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I'm creating some sort of frontend for a program. To launch the program I'm using the call CreateProcess(), which among other things receives a pointer to a STARTUPINFO structure. To initialize the structure I used to do:

STARTUPINFO startupInfo = {0}; // Or even '\0'.
startupInfo.cb = sizeof(startupInfo);

When compiling the program with GCC enabling these sets of warnings -Wall -Wextra it gives me a warning saying that there's a missing initializer pointing to the first line.

warning: missing initializer
warning: (near initialization for 'startupInfo.lpReserved')

So I ended up doing:

STARTUPINFO startupInfo;
memset(&startupInfo, 0, sizeof(startupInfo));
startupInfo.cb = sizeof(startupInfo);

And this way the compiler doesn't give any warning. The question is, what is the difference between these ways of initializing a structure? Using the first method, isn't the structure initialized? Which one would you recommend?

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  • 1
    The warning is just that: a warning. It's ok to ignore this specific warning on this specific occasion. The compiler emits the warning to help you in cases like: struct struct_with_four_fields x = {1, 2, 3}; where only 3 out of 4 members are initialized. Commented Oct 8, 2009 at 17:02
  • In my previous comment the 4th member is initialized to 0. Commented Oct 8, 2009 at 17:03
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    Warning about missing initializers is not unreasonable in general; if you have a structure with 4 members and you provide initializers for only 3 of them, it's likely to be a mistake. But { 0 } is a common and well-defined idiom for initializing all members to zero (defined recursively for each sub-member) -- which is why later versions of gcc have been modified not to warn about that particular case. Commented Sep 9, 2013 at 17:24
  • @KeithThompson what're you talking about? I am using gcc 4.8.2, and since the question five years passed. P.S. there was one more mail, which I am wanted to link in the end, but to my wonder it is missing. Probably mail server do not save all the messages, that's sad, the mail would be useful to one who meet the problem again. Commented Sep 12, 2014 at 10:07
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    @Hi-Angel: When I compile a small program with gcc-4.8.1 on Solaris, I get "warning: missing initializer". When I compile the same program with gcc-4.8.2 on Linux Mint, I don't get a warning. Incidentally, the line obj = {0}; in the message you linked to is not valid C, and gcc 4.8.2 rejects it as a syntax error. If you're compiling as C++, remember that it's a different language, and gcc uses a different front end; fixes in gcc's C compiler may or may not apply to g++. Commented Sep 12, 2014 at 20:41

5 Answers 5

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96

GCC is just being overly paranoid - for no good reason in my opinion, but then it's certainly true that the GCC maintainers know a lot more about the nuances of C that I do.

See this small thread of discussion about the problem on the GCC mailing list:

The bottom line though - initializing the struct with just {0} will in fact zero initialize the whole thing.

The C99 standard says the following in 6.7.8/21 "Initialization - Semantics":

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

C90 says essentially the same in 6.5.7 with a bit different wording (in other words, C99 didn't add something new here).

Also note that in C++ this was extended so that an empty set of braces, "{}", would perform value initialization on an object because there were situations (like templates) when you wouldn't even know what the members or how many members a type might have. So not only is it good practice, but necessary at times to have an initializer list that's shorter than the number of members an object might have.

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10 Comments

I had to add -Wno-missing-field-initializers and -Wno-missing-braces so that GGC stopped moaning about me putting = {0}; for my structures. Does anyone know if disabling these warning will miss warnings for other things other than = {0}; for structures?
The fix is present in gcc 4.7.0, but not gcc 4.6.3.
@splicer: Which "the" fix? Also I am compiling with mingw32-g++.exe (GCC) 4.7.2 and getting this warning on the idiom above (even the exact case of STARTUPINFO).
@Jan Hudec: Revision 172857. Unfortunately, the URLs from my previous comments no longer work. Here's the bug associated with the fix: gcc.gnu.org/bugzilla/show_bug.cgi?id=36750
I'm still getting the warning even with gcc 4.9.3 when using the empty brace {} or {0} initializers in my C++ classes :/
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This can be easily fixed for GCC in C++ programs by initializing the structure as

STARTUPINFO startupInfo = STARTUPINFO();
  • just did exactly this a few days ago
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4 Comments

A possible pitfall with this answer is that in C++98 this did not zero-initialize the fields. The zero-initialization behaviour was added in C++03.
@M.M Thank you for pointing out the possible pitfall but I think in December 2018 this shouldn't be in the code any more any dangerous situation. BTW, I find this answer the best!
@AlBundy Unfortunately there are still some compilers around that use C++98 initialization (e.g. C++Builder XE5 32-bit mode which I have open right now coincidentally)
I had to add 10+ lines of code to my file because of this. I am really angry. Tried to pass anonymous objects in a function call. If I used {} or {0} I got this warning (treated as an error). If I used () then gcc thinks it is a function call...
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You asked for as many warnings as possible using -Wall and -Wextra.

In this case, you get a warning that tells you you didn't specify all fields, which is perfectly valid, but could have been unintentional.

You can suppress this warning by adding -Wno-missing-field-initializers.

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1 Comment

"as many warnings as possible" — not correct; there are many more warnings that have to be turned on with other flags such as: -Weffc++.
17

The web page How to initialise structures to all-elements-zero-or-null discusses the underlying issue in great detail:

As a work-around, my current solution is to selectively suppress this warning:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmissing-field-initializers"
STARTUPINFO startupInfo = {0};
#pragma clang diagnostic pop

Sadly, this only works in Clang and doesn’t seem to work in GCC.

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3 Comments

I can confirm: #pragma GCC diagnostic ignored "-Wmissing-field-initializers" accepted by GCC compiler 4.2.1 but do nothing. unsure about new versions of gcc
@speakus, I can confirm it is working with the lastest (at time of write) gcc (7.3.0).
@cydef according to other comments and to the bug reports I've seen, GCC no longer warns (as of GCC 5) for this at all -- regardless of #pragma clang diagnostic ignored "-Wmissing-field-initializers". You should probably check if this is the case.
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In C++ you can use boost::initialized_value to get rid of this warning. I have warnings turned off for boost; so I don't know if this would cause any other warnings in your case. This way you don't have to disable the warning.

Example:

T bla = boost::initialized_value;
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2 Comments

But this presumes use of Boost(?). Is this covered by the standard library in some version?
OK, the OP has left the building ("Last seen more than 2 years ago "). Perhaps somebody else can chime it?

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