Variable-length array

In 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).

Programming languages that support VLAs include Ada, Algol 68 (for non-flexible rows), APL, C (added in C99; on some platforms, could be implemented previously with alloca() or similar functions) and C# (as unsafe-mode stack-allocated arrays), COBOL, Fortran 90.

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.

For example, the GNU C Compiler allocates memory for VLAs on the stack. If a VLA is too large, no error occurs immediately; instead, attempts to use the VLA may lead to segmentation faults later in the execution of the program.

Examples

The following C 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:

real function read_and_process(n)
 
    integer n, i
    real    vals(n), read_val, process
 
    do i = 1,n
       vals(i) = read_val()
    enddo
    read_and_process = process(vals, n)
 
end

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. Please note the "unsafe" keyword which would require 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.