使用malloc和realloc进行动态存储的最佳方法

Question

I'm trying to figure out what is the optimal way of using malloc and realloc for recieving unknown amount of characters from the user ,storing them, and printing them only by the end.

I've figured that calling realloc too many times wont be so smart. so instead, I allocate a set amount of space each time,lets say sizeof char*100 and by the end of file,i use realloc to fit the size of the whole thing precisely.

what do you think?is this a good way to go about? would you go in a different path?

Please note,I have no intention of using linked lists,getchar(),putchar(). using malloc and realloc only is a must.

Answer 1

If you realloc to fit the exact amount of data needed, then you are optimizing for memory consumption. This will likely give slower code because 1) you get extra realloc calls and 2) you might not allocate amounts that fit well with CPU alignment and data cache. Possibly this also causes heap segmentation issues because of the repeated reallocs, in which case it could actually waste memory.

It's hard to answer what's "best" generically, but the below method is fairly common, as it is a good compromise between reducing execution speed for realloc calls and lowering memory use:

You allocate a segment, then keep track of how much of this segment that is user data. It is a good idea to allocate size_t mempool_size = n * _Alignof(int); bytes and it is probably also wise to use a n which is divisible by 8.

Each time you run out of free memory in this segment, you realloc to mempool_size*2 bytes. That way you keep doubling the available memory each time.

Answer 2

I've figured that calling realloc too many times wont be so smart.

How have you figured it out? Because the only way to really know is to measure the performance.

Your strategy may need to differ based on how you are reading the data from the user. If you are using getchar() you probably don't want to use realloc() to increase the buffer size by one char each time you read a character. However, a good realloc() will be much less inefficient than you think even in these circumstances. The minimum block size that glibc will actually give you in response to a malloc() is, I think, 16 bytes. So going from 0 to 16 characters and reallocing each time doesn't involve any copying. Similarly for larger reallocations, a new block might not need to be allocated, it may be possible to make the existing block bigger. Don't forget that even at its slowest, realloc() will be faster than a person can type.

Most people don't go for that strategy. What can by typed can be piped so the argument that people don't type very fast doesn't necessarily work. Normally, you introduce the concept of capacity. You allocate a buffer with a certain capacity and when it gets full, you increase its capacity (with realloc() ) by adding a new chunk of a certain size. The initial size and the reallocation size can be tuned in various ways. If you are reading user input, you might go for small values eg 256 bytes, if you are reading files off disk or across the network, you might go for larger values eg 4Kb or bigger.

The increment size doesn't even need to be constant, you could choose to double the size for each needed reallocation. This is the strategy used by some programming libraries. For example the Java implementation of a hash table uses it I believe and so possibly does the Cocoa implementation of an array.

It's impossible to know beforehand what the best strategy in any particular situation is. I would pick something that feels right and then, if the application has performance issues, I would do testing to tune it. Your code doesn't have to be the fastest possible, but only fast enough.

However one thing I absolutely would not do is overlay a home rolled memory algorithm over the top of the built in allocator. If you find yourself maintaining a list of blocks you are not using instead of freeing them, you are doing it wrong. This is what got OpenSSL into trouble.