shmalloc

Like malloc(3), but for shared memory blocks.

Remember

• If someone shmallocs with some ID id, we have to tell if we need to create it or just attach it (probably a dirwalk).

Things we need as part of the requirements (i.e. error checking):

• Freeing a pointer not made by malloc
  • You malloc something and store it as ptr, but you free (ptr + 1)
  • Check the bit sequence
  • Freeing pointers that were not allocated
  • Freeing pointers made by malloc(3)
• Redundant freeing of the same pointer
  • Check the is_free flag
• Allocating all of the memory (oversaturation)
  • return NULL
• Fragmentation

IPC stuff:

• Make sure we initialize mutexes for the internal data structures
• Mutexes have special shared memory things (set some flags)
• Don't deadlock

When implementing shmfree:

• Keep track of the reference count; when the reference count hits zero we actually free and coalesce the memory with adjacent spots.
• Make sure we don't deadlock when we destroy the headers

In shmfree:

pthread_mutex_lock(header->mutex);
if (refcount <= 0) {
    // Destroy the header
}
pthread_mutex_unlock(header->mutex);

Have a canonical locking order for the mutexes in destroy_header:

pthread_lock(prev->mutex);
pthread_lock(next->mutex);
prev->next = next;
next->prev = prev;

Or something like that.

For the write-up:

• Don't forget a quick paragraph oh why we didn't do shmrealloc

SHMAT GIVES YOU A MEMORY SEGMENT FULL OF ZEROES Expectation: get malloc in shared memory to work AT ALL