is i=f(); defined when f modifies i?
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.
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.)
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:
- 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).
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