is i=f(); defined when f modifies i?

Question

Related question: Any good reason why assignment operator isn't a sequence point?

From the comp.lang.c FAQ I would infer that the program below is undefined. Strangely, it only mentions the call to f as a sequence point, between the computation of the arguments and the transfer of control to f. The transfer of control from f back to the calling expression is not listed as a sequence point.

int f(void) { i++; return 42; }
i = f();

Is it really undefined?

As an end-note that I add to many of my questions, I am interested in this in the context of static analysis. I am not writing this myself, I just want to know if I should warn about it in programs written by others.

Answer 1

The transfer of control from f back to the calling expression is not listed as a sequence point.

Yes it is.

at the end of the evaluation of a full expression

 

The complete expression that forms an expression statement, or one of the controlling expressions of an if, switch, while, for, or do/while statement, or the expression in an initializer or a return statement.

You have a return statement, therefore, you have a sequence point.

It doesn't even appear that

int f(void) { return i++; } // sequence point here, so I guess we're good
i = f();

is undefined. (Which to me is kind of weird.)

Answer 2

That's not undefined at all. One of the sequence points listed in Appendix C of C99 is the end of a full expression, of which one is the expression in a return statement.

Since you're returning 42, there's a sequence point immediately following that return statement.

For completeness, the C99 sequence points are listed here, with the relevant one bolded:

The following are the sequence points described in 5.1.2.3:

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• The call to a function, after the arguments have been evaluated (6.5.2.2).
• The end of the first operand of the following operators: logical AND && (6.5.13); logical OR || (6.5.14); conditional ? (6.5.15); comma , (6.5.17).
• The end of a full declarator: declarators (6.7.5);
• The end of a full expression: an initializer (6.7.8); the expression in an expression statement (6.8.3); the controlling expression of a selection statement (if or switch) (6.8.4); the controlling expression of a while or do statement (6.8.5); each of the expressions of a for statement (6.8.5.3); the expression in a return statement (6.8.6.4).
• Immediately before a library function returns (7.1.4).
• After the actions associated with each formatted input/output function conversion specifier (7.19.6, 7.24.2).
• Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call (7.20.5).