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Variable length arrays are not supported by Microsoft compilers.
GCC
Variable length arrays should be used on GCC with great care. Newer versions of GCC have incorporated variable length arrays but do not yet claim full C conformance. GCC has limited incomplete support for parts of this standard, enabled with -std=c11 or -std=iso9899:2011.
On an example Debian GNU/Linux Intel 32-bit test machine with GCC 4.2.2, the value of a variable length array's size is interpreted as a 32-bit signed integer. Passing in a negative number for the size will likely cause the program stack to become corrupted, and passing in a large positive number may cause a terminal stack overflow. It is important to note that this information may become outdated as GCC evolves.
Risk Assessment
Failure to properly specify the size of a variable length array may allow arbitrary code execution or result in stack exhaustion.
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