Adeclaration of an object may provide its initial value through the process known asinitialization.

For eachdeclarator, the initializer, if not omitted, may be one of the following:

whereinitializer-list is a non-empty comma-separated list ofinitializer s (with an optional trailing comma), where each initializer has one of three possible forms:

[edit]Explanation

The initializer specifies the initial value stored in an object.

[edit]Explicit initialization

If an initializer is provided, see

• scalar initialization for the initialization of scalar types
• array initialization for the initialization of array types
• struct initialization for the initialization of struct and union types.

[edit]Implicit initialization

If an initializer is not provided:

• objects with automaticstorage duration are initialized to indeterminate values (which may betrap representations)
• objects with static and thread-localstorage duration are empty-initialized

[edit]Empty initialization

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)

[edit]Notes

When initializing an object of static or thread-localstorage duration, everyexpression in the initializer must be aconstant expression orstring 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 afunction 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.int n;,n may not compare equal to itself and it may appear to change its value on subsequent reads)

[edit]Example

#include <stdlib.h>int a[2];// initializes a to {0, 0}int main(void){int i;// initializes i to an indeterminate valuestaticint j;// initializes j to 0int 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 variablechar* 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};}

[edit]References

[edit]See also

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