6-8.c

《linux应用开发技术详解》的配套代码

void *malloc(long numbytes) {
	/* Holds where we are looking in memory */
	void *current_location;
	/* This is the same as current_location, but cast to a
	 * memory_control_block
	 */
	struct mem_control_block *current_location_mcb;
	/* This is the memory location we will return.  It will
	 * be set to 0 until we find something suitable
	 */
	void *memory_location;
	/* Initialize if we haven't already done so */
	if(! has_initialized) 	{
		malloc_init();
	}
	/* The memory we search for has to include the memory
	 * control block, but the users of malloc don't need
	 * to know this, so we'll just add it in for them.
	 */
	numbytes = numbytes + sizeof(struct mem_control_block);
	/* Set memory_location to 0 until we find a suitable
	 * location
	 */
	memory_location = 0;
	/* Begin searching at the start of managed memory */
	current_location = managed_memory_start;
	/* Keep going until we have searched all allocated space */
	while(current_location != last_valid_address)
	{
		/* current_location and current_location_mcb point
		 * to the same address.  However, current_location_mcb
		 * is of the correct type, so we can use it as a struct.
		 * current_location is a void pointer so we can use it
		 * to calculate addresses.
		 */
		current_location_mcb =
			(struct mem_control_block *)current_location;
		if(current_location_mcb->is_available)
		{
			if(current_location_mcb->size >= numbytes)
			{
				/* Woohoo!  We've found an open,
				 * appropriately-size location.
				 */
				/* It is no longer available */
				current_location_mcb->is_available = 0;
				/* We own it */
				memory_location = current_location;
				/* Leave the loop */
				break;
			}
		}
		/* If we made it here, it's because the Current memory
		 * block not suitable; move to the next one
		 */
		current_location = current_location +
			current_location_mcb->size;
	}
	/* If we still don't have a valid location, we'll
	 * have to ask the operating system for more memory
	 */
	if(! memory_location)
	{
		/* Move the program break numbytes further */
		sbrk(numbytes);
		/* The new memory will be where the last valid
		 * address left off
		 */
		memory_location = last_valid_address;
		/* We'll move the last valid address forward
		 * numbytes
		 */
		last_valid_address = last_valid_address + numbytes;
		/* We need to initialize the mem_control_block */
		current_location_mcb = memory_location;
		current_location_mcb->is_available = 0;
		current_location_mcb->size = numbytes;
	}
	/* Now, no matter what (well, except for error conditions),
	 * memory_location has the address of the memory, including
	 * the mem_control_block
	 */
	/* Move the pointer past the mem_control_block */
	memory_location = memory_location + sizeof(struct mem_control_block);
	/* Return the pointer */
	return memory_location;
}