MEMORY CORRUPTION PART II HEAPS in Software

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Figure 67 The segment illustrated in Figure 67 contains two allocations (and associated metadata) followed by a range of uncommitted memory If another allocation request arrives, and no available free block is present in the free lists, the heap manager would commit additional memory from the uncommitted range, create a new heap block within the committed memory range, and return the block to the user Once a segment runs out of uncommitted space, the heap manager creates a new segment The size of the new segment is determined by doubling the size of the previous segment If memory is scarce and cannot accommodate the new segment, the heap manager tries to reduce the size by half If that fails, the size is halved again until it either succeeds or reaches a minimum segment size threshold in which case, an error is returned to the caller The maximum number of segments that can be active within a heap is 64 Once the new segment is created, the heap manager adds it to a list that keeps track of all segments being used in the heap Does the heap manager ever free memory associated with a segment The answer is that the heap manager decommits memory on a per-needed basis, but it never releases it (That is, the memory stays reserved) As Figure 67 depicts, each heap block in a given segment has metadata associated with it The metadata is used by the heap manager to effectively manage the heap blocks within a segment The content of the metadata is dependent on the status of the heap block For example, if the heap block is used by the application, the status of the block is considered busy Conversely, if the heap block is not in use (that is, has been freed by the application), the status of the block is considered free Figure 68 shows how the metadata is structured in both situations
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Busy Block: Allocation Metadata Current Block Size Size (in bytes) 2 Previous Block Size 2 Segment Index 1 Tag Index 1 User accessible part Suffix Bytes 16 Fill area (debug mode) Heap Extra 8
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Free Block: Allocation Metadata Current Block Size Size (in bytes) 2 Previous Block Size 2 Segment Index 1 Tag Index 1 User accessible part Suffix Bytes 16 Fill area (debug mode) Heap Extra 8
Flags 1
Unused 1
Preallocation metadata
Postallocation metadata
Figure 68 It is important to note that a heap block might be considered busy in the eyes of the back end allocator but still not being used by the application The reason behind this is that any heap blocks that go on the front end allocator s look aside list still have their status set as busy The two size fields represent the size of the current block and the size of the previous block (metadata inclusive) Given a pointer to a heap block, you can very easily use the two size fields to walk the heap segment forward and backward Additionally, for free blocks, having the block size as part of the metadata enables the heap manager to very quickly index the correct free list to add the block to The post-allocation metadata is optional and is typically used by the debug heap for additional bookkeeping information (see Attaching Versus Running under the debugger sidebar) The flags field indicates the status of the heap block The most important values of the flags field are shown in Table 61 Table 61
Value Description
6 MEMORY CORRUPTION PART II HEAPS
0x01 0x04 0x08 0x10
Indicates that the allocation is being used by the application or the heap manager Indicates whether the heap block has a fill pattern associated with it Indicates that the heap block was allocated directly from the virtual memory manager Indicates that this is the last heap block prior to an uncommitted range