Is an empty initializer list valid C code?

c gcc struct standards initializer

20,565

Solution 1

No, an empty initializer list is not allowed. This can also be shown by GCC when compiling with -std=c99 -pedantic:

a.c:4: warning: ISO C forbids empty initializer braces

The reason is the way the grammar is defined in §6.7.9 of the 2011 ISO C Standard:

initializer:
         assignment-expression
         { initializer-list }
         { initializer-list , }
initializer-list:
         designation(opt) initializer
         initializer-list , designation(opt) initializer

According to that definition, an initializer-list must contain at least one initializer.

Solution 2

According to the C99 standard, array creation with an empty initializer list is forbidden. In a previous answer, you can see that grammar does not describe this case.

But what happens if you declare an array without initialization? Well, it depends on the compiler which you use. Let's take a look at this simple example: int arr[5] = {}.

GCC

By default gcc does not produce any warnings/errors when you try to compile this code. Not even -Wall, but -Wpedantic does.

warning: ISO C forbids empty initializer braces

But anyway gcc fill members of an array with 0's exactly as if you specify it explicitly int arr[5] = {0} see assembly output godbolt.

CLANG

But default not showing warnings about this case, but with option -Wgnu-empty-initializer does:

warning: use of GNU empty initializer extension

Clang generates different assembly code godbolt but behaves the same.

20,565

Marcus Ahlberg

Embedded developer by day.

Updated on July 09, 2022

Comments

Marcus Ahlberg almost 2 years
It is common to use {0} to initialize a struct or an array but consider the case when the first field isn't a scalar type. If the first field of struct Person is another struct or array, then this line will result in an error (error: missing braces around initializer).
```
struct Person person = {0};
```
At least GCC allows me to use an empty initializer list to accomplish the same thing
```
struct Person person = {};
```
But is this valid C code?

Also: Is this line guaranteed to give the same behavior, i.e. a zero-initialized struct?
```
struct Person person;
```
- Montre almost 11 years
  
  C shouldn't guarantee initialization of anything except static variables, but I'm going off a single sentence in Wikipedia here.
- Marcus Ahlberg almost 11 years
  
  I might add that I built my code with GCC options pedantic and warnings-as-errors. Other compilers or configurations might allow the first syntax.
- NREZ almost 11 years
  
  Can you have a look at this: stackoverflow.com/questions/755305/empty-structure-in-c
- Hasturkun almost 11 years
  
  @NREZ: That's a different issue, being a struct with no members.
- ams almost 11 years
  
  @millimoose: variables that are explicitly initialized, even with empty braces, better had be guaranteed or something is badly broken! Local (auto) variables with an initializer are a different story though.
- NREZ almost 11 years
  
  @Hasturkun I was pointing out the different behavior of compilers for structures in C...
- Grijesh Chauhan almost 11 years
  
  Although this is a valid code struct X{}; int main(){ struct X x = {}; return 1; } Give it a try.
- Gorpik almost 11 years
  
  @Bentoy13 VC++ does not support C99, so it is not a good compiler for this specific question.
- Montre almost 11 years
  
  @ams I was referring to the last line. I should've said "shouldn't guarantee implicit initialization" to be more specific. Obviously skipping explicit initialization would be ludicrous
- Bentoy13 almost 11 years
  
  @millimoose & Gorpik: comment deleted; for all reasons, my comment is not pertinent
- Gabriel Staples about 4 years
  
  Related, and very useful: How to initialize all members of an array to the same value?.
torek almost 11 years

This is the correct answer (that an empty initializer-list is forbidden by the syntax). It's a bit unfortunate and it would be nice if the standards allowed it, but they don't really allow empty objects in general anyway, so it is not needed. The answer to the remaining part of the question (do you get as-if-zero if there is no = {...} part) is "yes if the object has static duration, no if not".
Marcus Ahlberg almost 11 years

This answers my question. I still don't have a fool-proof way of initializing the struct on a single line if I don't know the type of the first member (as is the case if the struct declaration is being auto-generated), but that is a different question.
Pavel Šimerda almost 9 years

@MarcusAhlberg: Indeed the way the standard is set is very silly in this case. The author of that must have forgotten logic at home to allow specifying 1 to N initializers instead of 0 to N or, looking from the opposite direction omitting 0 to N-1 initializers instead of 0 to N. GCC does a great job of not enforcing it.
Pavel Šimerda almost 9 years

@torek: Not supporting empty objects (because you can't allocate and address them) and setting an artificial limit to omit only up to N-1 initializers instead of N resulting in absence of universal zero initializer are two very different things.
torek almost 9 years

@PavelŠimerda: I'm confused by your comment: I didn't say empty initializers were not useful, just that they're not required to exist. If empty objects existed, then either they would need a special case ruling out initializers entirely (e.g., "empty objects may not be initialized") or they would require empty initializers. For that matter, allocating and addressing empty objects works fine in nonstandard GNU C, it just means that &a==&b is possible.
undercat about 3 years

The assembly code for Clang looks identical as well, perhaps it's something that has changed recently.