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# Custom Builds Of SQLite
or
Porting SQLite To New Operating Systems

## 1.0 Introduction

For most applications, the recommended method for building SQLite is to use the amalgamation code file, sqlite3.c, and its corresponding header file sqlite3.h. The sqlite3.c code file should compile and run on any Unix, Windows, OS/2, or Mac OS X system without any changes or special compiler options. Most applications can simply include the sqlite3.c file together with the other C code files that make up the application, compile them all together, and have working and well configured version of SQLite.

> Most applications work great with SQLite in its default configuration and with no special compile-time configuration. Most developers should be able to completely ignore this document and simply build SQLite from the amalgamation without any special knowledge and without taking any special actions.

However, highly tuned and specialized applications may want or need to replace some of SQLite's built-in system interfaces with alternative implementations more suitable for the needs of the application. SQLite is designed to be easily reconfigured at compile-time to meet the specific needs of individual projects. Among the compile-time configuration options for SQLite are these:

- Replace the built-in mutex subsystem with an alternative implementation.

- Completely disable all mutexing for use in single-threaded applications.

- Reconfigure the memory allocation subsystem to use a memory allocator other the malloc() implementation from the standard library.

- Realign the memory allocation subsystem so that it never calls malloc() at all but instead satisfies all memory requests using a fixed-size memory buffer assigned to SQLite at startup.

- Replace the interface to the file system with an alternative design. In other words, override all of the system calls that SQLite makes in order to talk to the disk with a completely different set of system calls.

- Override other operating system interfaces such as calls to obtain Zulu or local time.

Generally speaking, there are three separate subsystems within SQLite that can be modified or overridden at compile-time. The mutex subsystem is used to serialize access to SQLite resources that are shared among threads. The memory allocation subsystem is used to allocate memory required by SQLite objects and for the database cache. Finally, the Virtual File System subsystem is used to provide a portable interface between SQLite and the underlying operating system and especially the file system. We call these three subsystems the "interface" subsystems of SQLite.

We emphasis that most applications are well-served by the built-in default implementations of the SQLite interface subsystems. Developers are encouraged to use the default built-in implementations whenever possible and to build SQLite without any special compile-time options or parameters. However, some highly specialized applications may benefit from substituting or modifying one or more of these built-in SQLite interface subsystems. Or, if SQLite is used on an operating system other than Unix (Linux or Mac OS X), Windows (Win32 or WinCE), or OS/2 then none of the interface subsystems that come built into SQLite will work and the application will need to provide alternative implementations suitable for the target platform.

## 2.0 Configuring Or Replacing The Mutex Subsystem

In a multithreaded environment, SQLite uses mutexes to serialize access to shared resources. The mutex subsystem is only required for applications that access SQLite from multiple threads. For single-threaded applications, or applications which only call SQLite from a single thread, the mutex subsystem can be completely disabled by recompiling with the following option:

>     -DSQLITE_THREADSAFE=0

Mutexes are cheap but they are not free, so performance will be better when mutexes are completely disabled. The resulting library footprint will also be a little smaller. Disabling the mutexes at compile-time is a recommended optimization for applications where it makes sense.

When using SQLite as a shared library, an application can test to see whether or not mutexes have been disabled using the sqlite3_threadsafe() API. Applications that link against SQLite at run-time and use SQLite from multiple threads should probably check this API to make sure they did not accidentally get linked against a version of the SQLite library that has its mutexes disabled. Single-threaded applications will, of course, work correctly regardless of whether or not SQLite is configured to be threadsafe, though they will be a little bit faster when using versions of SQLite with mutexes disabled.

SQLite mutexes can also be disabled at run-time using the sqlite3_config() interface. To completely disable all mutexing, the application can invoke:

>     sqlite3_config(SQLITE_CONFIG_SINGLETHREAD);

Disabling mutexes at run-time is not as effective as disabling them at compile-time since SQLite still must do a test of a boolean variable to see if mutexes are enabled or disabled at each point where a mutex might be required. But there is still a performance advantage for disabling mutexes at run-time.

For multi-threaded applications that are careful about how they manage threads, SQLite supports an alternative run-time configuration that is half way between not using any mutexes and the default situation of mutexing everything in sight. This in-the-middle mutex alignment can be established as follows:

>     sqlite3_config(SQLITE_CONFIG_MULTITHREAD);
>     sqlite3_config(SQLITE_CONFIG_MEMSTATUS, 0);

There are two separate configuration changes here which can be used either togethr or separately. The SQLITE_CONFIG_MULTITHREAD setting disables the mutexes that serialize access to database connection objects and prepared statement objects. With this setting, the application is free to use SQLite from multiple threads, but it must make sure than no two threads try to access the same database connection or any prepared statements associated with the same database connection at the same time. Two threads can use SQLite at the same time, but they must use separate database connections. The second SQLITE_CONFIG_MEMSTATUS setting disables the mechanism in SQLite that tracks the total size of all outstanding memory allocation requests. This omits the need to mutex each call to sqlite3_malloc() and sqlite3_free(), which saves a huge number of mutex operations. But a consequence of disabling the memory statistics mechanism is that the sqlite3_memory_used(), sqlite3_memory_highwater(), and sqlite3_soft_heap_limit64() interfaces cease to work.

