Variable-length array

• The GNU C Compiler allocates memory for VLAs with automatic storage duration on the stack.[5] This is the faster and more straightforward option compared to heap-allocation, and is used by most compilers.
• VLAs can also be allocated on the heap and internally accessed using a pointer to this block.

The following C99 function allocates a variable-length array of a specified size, fills it with floating-point values, and then passes it to another function for processing. Because the array is declared as an automatic variable, its lifetime ends when read_and_process() returns.

float read_and_process(int n)
{
    float vals[n];

    for (int i = 0; i < n; ++i)
        vals[i] = read_val();

    return process(n, vals);
}

In C99, the length parameter must come before the variable-length array parameter in function calls.[1] In C11, a __STDC_NO_VLA__ macro is defined if VLA is not supported.[6] The C23 standard makes VLA types mandatory again. Only creation of VLA objects with automatic storage duration is optional.[7] GCC had VLA as an extension before C99, one that also extends into its C++ dialect.

Linus Torvalds has expressed his displeasure in the past over VLA usage for arrays with predetermined small sizes because it generates lower quality assembly code.[8] With the Linux 4.20 kernel, the Linux kernel is effectively VLA-free.[9]

Although C11 does not explicitly name a size-limit for VLAs, some believe it should have the same maximum size as all other objects, i.e. SIZE_MAX bytes.[10] However, this should be understood in the wider context of environment and platform limits, such as the typical stack-guard page size of 4 KiB, which is many orders of magnitude smaller than SIZE_MAX.

It is possible to have VLA-like syntax with dynamic storage by using a pointer to an array.

float read_and_process(int n)
{
    float (*vals)[n] = malloc(sizeof(float[n]));

    for (int i = 0; i < n; ++i)
        (*vals)[i] = read_val();

    float ret = process(n, *vals);
    
    free(vals);
    
    return ret;
}

The following is the same example in Ada. Ada arrays carry their bounds with them, so there is no need to pass the length to the Process function.

type Vals_Type is array (Positive range <>) of Float;

function Read_And_Process (N : Integer) return Float is
   Vals : Vals_Type (1 .. N);
begin
   for I in 1 .. N loop
      Vals (I) := Read_Val;
   end loop;
   return Process (Vals);
end Read_And_Process;

The equivalent Fortran 90 function is

function read_and_process(n) result(o)
    integer,intent(in)::n
    real::o

    real,dimension(n)::vals
    integer::i

    do i = 1,n
       vals(i) = read_val()
    end do
    o = process(vals)
end function read_and_process

when utilizing the Fortran 90 feature of checking procedure interfaces at compile time; on the other hand, if the functions use pre-Fortran 90 call interface, the (external) functions must first be declared, and the array length must be explicitly passed as an argument (as in C):

function read_and_process(n) result(o)
    integer,intent(in)::n
    real::o

    real,dimension(n)::vals
    real::read_val, process
    integer::i

    do i = 1,n
       vals(i) = read_val()
    end do
    o = process(vals,n)
end function read_and_process

The following COBOL fragment declares a variable-length array of records DEPT-PERSON having a length (number of members) specified by the value of PEOPLE-CNT:

DATA DIVISION.
WORKING-STORAGE SECTION.
01  DEPT-PEOPLE.
    05  PEOPLE-CNT          PIC S9(4) BINARY.
    05  DEPT-PERSON         OCCURS 0 TO 20 TIMES DEPENDING ON PEOPLE-CNT.
        10  PERSON-NAME     PIC X(20).
        10  PERSON-WAGE     PIC S9(7)V99 PACKED-DECIMAL.

The COBOL VLA, unlike that of other languages mentioned here, is safe because COBOL requires one to specify the maximal array size – in this example, DEPT-PERSON cannot have more than 20 items, regardless of the value of PEOPLE-CNT.

The following C# fragment declares a variable-length array of integers. Prior to C# version 7.2, a pointer to the array is required, requiring an "unsafe" context. The "unsafe" keyword requires an assembly containing this code to be marked as unsafe.

unsafe void DeclareStackBasedArrayUnsafe(int size)
{
    int *pArray = stackalloc int[size];
    pArray[0] = 123;
}

C# version 7.2 and later allow the array to be allocated without the "unsafe" keyword, through the use of the Span feature.[11]

void DeclareStackBasedArraySafe(int size)
{
    Span<int> stackArray = stackalloc int[size];
    stackArray[0] = 123;
}

Object Pascal dynamic arrays are allocated on the heap.[12]

In this language, it is called a dynamic array. The declaration of such a variable is similar to the declaration of a static array, but without specifying its size. The size of the array is given at the time of its use.

program CreateDynamicArrayOfNumbers(Size: Integer);
var
  NumberArray: array of LongWord;
begin
  SetLength(NumberArray, Size);
  NumberArray[0] := 2020;
end.

Removing the contents of a dynamic array is done by assigning it a size of zero.

...
SetLength(NumberArray, 0);
...