Implicit int return value of C function

Question

I've googled and just can't seem to find the answer to this simple question.

Working on a legacy code base (ported to Linux recently, and slowly updating to a new compiler) and I see a lot of

int myfunction(...)
{
// no return...
}

I know the implicit return TYPE of a function is int, but what is the implicit return VALUE when no return is specified. I've tested and gotten 0, but that's only with gcc. Is this compiler specific or is it standard defined to 0?

EDIT: 12/2017 Adjusted accepted answer based upon it referencing a more recent version of the standard.

try to compile with -Wall -O3 and without -g . Try to see if return value is still 0 and if some warning exists — kappa, Apr 09 '12 at 20:10
Try to compile with `-Wall -Werror` and see if it still compiles. — dreamlax, Apr 09 '12 at 20:15
The accepted answer should be changed. Quoting 89 version of a standard is out of its place in 2016. — Antti Haapala -- Слава Україні, Mar 13 '16 at 16:19
Really old school code tend to mix assembly and C code. In some cases, the assembly code would set the returning value. Be sure to check if any assembly code is present, either through an extern or _asm keyword. — Dan, Sep 07 '18 at 12:37

score 25 · Answer 1 · answered Apr 09 '12 at 20:12

25

From the '89 standard as quoted in the new testament:

Flowing off the end of a function is equivalent to a return with no expression. In either case, the return value is undefined.

That standard usually expresses the on-the-ground behavior of pre-existing implementations.

answered Apr 09 '12 at 20:12

dmckee --- ex-moderator kitten

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Do you mean that only the value is undefined, or that behavior of the program is undefined after that point? – Pacerier May 13 '15 at 04:22
The 89 standard doesn't specify. I would assume that actual compilers won't throw you any surprises if you don't attempt to use the return value. But no one is guaranteeing that. – dmckee --- ex-moderator kitten May 13 '15 at 04:26
Well, as Jens quotes it that standard seems to guarantee good behavior as long as you don't use the undefined return value. – dmckee --- ex-moderator kitten May 13 '15 at 14:16
@Pacerier it just means the value is undefined. The behavior of the program will still be defined, but any expression that uses that value will be undefined. – SarcasticSully Aug 31 '16 at 17:07
If a function is `int isempty(struct Node* top){ return !top; }` What will it return? What **'!'** does while returning? – Vibhav Chaddha Feb 05 '17 at 11:34

score 18 · Accepted Answer · answered Apr 09 '12 at 20:30

18

Such a thing is possible, but only under the assumption that the return value of the function is never used. The C11 standard says in para 6.9.1:

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

(AFAIR previous version of the standard had similar wording)

So it would be a good idea to transform all the functions of that sort that you have to void functions, so no user of such a function could be tempted to use the return value.

answered Apr 09 '12 at 20:30

Jens Gustedt

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Ok, are you saying that **as long as we don't read it**, behavior of the program is still OK? – Pacerier May 13 '15 at 04:23
Yes, the standard says that. – Jens Gustedt May 13 '15 at 07:17
Well, the first answer by dmckee seems to contradict this. – Pacerier May 13 '15 at 08:33
@Pacerier, no I don't think it does, and ouah's answer also is saying the same thing. – Jens Gustedt May 13 '15 at 10:10

score 15 · Answer 3 · edited Sep 15 '20 at 10:24

15

The return statement is never mandatory at the end of a function, even if the function return type is not void. No diagnostic is required and it is not undefined behavior.

Example (defined behavior):

int foo(void)
{
}

int main()
{
    foo();
}

But reading the return value of foo is undefined behavior:

int bla = foo();  // undefined behavior

From the C Standard:

(C99, 6.9.1 on p.12) "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

The main function is an exception to this rule as if the } is reached in main it is equivalent as if there was a return 0; statement.

(C99 draft, 5.1.2.2.3 on p.13) ...reaching the } that terminates the main function returns a value of 0.

edited Sep 15 '20 at 10:24

0 _

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answered Apr 09 '12 at 20:32

ouah

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*"and it is not undefined behavior"* unless the caller uses the return value. – dmckee --- ex-moderator kitten Apr 09 '12 at 23:16
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@dmckee that's why I wrote: *"But reading the return value of foo is undefined behavior"* – ouah Apr 10 '12 at 07:41
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@ouah Can we do this in cpp ? – asn Jan 23 '19 at 14:19

score 9 · Answer 4 · edited May 23 '17 at 10:29

9

That's simply undefined behaviour; if you don't populate the return area (which for example is generally eax/rax on x86 family processors), it'll have the value last set up through some side-effect in your function.

See Is a return statement mandatory for C++ functions that do not return void? which is basically a duplicate of this question (except that it's tagged as C++).

edited May 23 '17 at 10:29

Community

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answered Apr 09 '12 at 20:12

modelnine

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Not quite, this is only UB if the return value is used, which probably is not the case here, please see my answer. – Jens Gustedt Apr 09 '12 at 20:31
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yes, it is not undefined behavior unless you read the return's value. See my answer. – ouah Apr 09 '12 at 20:32

Jonathan Leffler · Answer 5 · 2012-04-09T20:26:51.077

If the return statements consistently do not return a value, the function is best converted to, and declared as, returning void:

void myfunction(...)
{
    ...
    return;
    ...
}

If there are some some return expr; and some return; statements in the function, then you need to decide which is the better behaviour, and make them consistent — either always return a value and leave the type as int or never return a value and change the type to void.

Note that you'll need to declare functions modified to return void (in a header, unless they are — or should be — static and hidden in a single source file) since the default return type (assumed return type) of int is no longer valid.

score 1 · Answer 6 · answered Apr 09 '12 at 20:13

It may always be zero in that function, but on most architectures (certainly x86) the return statement moves the contents of a specific register to a specific place on the stack which the caller will retrieve and use as its return function.

The return statement will place the variable passed to it in that location so that it'll be a different value. My experience with not placing a specific return statement is that its fairly random what gets returned and you can't rely on it being the same thing.

score 0 · Answer 7 · answered Apr 09 '12 at 20:12

I'm certain that it is undefined behaviour for a function whose return type is not void to omit a return statement. In C99 there is an exception for main, where if the return statement is omitted, it is assumed to return 0 implicitly, but this doesn't apply to any other function.

It may work on a specific platform/compiler combination but you should never rely on such specifics. Using any kind of undefined behaviour in your code makes it unportable. It is common to see undefined behaviour in legacy code though.

Implicit int return value of C function

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