How Does Partial Initialization of Structures and Arrays Work in C and C ?

Partial Initialization of Automatic Structures and Arrays in C and C

Partial Initialization

In structured programming languages like C and C , initializers can be used to assign values to data members at the time of object declaration. However, unlike case 1 in the code snippet provided below, where fewer initializers than the number of members are provided, standards only define two types of initialization:

struct someStruct {
    int a;
    int b;
    int c;
};

// Partial Initialization
struct someStruct s = {123,};

// Complete Initialization
struct someStruct s = {123, 456, 789};

// No Initialization
struct someStruct s;

Rules for Partial Initialization of Arrays and Structures

In practice, the term "Partial Initialization" is often used to describe situations like in case 1 above, where initializers are provided for only a subset of the members. While both C and C standards do not explicitly refer to partial initialization, they do define the behavior for both complete and no initialization, with the following rules applying to partial initialization as well:

• If an array or structure is partially initialized, any uninitialized elements will be zero-initialized, regardless of its storage type.
• This behavior is guaranteed by the C99 Standard (6.7.8.21) for C and the C 03 Standard (8.5.1 and 8.5) for C .

Ensuring Compatibility

To ensure compatibility with various compilers, including gcc/g , it is recommended to follow the standard rules when initializing structures and arrays, avoiding partial initialization as it may lead to unexpected behavior.

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