Programming III SPRING 2015 School of Computer and Information Sciences Francisco R. Ortega, Ph.D. McKnight Fellow and GAANN Fellow LECTURE #6C Pointers, and a bit More

Programming III Remember to Check – –

Remember malloc calloc realloc free

Example (8.7) Storage Allocator

Storage Allocator Example Uses First Fit – Best fit will look for smallest chunk needed. – First fit looks for first space that has enough space If exact space found, it is unlinked and return to user If the block is too big, it is split – The proper amount is returned to user – Residual is left on the free list If not big enough, another chunk is obtain from OS and linked into the free list

Storage Allocator Example If space being free is adjacent to free block in either side – it is coalesced into single bigger block Most restrictive type is used – Some machines may be a double – And others long or int A free block contains – a pointer to the next block in the chain. – a record of the size of the block – free space If the block is too big, it is split – The proper amount is returned to user – Residual is left on the free list If not big enough, another chunk is obtain from OS and linked into the free list

Align, Size, and More Align field is never used. – Forces each header to be aligned on a worst-case boundary The size field is necessary because the blocks need to be contiguous – Why is not possible to use pointer arithmetic? The block contains one more unit – The header If programmer messes with reserved blocks, – list will be scrambled

Allocation (more) Variable base is used to get started – empty list if freep is NULL, it is the first call to malloc then a degenerate free list is created – Contains one block of size zero. – Points to itself Then List is search The free space is return to the user

malloc The function morecore obtains storage from OS – It is expensive. We want to get at least NALLOC units After setting the size field – morecore inserts the additional memory Unix system call is sbrk(n) – Returns a pointer to n more buts of storage – returns -1 if no space – -1 must be cast to (char *) to compared – This code is portable among machines that general pointer comparison is meaningful

Free Scans free list starting at freep looking for the place to insert free block – Either between two existing blocks or at one end of the list Pointers must be updated to point to correct places

Allocation Example While this allocation is machine dependent – illustrate the concepts This can be used in other situations – Hint (Homework) Suggested Exercises 8-6, 8-7,8-8