malloc - systemcall - how it's made?

Question

Concerning malloc and systemcall - I would like to know what's happening when the systemcall is made. Malloc is just a library-function right?

So - when calling malloc and there are unmapped memory left on the heap - no system call is made, I guess. But, let say the allocated heap becomes full - the heap has to grow. Here, I do not know how a trap is triggered?

A trap is needed to make the system call, so a kernel function such as brk() or mmap() can be made - but how is it made? Is it through some kind of exception internally?

I am interested to know!!!

Edit: concerning the other question - I have looked at it and cannot se (by the moment) anything about systemcall functions, traps in the cpu - instead its much about why the program crashes

Answer 1

No special traps or exceptions are needed. Pseudocode for malloc is:

void *malloc(size_t size)
{
    search for 'size' free space in available blocks;
    if(no block found}
        {
        request additional memory from OS;
        construct new block;
        }
    mark 'size' bytes used in block;
    return pointer into block;
}

The step you're wondering about is request additional memory from OS; and it is, as you speculated, typically a straightforward function call to sbrk.

(It's true, when you call sbrk, somewhere in its implementation there's going to be a special mechanism, such as a trap, to perform the context switch into the operating system so that it can do its work for you, but this will be the same sort of mechanism as is used for all system calls. It's not something you generally need to worry about: you just call sbrk() like any other function.)

Answer 2

In order to have an idea how malloc is made, you can write a program in which you call malloc and then use this command : strace yourprogram.

With this command, you can see in depth the trace system calls and signals when you are calling malloc and have an idea how it's made.