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Version Control with Subversion
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Version Control with Subversion - Using the APIs

Using the APIs

Developing applications against the Subversion library APIs is fairly straightforward. All of the public header files live in the subversion/include directory of the source tree. These headers are copied into your system locations when you build and install Subversion itself from source. These headers represent the entirety of the functions and types meant to be accessible by users of the Subversion libraries.

The first thing you might notice is that Subversion's datatypes and functions are namespace protected. Every public Subversion symbol name begins with svn_, followed by a short code for the library in which the symbol is defined (such as wc, client, fs, etc.), followed by a single underscore (_) and then the rest of the symbol name. Semi-public functions (used among source files of a given library but not by code outside that library, and found inside the library directories themselves) differ from this naming scheme in that instead of a single underscore after the library code, they use a double underscore (__). Functions that are private to a given source file have no special prefixing, and are declared static. Of course, a compiler isn't interested in these naming conventions, but they help to clarify the scope of a given function or datatype.

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 are practically no OS-specific code portions in Subversion itself. Also, it means that the Subversion client compiles and runs anywhere that the 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, [43] APR gives Subversion immediate access to many custom datatypes, such as dynamic arrays and hash tables. Subversion uses these types extensively throughout the codebase. 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 schema for data passed through its API), [44] and while a person coding against the Subversion APIs is not required to do the same, they are 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 acquire a pool for use with Subversion API calls. See the section called “Programming with Memory Pools” for more information.


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Version Control with Subversion
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