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In this exercise, we simulate a memory allocation (we just want to see the effect of fragmentation and of compacting memory fragments into larger ones). For this, we manage a list of memory segments in our program:
typedef struct segment {
int start ;
int size ;
int status ;
struct segment * next ;
} segment_t ;
The above segment descriptor describes a memory segment in terms of its start address, its size and whether it is ALLOCATED or FREE. This descriptor is part of a list (contains a pointer to next segment descriptor). Initially, there is only one segment with status == FREE, describing the complete memory as one segment.
Two functions have to be implemented: mem_alloc() and mem_free(). With mem_alloc(), a junk of memory is allocated: it takes as a parameter the size of memory required and returns a pointer to the segment created or NULL, if not enough free memory can be found. With mem_free(), a segment can be freed up again. If there are two free segments adjacent to each other, they have to be combined into one segment (one of the segment nodes has to be removed from the list).
The prototype for mem_alloc(): segment_t * mem_alloc( int size ) A pointer to the allocated segment is returned.
The prototype for mem_free(): void mem_free ( segment_t * segment ) It takes the segment to be freed as a parameter.
Over time, the memory will become fragmented and a call to mem_alloc() will fail, as no free segment large enough may be available. The function mem_alloc() therefore has to initiate a compaction of free memory : all allocated segments should be moved towards the top of the list and all free segments to the bottom of the list. When all free segments are collected in one place, they all should be replaced by one single segment descriptor describing one free memory segment.
Test mem_alloc() and mem_free(). mem_alloc() takes a size of memory as a parameter and returns a pointer to the allocated memory segment. This pointer to the allocated segment has to be recorded in your test environment (create a list). You can implement a little menu that runs in an endless loop and lets you allocate memory. If you want to free memory from this menu, you need an extra command that lets you first list the allocated segments, so that you can select one of them to be freed.
Hints for implementation: Implement your application in the form of three files: start with memalloc.h, in which you specify the segment struct and extra parameters such as the maximum size of the memory and the function prototypes. Continue with memalloc.c, that includes this header file and implements the two functions mem_alloc() and mem_free(). Create a third file memtest.c that includes the header file and implements the main() function of your program
Here is a first go at the memalloc.h
#ifndef MEMALLOC_H
#define MEMALLOC_H
#define MAXMEMORY 1024
typedef struct segment {
int start ;
int size ;
int status ;
struct segment * next ;
} segment_t ;
segment_t * mem_alloc ( int ) ;
void mem_free( segment_t * ) ;