One silly question . . .
As far as I remember we can take only constant index of an array in C. In other words we can't take variable value as an index of array at the time of DECLARATION.

#include <stdio.h>

int main()
{
int i;
for(i=1; i < 10; i++)
{
char ch[i]; //Compile time error should come here
printf("\n Size of an array is: %d\n", sizeof(ch));
}
return 0;
}

And so this code should give Compile time Error... But it is not giving ... WHY ? Using GCC to compile

It is called a variable length array(VLA) and is part of the c99 standard. http://c0x.coding-guidelines.com/6.7.5.2.html

But so I have another question ...
This is only allowed when I am using GCC... is that correct ?

If I use some other compiler like VC compiler, this feature may not be available...
And in short, this is specific to "Compiler Implementation"... Is that right ?

Maybe you are having a bad day so I will just quote myself "...part of the c99 standard".
http://www.comeaucomputing.com/techt...lelengtharrays more info if you require it.

Come on dmail...
You are making puzzles for me ...
I understand tht this is part of C99

But, is this still under implementation/design ?
Or every compiler should follow the c99 statndard ?

In general not all compilers obey the same standards, or at least equally well. Even if a compiler "follows a certain standard", it may just mean it follows some parts of it and not all. Shortly put, if the code is ok by some standard, it will compile on compilers that follow that standard fully or by those parts that your code uses. With very new standards it could be that only parts of it are implemented in a compiler (say GCC compiler), and the rest come afterwards.

In short: use compilers that say they follow the standard as well as possible.

Come on, vkmgeek...

DMail was just saying that you happened to be using a C99 feature on what happened to be a C99 conformant compiler (GCC). What's so puzzling about that ;-)?

Personally, I don't know how many other compilers out there are C99 conformant. Recent offerings from Sun and IBM - certainly. Microsoft? I have the new VS2008 - and frankly, I hardly even recognize standard C++ anywhere in all the .Net-interoperability-this and ATL-enhancements-that junk the IDE likes to throw in, much less what part of "C" is or isn't supported. My guess is that VS2008 has "token support" for C95, and not much more than that.

Your best bet is to look at the documentation for individual vendors to check:

a) Whether or not they claim conformance to any standard
... and, if so ...
b) which standards, at which levels

IMHO .. PSM

PS:
This is a good link:
http://en.wikipedia.org/wiki/C_(prog..._language)#C99