Array declaration

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Array is a type consisting of a contiguously allocated nonempty sequence of objects with a particular element type. The number of those objects (the array size) never changes during the array lifetime.

Syntax

In the declaration grammar of an array declaration, the type-specifier sequence designates the element type (which must be a complete object type), and the declarator has the form:

[ static(optional) qualifiers(optional) expression(optional) ] (1)
[ qualifiers(optional) static(optional) expression(optional) ] (2)
[ qualifiers(optional) * ] (3)
1,2) General array declarator syntax
3) Declarator for VLA of unspecified size (can appear in function prototype scope only) where
expression - any expression other than comma operator, designates the number of elements in the array
qualifiers - any combination of const, restrict, or volatile qualifiers, only allowed in function parameter lists; this qualifies the pointer type to which this array parameter is transformed 
float fa[11], *afp[17]; // fa is an array of 11 floats
                        // afp is an array of 17 pointers to floats

Explanation

There are several variations of array types: arrays of known constant size, variable-length arrays, and arrays of unknown size.

Arrays of constant known size

If expression in an array declarator is an integer constant expression with a value greater than zero and the element type is a type with a known constant size (that is, elements are not VLA) (since C99), then the declarator declares an array of constant known size: 

int n[10]; // integer constants are constant expressions
char o[sizeof(double)]; // sizeof is a constant expression
enum { MAX_SZ=100 };
int n[MAX_SZ]; // enum constants are constant expressions

Arrays of constant known size can use array initializers to provide their initial values: 

int a[5] = {1,2,3}; // declares int[5] initalized to 1,2,3,0,0
char str[] = "abc"; // declares char[4] initialized to 'a','b','c','\0'

In function parameter lists, additional syntax elements are allowed within the array declarators: the keyword static and qualifiers, which may appear in any order before the size expression (they may also appear even when the size expression is omitted).

In each function call to a function where a parameter of array type uses the keyword static between [ and ], the value of the actual parameter must be a valid pointer to the first element of an array with at least as many elements as specified by expression: 

void fadd(double a[static 10], const double b[static 10])
{ 
    for (int i = 0; i < 10; i++) {
        if (a[i] < 0.0) return;
        a[i] += b[i];
    }
}
// a call to fadd performs compile-time bounds checking
// and also permits optimizations such as prefetching 10 doubles 
int main(void)
{
    double a[10] = {0}, b[20] = {0};
    fadd(a, b); // OK
    double x[5] = {0};
    fadd(x, b); // error: array argument is too small
}

If qualifiers are present, they qualify the pointer type to which the array parameter type is transformed: 

int f(const int a[20])
{
 // in this function, a has type const int* (pointer to const int)
}
int g(const int a[const 20])
{
 // in this function, a has type const int* const (const pointer to const int)
}

This is commonly used with the restrict type qualifier: 

void fadd(double a[static restrict 10],
          const double b[static restrict 10])
{
    for (int i = 0; i < 10; i++) { // loop can be unrolled and reordered
        if (a[i] < 0.0) break;
        a[i] += b[i];
    }
}

Variable-length arrays

If expression is not an integer constant expression, the declarator is for an array of variable size.

Each time the flow of control passes over the declaration, expression is evaluated (and it must always evaluate to a value greater than zero), and the array is allocated (correspondingly, lifetime of a VLA ends when the declaration goes out of scope). The size of each VLA instance does not change during its lifetime, but on another pass over the same code, it may be allocated with a different size. 

{
   int n = 1;
label:
   int a[n]; // re-allocated 10 times, each with a different size
   printf("The array has %zu elements\n", sizeof a / sizeof *a);
   if (n++ < 10) goto label; // leaving the scope of a VLA ends its lifetime
}

If the size is *, the declaration is for a VLA of unspecified size. Such declaration may only appear in a function prototype scope, and declares an array of a complete type. In fact, all VLA declarators in function prototype scope are treated as if expression were replaced by *. 

void foo(size_t x, int a[*]);
void foo(size_t x, int a[x]) 
{
    printf("%zu\n", sizeof a); // same as sizeof(int*)
}

Variable-length arrays and the types derived from them (pointers to them, etc) are commonly known as "variably-modified types" (VM). Objects of any variably-modified type may only be declared at block scope or function prototype scope. 

extern int n;
int A[n];            // Error: file scope VLA
extern int (*p2)[n]; // Error: file scope VM
int B[100];          // OK: file-scope array of constant known size
void fvla(int m, int C[m][m]); // OK: prototype-scope VLA

VLA must have automatic storage duration. Pointers to VLA, but not VLA themselves may also have static storage duration. No VM type may have linkage. 

