Is an empty initializer list valid C code?

Question

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;

Answer 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.

Answer 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.

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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.

Answer 3

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.

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