SQLite uses pthreads for its mutex implementation on Unix and SQLite requires a recursive mutex. Most modern pthread implementations support recursive mutexes, but not all do. For systems that do not support recursive mutexes, it is recommended that applications operate in single-threaded mode only. If this is not possible, SQLite provides an alternative recursive mutex implementation built on top of the standard "fast" mutexes of pthreads. This alternative implementation should work correctly as long as pthread_equal() is atomic and the processor has a coherent data cache. The alternative recursive mutex implementation is enabled by the following compiler command-line switch:

>     -DSQLITE_HOMEGROWN_RECURSIVE_MUTEX=1

When porting SQLite to a new operating system, it is usually necessary to completely replace the built-in mutex subsystem with an alternative built around the mutex primitives of the new operating system. This is accomplished by compiling SQLite with the following option:

>     -DSQLITE_MUTEX_APPDEF=1

When SQLite is compiled with the SQLITE_MUTEX_APPDEF=1 option, it completely omits the implementation of its mutex primitive functions. But the SQLite library still attempts to call these functions where necessary, so the application must itself implement the mutex primitive functions and link them together with SQLite.

## 3.0 Configuring Or Replacing The Memory Allocation Subsystem

By default, SQLite obtains the memory it needs for objects and cache from the malloc()/free() implementation of the standard library. There is also on-going work with experimental memory allocators that satisfy all memory requests from a single fixed memory buffer handed to SQLite at application start. Additional information on these experimental memory allocators will be provided in a future revision of this document.

SQLite supports the ability of an application to specify an alternative memory allocator at run-time by filling in an instance of the sqlite3_mem_methods object with pointers to the routines of the alternative implementation then registering the new alternative implementation using the sqlite3_config() interface. For example:

>     sqlite3_config(SQLITE_CONFIG_MALLOC, &my_malloc_implementation);

SQLite makes a copy of the content of the sqlite3_mem_methods object so the object can be modified after the sqlite3_config() call returns.

## 4.0 Adding New Virtual File Systems

Since version 3.5.0, SQLite has supported an interface called the virtual file system or "VFS". This object is somewhat misnamed since it is really an interface to the whole underlying operating system, not just the filesystem.

One of the interesting features of the VFS interface is that SQLite can support multiple VFSes at the same time. Each database connection has to choose a single VFS for its use when the connection is first opened using sqlite3_open_v2(). But if a process contains multiple database connections each can choose a different VFS. VFSes can be added at run-time using the sqlite3_vfs_register() interface.

The default builds for SQLite on Unix, Windows, and OS/2 include a VFS appropriate for the target platform. SQLite builds for other operating systems do not contain a VFS by default, but the application can register one or more at run-time.

## 5.0 Porting SQLite To A New Operating System

In order to port SQLite to a new operating system - an operating system not supported by default - the application must provide...

- a working mutex subsystem (but only if it is multithreaded),
- a working memory allocation subsystem (assuming it lacks malloc() in its standard library), and
- a working VFS implementation.

All of these things can be provided in a single auxiliary C code file and then linked with the stock "sqlite3.c" code file to generate a working SQLite build for the target operating system. In addition to the alternative mutex and memory allocation subsystems and the new VFS, the auxiliary C code file should contain implementations for the following two routines:

- sqlite3_os_init()
- sqlite3_os_end()

The "sqlite3.c" code file contains default implementations of a VFS and of the sqlite3_initialize() and sqlite3_shutdown() functions that are appropriate for Unix, Windows, and OS/2. To prevent one of these default components from being loaded when sqlite3.c is compiled, it is necessary to add the following compile-time option:

>     -DSQLITE_OS_OTHER=1

The SQLite core will call sqlite3_initialize() early. The auxiliary C code file can contain an implementation of sqlite3_initialize() that registers an appropriate VFS and also perhaps initializes an alternative mutex system (if mutexes are required) or does any memory allocation subsystem initialization that is required. The SQLite core never calls sqlite3_shutdown() but it is part of the official SQLite API and is not otherwise provided when compiled with -DSQLITE_OS_OTHER=1, so the auxiliary C code file should probably provide it for completeness.