void fvla(int m, int C[m][m]) // OK: block scope/auto duration pointer to VLA
{
    typedef int VLA[m][m]; // OK: block scope VLA
    int D[m];              // OK: block scope/auto duration VLA
//  static int E[m]; // Error: static duration VLA
//  extern int F[m]; // Error: VLA with linkage
    int (*s)[m];     // OK: block scope/auto duration VM
//  extern int (*r)[m]; // Error: VM with linkage
    static int (*q)[m] = &B; // OK: block scope/static duration VM}
}

Variably-modified types cannot be members of structs or unions. 

struct tag {
    int z[n]; // Error: VLA struct member
    int (*y)[n]; // Error: VM struct member
};
 (since C99)

If the compiler defines the macro constant __STDC_NO_VLA__ to integer constant 1, then VLA and VM types are not supported.

(since C11)

Arrays of unknown size

If expression in an array declarator is omitted, it declares an array of unknown size. Except in function parameter lists (where such arrays are transformed to pointers) and when an initializer is available, such type is an incomplete type (note that VLA of unspecified size, declared with * as the size, is a complete type) (since C99): 

extern int x[]; // the type of x is "array of unknown bound of int"
int a[] = {1,2,3}; // the type of a is "array of 3 int"

Within a struct definition, an array of unknown size may appear as the last member (as long as there is at least one other named member), in which case it is a special case known as flexible array member. See struct for details: 

struct s { int n; double d[]; }; // s.d is a flexible array member 
struct s *s1 = malloc(sizeof (struct s) + (sizeof (double) * 8)); // as if d was double d[8]
 (since C99)

Qualifiers

If an array type is declared with a const, volatile, restrict (since C99), or _Atomic (since C11) qualifier (which is possible through the use of typedef), the array type is not qualified, but its element type is: 

typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
int* pi = a[0]; // Error: a[0] has type const int*

Assignment

Objects of array type are not modifiable lvalues, and although their address may be taken, they cannot appear on the left hand side of an assignment operator. However, structs with array members are modifiable lvalues and can be assigned: 

int a[3] = {1,2,3}, b[3] = {4,5,6};
int (*p)[3] = &a; // okay, address of a can be taken
// a = b;            // error, a is an array
struct { int c[3]; } s1, s2 = {3,4,5};
s1 = s2; // okay: can assign structs holding array members

Array to pointer conversion

Any lvalue expression of array type, when used in any context other than

(since C11)

undergoes an implicit conversion to the pointer to its first element. The result is not an lvalue.

If the array was declared register, the behavior of the program that attempts such conversion is undefined. 

int a[3] = {1,2,3};
int* p = a;
printf("%zu\n", sizeof a); // prints size of array
printf("%zu\n", sizeof p); // prints size of a pointer

When an array type is used in a function parameter list, it is transformed to the corresponding pointer type: int f(int a[2]) and int f(int* a) declare the same function. Since the function's actual parameter type is pointer type, a function call with an array argument performs array-to-pointer conversion; the size of the argument array is not available to the called function and must be passed explicitly: 

void f(int a[], int sz) // actually declares int f(int* a, int sz)
{
    for(int i = 0; i < sz; ++i)
       printf("%d\n", a[i]);
}
int main(void)
{
    int a[10];
    f(a, 10); // converts a to int*, passes the pointer
}

Multidimensional arrays

When the element type of an array is another array, it is said that the array is multidimensional: 

// array of 2 arrays of 3 ints each
int a[2][3] = {{1,2,3},  // can be viewed as a 2x3 matrix
               {4,5,6}}; // with row-major layout

Note that when array-to-pointer conversion is applied, a multidimensional array is converted to a pointer to its first element, e.g., pointer to the first row: 

int a[2][3]; // 2x3 matrix
int (*p1)[3] = a; // pointer to the first 3-element row
int b[3][3][3]; // 3x3x3 cube
int (*p2)[3][3] = b; // pointer to the first 3x3 plane

Multidimensional arrays may be variably modified in every dimension: 

int n = 10;
int a[n][2*n];
 (since C99)

Notes

Zero-length array declarations are not allowed, even though some compilers offer them as extensions (typically as a pre-C99 implementation of flexible array members).

If the size expression of a VLA has side effects, they are guaranteed to be produced except when it is a part of a sizeof expression whose result doesn't depend on it: 

int n = 5;
int m = 7;
size_t sz = sizeof(int (*)[n++]); // may or may not increment n

References

  • C11 standard (ISO/IEC 9899:2011):
  • 6.7.6.2 Array declarators (p: 130-132)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.7.5.2 Array declarators (p: 116-118)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.5.4.2 Array declarators

See also

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