Variable-length array

In computer programming, a variable-length array (VLA), also called variable-sized, runtime-sized, 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, C99 (although subsequently relegated in C11 to a conditional feature which implementations are not required to support; on some platforms, could be implemented previously with alloca() or similar functions) and C# (as unsafe-mode stack-allocated arrays), COBOL, Fortran 90, and J.

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. VLAs, like all objects in C, are limited to SIZE_MAX bytes.

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 incomplete types.

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.

In C99, the length parameter must come before the variable-length array parameter in function calls.

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