what is _alloca()?

[earlz]

I was trying to add some c++ to my kernel and got a linker error for _alloca so what is it, the parameters, and is there a dealloca because i used my generic sbrk() as alloca but it didn't give me an error and i have no dealloc

[paulbarker]

alloca should allocate memory on the stack, so that when the function returns the memory is automatically freed.

Now, the question of how thats done is not something I can answer. Its usually compiler magic.

[nick8325]

It's in the FAQ

How kernel, compiler, and C library work together (near the bottom)

It'll appear if you have a function with more than 4KB of local variables, or (possibly) if you have a local variable which doesn't have a constant size (e.g. an array with a variable sized number of elements).

You might be able to find a proper implementation (for GCC) in libgcc, but I can't find it on the GCC website any more... ???

[earlz]

I understand what it does now but how the crap would you make it and how is the memory automatically freed at the exit of a function without calling another function to free

btw

The alloca function is machine and compiler dependent. On many systems its implementation is buggy. Its use is discouraged.

so is there anyway to avoid using it

[paulbarker]

What compiler are you using?

[Colonel Kernel]

AFAIK, you would implement it on x86 just by subtracting a certain amount from esp. When the function returns, it will restore the stack pointer of the calling frame, thus "freeing" the alloca'd memory just like local variables are freed.

[Solar]

so is there anyway to avoid using it

• How kernel, compiler and C library work together
• GCC Cross-Compiler

A lot of effort has gone into the FAQ. Many questions are answered there - including those about alloca().