Variable-length array

• The GNU C Compiler allocates memory for VLAs with automatic storage duration on the stack.[4] 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.[5] GCC had VLA as an extension before C99.

Linus Torvalds has expressed his displeasure in the past over VLA usage for arrays with predetermined small sizes, with comments like "USING VLA'S IS ACTIVELY STUPID! It generates much more code, and much slower code (and more fragile code), than just using a fixed key size would have done." [6] With the Linux 4.20 kernel, Linux kernel is effectively VLA-free.[7]

Although C11 does not explicitly name a size-limit for VLAs, some readings believe it should have the same maximum size as all other objects, i.e. SIZE_MAX bytes.[8] As a size many orders of magnitude larger than the typical stack guard-page size of 4 KiB and typical total stack size limits on the order of a few megabytes, this reading should not be taken literally.

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.[9]

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