arguments in a function call are not comma operators, and they do not create sequence points. The rule for function arguments and the rule for operands are different (see Ordering of Operands).

• Just before calling a function. All side effects specified by the argument expressions are carried out before calling the function.

  If the function to be called is not constant—that is, if it is computed by an expression—all side effects in that expression are carried out before calling the function.

The ordering imposed by a sequence point applies locally to a limited range of code, as stated above in each case. For instance, the ordering imposed by the comma operator does not apply to code outside the operands of that comma operator. Thus, in this code,

(x = 5, foo (x)) + x * x

the sequence point of the comma operator orders x = 5 before foo (x), but x * x could be computed before or after them.