CITS2002 Systems Programming  
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Allocating new memory

Let's first address the first of these problems - we do not know, until the function is called, how big the array should be.

It is often the case that we do not know, until we execute our programs, how much memory we'll really need!

Instead of using a fixed sized array whose size may sometimes be too small, we must dynamically request some new memory at runtime to hold our desired result.

This is a fundamental (and initially confusing) concept of most programming languages - the ability to request from the operating system additional memory for our programs.

C11 provides a small collection of functions to support memory allocation.
The primary function we'll see is named malloc(), which is declared in the standard <stdlib.h> header file:

#include <stdlib.h>

extern void *malloc( size_t nbytes );

  • malloc() is a function (external to our programs) that returns a pointer.
    However, malloc() doesn't really know what it's returning a pointer to - it doesn't know if it's a pointer to an integer, or a pointer to a character, or even to one our own user-defined types.

    For this reason, we use the generic pointer, pronounced "void star" or "void pointer".

    It's a pointer to "something", and we only "know" what that is when we place an interpretation on the pointer.

  • malloc() needs to be informed of the amount of memory that it should allocate - the number of bytes we require.

    We use the standard datatype size_t to hold an integer value that may be 0 or positive (we obviously can't request a negative amount of memory!).

    We have used, but skipped over, the use of size_t before - it's the datatype of values returned by the sizeof operator, and the pedantically-correct type returned by the strlen() function.

 


CITS2002 Systems Programming, Lecture 12, p4, 28th August 2024.