CS 214: Systems Programming, Spring 2018
Completed by Frederick William Lau and Jaroor Modi
Assignment 1: To Build A Better Malloc
- Introduction
We have implemented malloc() and free() library calls for dynamic memory allocation that detect common programming and usage errors.
Malloc( size_t size ) is a system call that returns a pointer to a block of memory of at least the requested size. This memory comes from a main memory resource managed by the operating system. The free( void * ) function informs the operating system that you are done with a given block of dynamically-allocated memory, and that it can reclaim it for other uses.
We use a large array to simulate main memory (static char myblock[5000]). The malloc() function will return pointers to this large array and your free() function let's the code know that a previously-allocated region can be reclaimed and used for other purposes by changing pointers to regions into NULL. Our malloc() function employs a “first free” algorithm to select blocks of memory to allocate.
- Detectable Errors
Our malloc() and free() can catch the following errors:
A: Free()ing addresses that are not pointers:
int x;
free( (int*)x );
B: Free()ing pointers that were not allocated by malloc():
p = (char *)malloc( 200 );
free( p + 10 );
- or -
int * x;
free( x );
C: Redundant free()ing of the same pointer:
p = (char*)malloc(100);
free( p );
free( p );
... is an error, but:
p = (char *)malloc( 100 );
free( p );
p = (char *)malloc( 100 );
free( p );
... is perfectly valid, even if malloc() returned the same pointer both times.
D: Saturation of dynamic memory:
p = (char*)malloc(5001);
- or -
p = (char*)malloc(5000);
q = (char*)malloc(1);
... your code must gracefully handle being asked for more memory than it can
allocate.
- Responding to Detected Errors
Your modified malloc() and free() should report the precise calls that caused dynamic memory problems during program execution. Your code should use the preprocessor LINE and FILE printf directives to print informative messages:
#define malloc( x ) mymalloc( x, FILE, LINE )
#define free( x ) myfree( x, FILE, LINE )
- Testing and Instrumentation
We have implemented the following workloads:
A: malloc() 1 byte and immediately free it - does this 150 times
B: malloc() 1 byte, store the pointer in an array - does this 150 times.
Once you've malloc()ed 150 byte chunks, then free() the 150 1 byte pointers one by one.
C: Randomly choose between a 1 byte malloc() or free()ing a 1 byte pointer - do this 150 times
-
Keep track of each operation so that you eventually malloc() 150 bytes, in total
-
Keep track of each operation so that you eventually free() all pointers
(don't allow a free() if you have no pointers to free())
D: Randomly choose between a randomly-sized malloc() or free()ing a pointer – do this many times (see below)
-
Keep track of each malloc so that all mallocs do not exceed your total memory capacity
-
Keep track of each operation so that you eventually malloc() 150 times
-
Keep track of each operation so that you eventually free() all pointers
-
Choose a random allocation size between 1 and 64 bytes
E,F: Two more workloads of your choosing
- Describe both workloads in your testplan.txt
Your memgrind.c should run all the workloads, one after the other, 100 times. It should record the run time for each workload and store it. When all 100 iterations of all the workloads have been run, memgrind.c should calculate the mean time for each workload to execute and output them in sequence.
You might find the gettimeofday(struct timeval * tv, struct timezone * tz) function in the time.h library useful.
You should run memgrind yourself and include its results in your readme.pdf. Be sure to discuss your findings, especially any interesting or unexpected results.
- Submission
You should submit a Asst1.tar.gz containing:
A: readme.pdf documenting your design and workload data and findings
B: testcases.txt that describes your two workloads and why you included them
C: mymalloc.h with your malloc headers and definitions
D: mymalloc.c with your malloc function implementations
E: memgrind.c with your memory test and profiling code as described above
F: Makefile that builds and cleans memgrind with your mymalloc library