The concept of a variable-length array (VLA) was introduced in the 1999 revision of the C standard, C99. VLAs involve several changes to the language syntax to facilitate passing them to functions.
Any array whose number of elements is not a constant, will be implemented as a VLA by a C99 compiler.
#define a 1 int b = 1; const int c = 1; double foo[a]; /* normal fixed-length array */ double bar[b]; /* variable-length array */ double baz[c]; /* variable-length array */
As you can see in this example, in C the keyword const does not make an expression into a compile-time constant.