c arrays: setting size dynamically?

Question

I am new to C programming. I am trying to set the size of the array using a variable but I am getting an error: Storage size of 'array' isn't constant !!

01 int bound = bound*4;

02 static GLubyte vertsArray[bound];

I have noticed that when I replace bounds (within the brackets on line 02) with the number say '20', the program would run with no problems. But I am trying to set the size of the array dynamically ...

Any ideas why I am getting this error ? thanks much,

Answer 1

You are getting this error because, as the compiler told you, your array size is not constant. In C89/90 version of C language array size has to be a constant. You can't "set the size of the array dynamically". If you need a run-time sized array, you have to either allocate it manually with malloc or use some non-standard compiler-specific approach (like alloca function).

In C99 version of C language support for so called Variable-Length Arrays (VLA) was added. A C99 compiler would accept run-time sized array declaration for an automatic array. Yet even in C99 you can't declare a static array of run-time size, as you are trying to.

Answer 2

To create an array of a non-constant size (i.e. known at compile time), you need to dynamically allocate space for it using malloc() (and correspondingly deallocate it using free() when it is no longer required).

As others have noted, the ability to declare dynamic arrays is available in C99 compliant compilers.