Descriptions of test program:

Overview

memtest.c tests:

• Correctness of mymalloc() and myfree()
• Error detection for invalid free() calls
• Proper coalescing of free chunks
• Edge cases such as malloc(0) and free(NULL)

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Test Cases

1. Basic Allocation and Free Test

• Purpose: Checks whether mymalloc() properly allocates memory and myfree() correctly deallocates it.
• Procedure:
  1. Allocate OBJECTS small memory chunks.
  2. Fill each chunk with a unique byte pattern.
  3. Verify the memory is correctly assigned.
  4. Free all allocated memory.
• Expected Behavior:
  • Each chunk should retain its value.
  • Memory should be correctly freed without corruption.
  • Leak detector should report no leaks.

2. mymalloc() Coalescing Free Chunks (Forward Only)

• Purpose: Tests whether mymalloc() correctly merges adjacent free chunks when a large allocation is requested.
• Procedure:
  1. Allocate and free OBJECTS small chunks sequentially.
  2. Attempt to allocate a large chunk equal to the total size of all freed memory.
• Expected Behavior:
  • Allocation should succeed only if forward coalescing is implemented.
  • If mymalloc() does not coalesce properly, allocation will fail.

3. myfree() Coalescing Free Chunks

• Purpose: Ensures myfree() correctly merges adjacent free chunks.
• Procedure:
  1. Allocate OBJECTS small memory chunks.
  2. Free them in reverse order.
  3. Attempt to allocate a single large chunk.
• Expected Behavior:
  • Allocation should succeed if myfree() coalesces forward chunks.
  • If myfree() does not merge properly, allocation will fail.

4. Large Allocation Detection

• Purpose: Ensures mymalloc() correctly detects and rejects requests larger than available memory.
• Procedure:
  1. Request a block larger than MEMLENGTH - HEADERSIZE.
  2. Request a block equal to MEMLENGTH.
  3. Request malloc(0).
• Expected Behavior:
  • Large allocations should return NULL with an error message.
  • malloc(0) should return NULL.

5. Normal Unallocatable Cases

• Purpose: Ensures mymalloc() does not allocate when memory is exhausted.
• Procedure:
  1. Allocate OBJECTS + 1 chunks.
  2. Free all OBJECTS valid chunks.
  3. Check the last allocation.
• Expected Behavior:
  • malloc() should fail when requesting memory beyond capacity.
  • If it succeeds, there's a bug in allocation logic.

6. Invalid free() - Out of Heap

• Purpose: Ensures myfree() detects and prevents freeing non-heap pointers.
• Procedure:
  1. Call free() on an unallocated stack variable.
  2. Call free(NULL).
  3. Call free() on an arbitrary address.
• Expected Behavior:
  • free(&x) should terminate with an error.
  • free(NULL) should be ignored (no crash).
  • Freeing an invalid pointer should terminate with an error.

7. Invalid free() - Not at Chunk Start

• Purpose: Ensures myfree() rejects freeing an address that is not at the start of a chunk.
• Procedure:
  1. Allocate a chunk.
  2. Try to free(ptr + 1).
• Expected Behavior:
  • myfree() should detect and reject the invalid free.

8. Double Free Detection

• Purpose: Ensures myfree() detects double frees.
• Procedure:
  1. Allocate a chunk.
  2. Free it twice.
• Expected Behavior:
  • The second free() should trigger an error and terminate.

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mymalloc.c Design Notes

1. Chunk Splitting and Allocation Behavior

• mymalloc() splits free chunks when possible.
• If the chunk is only slightly larger than requested, it allocates the entire chunk.

2. Forward-Only Coalescing

• myfree() only merges free chunks in the forward direction.
• It does not merge with previous chunks.

3. Behavior of Edge Cases

Case	Behavior
malloc(0)	Returns NULL
free(NULL)	Does nothing
malloc(large_size)	Fails with error message
free(outside_heap_ptr)	Fails with error and exits
free(not_chunk_start_ptr)	Fails with error and exits
double free()	Fails with error and exits

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