Variable-length array

In computer programming, a variable-length array (or VLA) is an array data structure of automatic storage duration whose length is determined at run time (instead of at compile time).^[1]

Programming languages that support VLAs include Ada, Algol 68 (for non-flexible rows), APL, C99 (although subsequently relegated in C11 to a conditional feature which implementations are not required to support;^[2][3][4] on some platforms, could be implemented previously with alloca() or similar functions) and C# (as unsafe-mode stack-allocated arrays), COBOL, Fortran 90, J.

Memory

Allocation

One problem that may be hidden by a language's support for VLAs is that of the underlying memory allocation: in environments where there is a clear distinction between a heap and a stack, it may not be clear which, if any, of those will store the VLA.^[5]

For example, the GNU C Compiler allocates memory for VLAs on the stack.^[6] VLAs, like all objects in C, are limited to SIZE_MAX bytes.^[7]

Variable access

In some programming languages VLAs can be accessed via pointers, but the size can no longer be obtained when de-referenced as they are considered complete types.^[8]

Examples

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

float read_and_process(int n)
{
    float vals[n];
 
    for (int i = 0; i < n; i++)
        vals[i] = read_val();
    return process(vals, n);
}

Following is the same example in Ada. Note that Ada arrays carry their bounds with them, 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 following C# fragment declares a variable-length array of integers. The "unsafe" keyword would require an assembly containing this code to be marked as unsafe.

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

Dynamic vs. automatic

Languages such as Java technically do not provide variable-length arrays, because all array objects in those languages are dynamically allocated on the heap, and therefore do not have automatic storage duration for arrays.

References