View source for c/language/initialization

{{title|Initialization}}
{{c/language/initialization/navbar}}

A {{rlp|declarations|declaration}} of an object may provide its initial value through the process known as ''initialization''.

For each {{rlp|declarations|declarator}}, the initializer, if not omitted, may be one of the following:

{{sdsc begin}}
{{sdsc|num=1|1=
{{ttb|1==}} {{spar|expression}}
}}
{{sdsc|num=2|1=
{{ttb|1==}} {{ttb|{}} {{spar|initializer-list}} {{ttb|}<nowiki/>}}
}}
{{sdsc|num=3|notes={{mark since c23}}|1=
{{ttb|1==}} {{ttb|{}} {{ttb|}<nowiki/>}}
}}
{{sdsc end}}

where {{spar|initializer-list}} is a non-empty comma-separated list of {{spar|initializer}}{{sep}}s (with an optional trailing comma), where each initializer has one of three possible forms:

{{sdsc begin}}
{{sdsc|num=1|1=
{{spar|expression}}
}}
{{sdsc|num=2|1=
{{ttb|{}} {{spar|initializer-list}} {{ttb|}<nowiki/>}}
}}
{{sdsc|num=3|notes={{mark since c23}}|1=
{{ttb|{}} {{ttb|}<nowiki/>}}
}}
{{sdsc|num=4|notes={{mark since c99}}|1=
{{spar|designator-list}} {{ttb|1==}} {{spar|initializer}}
}}
{{sdsc end}}

{{rrev|since=c99|
where {{spar|designator-list}} is a list of either array designators of the form {{ttb|[}} {{spar|constant-expression}} {{ttb|]}} or struct/union member designators of the form {{ttb|.}} {{spar|identifier}}{{sep}}; see 
{{rlp|array initialization}} and {{rlp|struct initialization}}.

Note: besides initializers, brace-enclosed {{spar|initializer-list}} may appear in {{rlp|compound literal}}s, which are expressions of the form:
{{sdsc begin}}
{{sdsc | notes=<nowiki/> | {{ttb|(}} {{spar|type}} {{ttb|)}} {{ttb|{}} {{spar|initializer-list}} {{ttb|}<nowiki/>}} }}
{{sdsc | notes={{mark since c23}} | {{ttb|(}} {{spar|type}} {{ttb|)}} {{ttb|{}} {{ttb|}<nowiki/>}} }}
{{sdsc end}}
}}

===Explanation===
The initializer specifies the initial value stored in an object.

====Explicit initialization====
If an initializer is provided, see
* {{rlp|scalar initialization}} for the initialization of scalar types
* {{rlp|array initialization}} for the initialization of array types
* {{rlp|struct initialization}} for the initialization of struct and union types.

====Implicit initialization====
If an initializer is not provided:
* objects with automatic {{rlp|storage duration}} are initialized to indeterminate values (which may be {{rlp|object|trap representations}})
* objects with static and thread-local {{rlp|storage duration}} are empty-initialized

====Empty initialization====
{{rrev|since=c23|
An object is empty-initialized if it is explicitly initialized from initializer {{c|1== {} }}.
}}

In some cases, an object is empty-initialized if it is not initialized explicitly, that is:
* pointers are initialized to null pointer values of their types
* objects of integral types are initialized to unsigned zero
* objects of floating types are initialized to positive zero
* all elements of arrays, all members of structs, and the first members of unions are empty-initialized, recursively, plus all padding bits are initialized to zero
:(on platforms where null pointer values and floating zeroes have all-bit-zero representations, this form of initialization for statics is normally implemented by allocating them in the .bss section of the program image)

===Notes===
When initializing an object of static or thread-local {{rlp|storage duration}}, every {{spar|expression}} in the initializer must be a {{rlp|constant expression}} or {{rlp|string literal}}.

Initializers cannot be used in declarations of objects of incomplete type, VLAs, and block-scope objects with linkage.

The initial values of function parameters are established as if by assignment from the arguments of a {{rlp|operator_other#Function_call|function call}}, rather than by initialization.

If an indeterminate value is used as an argument to any standard library call, the behavior is undefined. Otherwise, the result of any expression involving indeterminate values is an indeterminate value (e.g. {{c|int n;}}, {{c|n}} may not compare equal to itself and it may appear to change its value on subsequent reads)

{{rrev multi
|rev1=
There is no special construct in C corresponding to [[cpp/language/value initialization|value initialization]] in C++; however, {{c|1== {0} }}{{rev inl|since=c99| (or {{c|1=(T){0} }} in compound literals)}} can be used instead, as the C standard does not allow empty structs, empty unions, or arrays of zero length.
|since2=c23|rev2=
The empty initializer {{c|1== {} }} (or {{c|(T){} }} in compound literals) can be used to achieve the same semantics as [[cpp/language/value initialization|value initialization]] in C++.
}}

===Example===
{{example|code= 
#include <stdlib.h>
int a[2]; // initializes a to {0, 0}
int main(void)
{
    int i;          // initializes i to an indeterminate value
    static int j;   // initializes j to 0
    int k = 1;      // initializes k to 1

    // initializes int x[3] to 1,3,5
    // initializes int* p to &x[0]
    int x[] = { 1, 3, 5 }, *p = x;

    // initializes w (an array of two structs) to
    // { { {1,0,0}, 0}, { {2,0,0}, 0} }
    struct {int a[3], b;} w[] = {[0].a = {1}, [1].a[0] = 2};

    // function call expression can be used for a local variable
    char* ptr = malloc(10);
    free(ptr);

//  Error: objects with static storage duration require constant initializers
//  static char* ptr = malloc(10);

//  Error: VLA cannot be initialized
//  int vla[n] = {0};
}
}}

===References===
{{ref std c17}}
{{ref std | section=6.7.9 | title=Initialization | p=100-105}}
{{ref std end}}
{{ref std c11}}
{{ref std | section=6.7.9 | title=Initialization | p=139-144}}
{{ref std end}}
{{ref std c99}}
{{ref std | section=6.7.8 | title=Initialization | p=125-130}}
{{ref std end}}
{{ref std c89}}
{{ref std | section=6.5.7 | title=Initialization}}
{{ref std end}}

===See also===
{{dsc begin}}
{{dsc see cpp | cpp/language/initialization |Initialization | nomono=true}}
{{dsc end}}

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