this code is supposed to fail but it works, why?

Question

Folks I think I will throw all my modest C lore away. Look at this code please:

int main( int argc, char ** argv, char ** envp )
{
int aa;

srand(time(NULL));

int Num=rand()%20;

int Vetor[Num];

for (aa=0; aa<Num; aa++)
  {
    Vetor[aa]=rand()%40;
    printf("Vetor [%d] = %d\n",aa,Vetor[aa]);
  }
}

I would think that this should throw an error for two reasons - first that I am declaring both Num and Vetor after executing a command (srand), second because I am declaring Vetor based on Num, this should not be possible right? because those array sizes should not be decided at runtime but at compile time right?

I am really surprised that his works and if you guys could point to some references why I can actually use stuff like this would be great. I think I lost some big changes to C in the last years...

oh by the way, this is using gcc

Answer 1

These are C99 features, and it seems your compiler supports them. That's all ;)

From Wikipedia:

C99 introduced several new features, many of which had already been implemented as extensions in several compilers:

• inline functions
• intermingled declarations and code, variable declaration no longer restricted to file scope or the start of a compound statement (block)
• several new data types, including long long int, optional extended integer types, an explicit boolean data type, and a complex type to represent complex numbers
• variable-length arrays
• support for one-line comments beginning with //, as in BCPL or C++
• new library functions, such as snprintf
• etc (more)

Answer 2

C99 supports declarations anywhere in the code, as well as VLAs. What compiler are you using?