C language dynamic memory allocation

First, understand why dynamic memory allocation is required , be familiar with C Language readers should be familiar with this , It is used when a piece of memory is needed malloc Function to request the required amount of memory , Function returns the first address of a section of memory . Simply speaking , The advantage of dynamic memory allocation is that when memory is needed, it can be allocated on demand , Memory can be released when it is not needed , This makes efficient use of memory . The following article implements a complete dynamic memory allocation from scratch .

Simple dynamic memory allocation implementation
Memory allocation is to allocate unused memory blocks to required variables （ Common variables 、 Pointer to the variable 、 Structural variables, etc ） Use , Because it needs to be released after use , This will cause the free memory to be scattered in the memory pool . therefore , Memory must be managed , That is, mark the memory usage .

The above figure shows a certain time after a memory pool is used , You can see , Used blocks and unused blocks are not contiguous , So you need to tag it with a table , This watch is called BitMap. Suppose that the memory is now divided into each Byte division , Then use one bit Mark blocks ,1 Indicates that... Has been used ,0 Indicates not used , Such a block requires a bit.

Make sure you use C Language to implement this simple dynamic memory allocation .

The final terminal output results are as follows ：

The above has implemented a simple dynamic memory allocation , Can complete the allocation and release of memory, output utilization and view bitmap . The dynamic memory allocation implemented in this way will not generate internal fragments , That's the advantage of it , But the obvious disadvantage is that the utilization rate is too low .
Practical dynamic memory allocation
Careful readers may have noticed that the simple dynamic memory allocation above has a drawback , It's just one. bit Only one byte can be represented , That is to say, it means 8 A byte requires a byte bitmap , This mapping causes its memory to

This is a waste in many cases . In order to improve utilization , You must increase the granularity of the mapping block , That's one Bit The mapping range of corresponds to multiple bytes .

The picture above shows a bit Mapping to 64Byte, such ：

Although the utilization rate has become higher , But it can produce internal debris , The so-called internal fragmentation is the memory space that cannot be used within the minimum granularity , Why is this space unusable , The reason is that when applying for memory blocks , Its memory can only be 64B The alignment of the , Even if less than 64B, You have to press 64B Come and see , Because this granularity has been bitmap The tag uses , When next used , It cannot be assigned . therefore , You can see , The larger the particle size , The larger the internal memory fragments it may generate , Memory utilization and fragmentation need to be balanced , A good algorithm can only solve the problem of external fragmentation , Unable to resolve internal fragmentation problem , Therefore, the dynamic memory allocation must be balanced , To achieve the best results .

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