CODEWITHSUNDEEP
cgccstructstandardsinitializer
Marcus Ahlberg
Marcus Ahlberg

Reputation: 1389

Is an empty initializer list valid C code?

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;

Upvotes: 57

Views: 27307

Answers (4)

Learner
Learner

Reputation: 211

Yes from C23 empty initialization is allowed. If the initializer is the empty initializer, the initial value is the same as the initialization of a static storage duration object.

struct Person person = {}; // Valid C23

If an object is initialized with an empty initializer, then:

  • If it has pointer type, it is initialized to a null pointer

  • If it has decimal floating type, it is initialized to (positive or unsigned) zero, and the quantum exponent is implementation-defined166)

  • If it has arithmetic type, and it does not have decimal floating type, it is initialized to (positive or unsigned) zero

  • If it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits

  • If it is a union, the first-named member is initialized (recursively) according to these rules, and any padding is initialized to zero bit

Reference: ISO/IEC 9899:202x (E)

Upvotes: 21

Ayka
Ayka

Reputation: 51

It depends. For ISO C standard, before ISO C23, the empty initialization of arrays, structs, or unions is not allowed; since ISO C23 (see 6.7.10 Initialization), it is allowed:

braced-initializer:
                    { }
                    { initializer-list }
                    { initializer-list , }

An empty brace pair ({}) is called an empty initializer and is referred to as empty initialization

However, GCC provides GNU C extensions that allow empty initialization of arrays or structs. Unless -Wpedantic, -pedantic, or -pedantic-errors options are given, GCC will not throw warnings or errors for this.

Upvotes: 1

funnydman
funnydman

Reputation: 11366

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.

Upvotes: 9

interjay
interjay

Reputation: 110146

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.

Upvotes: 68

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