gcc error generation

Why does neither gcc or clang generate an error when I try to compile code containing the following two lines?

int palindrome(char s[]){
    char s2[strlen(s)];

I thought in such an instance you would have 开发者_Python百科to dynamically allocate memory to s2.

GCC has an extension for this behavior, and it's also standard in C99, known as variable length arrays.

http://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html

Clang supports it due to GCC C and C99: http://clang.llvm.org/compatibility.html#vla

Section 6.7.5.2 Array declarators:

If the size is not present, the array type is an incomplete type. If the size is * instead of being an expression, the array type is a variable length array type of unspecified size, which can only be used in declarations with function prototype scope) such arrays are nonetheless complete types. If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type.

And an example from 6.5.3.4:

#include <stddef.h>
size_t fsize3(int n)
{
      char b[n+3];     // variable length array
      return sizeof b; // execution time sizeof
}

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf (not the standard, but a draft, and free. :)