Why does GCC give me the -Wjump-misses-init warning?

Question

I have a piece of code that compiles fine when I write it in this form (with the -Wjump-misses-init flag):

int some_function(void) {

        ...

        if (is_error)
                goto error;

        int a;

        a = 1;

        return a;

error:
        return 666;
}

But when I write the same function in this form I get the below warning when I compile (:

int some_function(void) {

        ...

        if (is_error)
                goto error;

        int a = 1;

        return a;

error:
        return 666;
}

test.c: In function 'some_function':
test.c:15:17: warning: jump skips variable initialization [-Wjump-misses-init]
test.c:21:1: note: label 'error' defined here
test.c:17:13: note: 'a' declared here

Why does GCC give me that warning when I declare and initialize a on the same line? Seems a bit odd to me? These examples are nonsensical but I'm afraid I'm not at liberty to divulge the real code snippet. I am running GCC 4.7.2 on Debian Wheezy 7.3.

EDIT: void typo

In C++ you [are not allowed to jump past a declaration with initialization](http://stackoverflow.com/questions/20963288/c-conditional-storage-allocation-for-object-caused-by-scope/20963326#20963326) but I am having trouble finding the same statement in C99. — Shafik Yaghmour, Jan 08 '14 at 14:18
I wonder if the error hold with different optimization flags. Or if the assembly differs. Maybe in one case the order of operations is slightly different. It's just a wild guess though. — luk32, Jan 08 '14 at 14:19
@ShafikYaghmour Oh, ok I edited away this part out of my comment. — luk32, Jan 08 '14 at 14:24
@PascalCuoq indeed, I just discovered that, I am always learning new nooks and crannies. — Shafik Yaghmour, Jan 08 '14 at 14:44
See also http://stackoverflow.com/questions/2821663/c99-goto-past-initialization — ecatmur, Jan 08 '14 at 14:45

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

In C++ you are not allowed to bypasses declarations with initialization but it seems to be allowed in C99. You can have gcc warn you about if you either use -Wjump-misses-init or -Wc++-compat. This is covered in the gcc docs section Options to Request or Suppress Warnings and says:

Warn if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.

Note, this also applies to declarations inside a switch statement. One way to work around this is to create a new scope using {}.

In the Annex I the draft C99 standard suggests this as warning, it says:

An implementation may generate warnings in many situations, none of which are specified as part of this International Standard. The following are a few of the more common situations.

and includes the following bullet:

A block with initialization of an object that has automatic storage duration is jumped into (6.2.4).

Interesting, `-Wjump-misses-init` isn't in `-Wc++-compat` for my version of gcc (4.8.1, Ubuntu 12.04). Has it been removed? — ecatmur, Jan 08 '14 at 14:44
@ecatmur hmmm, seems to be so in `4.8` although `4.7` still has it. — Shafik Yaghmour, Jan 08 '14 at 14:51

score 1 · Answer 2 · answered Jan 08 '14 at 14:38

If you were to use the a variable after jumping to the error: label, its value would be indeterminate (6.2.4p6); this is potentially confusing, which is why gcc warns about it. (It is also illegal in C++.)

To avoid the warning and still use declaration-initialization, you can wrap code in a block:

int some_function(void) {
    {
        ...

        if (is_error)
                goto error;

        int a = 1;

        return a;
    }
error:
    return 666;
}

In this case, you would need to declare outside the block any variables that you use after the error: label.

Why does GCC give me the -Wjump-misses-init warning?

2 Answers2