The Apache Portable Runtime Library

Along with Subversion’s own datatypes, you will see many references to datatypes that begin with apr_—symbols from the Apache Portable Runtime (APR) library. APR is Apache’s portability library, originally carved out of its server code as an attempt to separate the OS-specific bits from the OS-independent portions of the code. The result was a library that provides a generic API for performing operations that differ mildly—or wildly—from OS to OS. While the Apache HTTP Server was obviously the first user of the APR library, the Subversion developers immediately recognized the value of using APR as well. This means that there is practically no OS-specific code in Subversion itself. Also, it means that the Subversion client compiles and runs anywhere that the Apache HTTP Server does. Currently, this list includes all flavors of Unix, Win32, BeOS, OS/2, and Mac OS X.

In addition to providing consistent implementations of system calls that differ across operating systems,[54] APR gives Subversion immediate access to many custom datatypes, such as dynamic arrays and hash tables. Subversion uses these types extensively. But perhaps the most pervasive APR datatype, found in nearly every Subversion API prototype, is the apr_pool_t—the APR memory pool. Subversion uses pools internally for all its memory allocation needs (unless an external library requires a different memory management mechanism for data passed through its API),[55] and while a person coding against the Subversion APIs is not required to do the same, she is required to provide pools to the API functions that need them. This means that users of the Subversion API must also link against APR, must call apr_initialize() to initialize the APR subsystem, and then must create and manage pools for use with Subversion API calls, typically by using svn_pool_create(), svn_pool_clear(), and svn_pool_destroy().

Programming with Memory Pools

Almost every developer who has used the C programming language has at some point sighed at the daunting task of managing memory usage. Allocating enough memory to use, keeping track of those allocations, freeing the memory when you no longer need it—these tasks can be quite complex. And of course, failure to do those things properly can result in a program that crashes itself, or worse, crashes the computer.

Higher-level languages, on the other hand, either take the job of memory management away from you completely or make it something you toy with only when doing extremely tight program optimization. Languages such as Java and Python use garbage collection, allocating memory for objects when needed, and automatically freeing that memory when the object is no longer in use.

APR provides a middle-ground approach called pool-based memory management. It allows the developer to control memory usage at a lower resolution—per chunk (or “pool”) of memory, instead of per allocated object. Rather than using malloc() and friends to allocate enough memory for a given object, you ask APR to allocate the memory from a memory pool. When you’re finished using the objects you’ve created in the pool, you destroy the entire pool, effectively deallocating the memory consumed by all the objects you allocated from it. Thus, rather than keeping track of individual objects that need to be deallocated, your program simply considers the general lifetimes of those objects and allocates the objects in a pool whose lifetime (the time between the pool’s creation and its deletion) matches the object’s needs.


[54] Subversion uses ANSI system calls and datatypes as much as possible.

[55] Neon and Berkeley DB are examples of such libraries.

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