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Writing Apache Modules with Perl and C by Lincoln Stein, Doug MacEachern

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10.3. Memory Management and Resource Pools

If you've ever developed a moderately complex C-language program, you've struggled with memory management. It's not easy to manage memory in C: failing to deallocate a data structure when you're through with it gives rise to memory leaks, and conversely, disposing of the same data structure twice is likely to lead to a crash. It's one thing to have a small memory leak in your own program. Unless the leak is very severe, the program will probably finish execution and exit normally before memory becomes tight. However, it's quite another issue to have memory management problems in a network server, which is expected to run for weeks or months at a time. Even small leaks can add up over time, and a dangling pointer or a doubly deallocated block can make the whole server crash.

The Apache server developers were aware of the challenge of memory management, and so they devised a system to make life easier both for themselves and for module writers. Instead of managing memory directly, Apache module developers take the memory they need from one or more resource pools. An Apache pool structure keeps track of all module memory allocations and releases all allocated blocks automatically when the lifetime of the pool has ended.

Different pools have different lifetimes: one lasts for the lifetime of a child, one for the lifetime of a request, another for the module configuration phase, and so forth. However, the nicest feature about pools is ...

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