Initialize struct members to 0 (gcc -Wextra)

Question

I want to initialize all struct members to 0. Common solution is to make something like this:

struct foo bar = {0}

I create this example:

#include <stdio.h>

struct Stru2 {
    int c;
    int d;
};

struct Stru1 {
    int a;
    Stru2 b;
};

int main()
{
    struct Stru1 aaa = { 0 };
    return aaa.b.c;
}

And I compile (gcc 4.6.3) it whit this parameters, to make sure how ANSI handle this

gcc -Wall -Wextra -pedantic -ansi main.cpp

And I got following warnings:

main.cpp: In function ‘int main()’: 
main.cpp:36:28: warning: missing initializer for member ‘Stru1::b’ [-Wmissing-field-initializers]

The question is, why -Wextra, generate this warning? Maybe not always "= {0}", set all members to 0?

Does it also generate the warning when you initialize with `{}`? (This would be the more idiomatic form, I think.) — James Kanze, Nov 05 '12 at 19:36
@pmg The programme is C++. The member of `struct Stru1` is typed `Stru2 b;` but there's no `typedef struct Stru2 Stru2;`. — Daniel Fischer, Nov 05 '12 at 22:07
@DanielFischer: you're correct. I just looked at the tags and neglected the code. — pmg, Nov 05 '12 at 22:13
@pmg (I have to admit that I didn't notice either until my compiler complained.) — Daniel Fischer, Nov 05 '12 at 22:15
This was just fixed in the GCC source repo and should be gone in the next release (4.9.1? 4.10.0?) — R.. GitHub STOP HELPING ICE, Jun 05 '14 at 19:43

score 12 · Accepted Answer · answered Nov 05 '12 at 19:08

It's just a nanny warning.. = {0} definitely sets all of the members to zero. The reason it's included as a warning at all is for the case where you might add a field to a structure and you want to make sure it gets initialized correctly. Having the warning lets you find those places quickly and fix them. For example, let's say you fix have an initializer something like:

struct Stru1 aaa = { 1, { 2, 3 } };

Now, add a field int e to Stru2:

struct Stru2 {
    int c;
    int d;
    int e;
};

And let's pretend it's very important that e get initialized. Well, without -Wmissing-field-initializers, you'd have to hunt around in your code to find every place you care about. With the warning, you get a diagnostic message like the one you saw, and you can find and fix them very easily:

struct Stru1 aaa = { 1, { 2, 3, 4 } };

To get the advantage of this warning, you do have to go through and fully initialize all of your structures, though. In your case, that means something like:

struct Stru1 aaa = { 0, { 0, 0 } };

`-Wextra` has other "nanny" warnings in addition to this one. Sometimes they're annoying, but other times the author of the code can be quite thankful for them. — David Hammen, Nov 05 '12 at 19:18
+1 @DavidHammen. I also like it detecting accidental use of `=` rather than `==` in conditionals. I don't know if that's one of the `-Wextra` family or not, but it's a nice one to have. — Carl Norum, Nov 05 '12 at 22:50
That's a `-Wparentheses` warning, which is enabled by `-Wall`. — David Hammen, Nov 05 '12 at 23:36

score 5 · Answer 2 · edited May 23 '17 at 12:22

5

Yes, {0} sets all struct members to 0, because it sets the first member to 0, and all other members are by default set to 0 as long as at least one member is initialized. See Are uninitialized struct members always set to zero?.

edited May 23 '17 at 12:22

Community

1
1

answered Nov 05 '12 at 19:11

1''

26,823
32
143
200

score 3 · Answer 3 · answered Nov 05 '12 at 21:33

This is related to GCC bug #53119, --Wmissing-braces wrongly warns about universal zero initializer {0}, but I think your question indicates that there may be at least one other warning option adding to the problem. Posting follow-ups to the bug thread would probably help to get the issue addressed.

Initialize struct members to 0 (gcc -Wextra)

3 Answers3