sysreq.html

sqlite的帮助文档

<a name="S30000"></a>
<p>
  SQLite is designed to work well within embedded devices with 
  very limited resources.  To this end, it expects to confront situations
  where memory is unavailable and where I/O operations fail and it is designed
  to handle such situations with ease and grace.  SQLite also avoids aggravating
  low-resource situations by correctly freeing rather than leaking 
  resources it uses itself.
</p><blockquote><b>S30000:</b>
  The SQLite library shall be safe for use in long-running,
  low-resource, high-reliability applications.
</blockquote><a name="S30100"></a>
<p>
  A "Proper shutdown" means that all resources that the application
  has allocated from SQLite have been released by the application.
  The leak-free operation guarantee of SQLite applies even if there
  have been memory allocation errors or I/O errors during operation.
</p><blockquote><b>S30100:</b>
  The SQLite library shall release all system resources it holds
  when it is properly shutdown.
</blockquote><a name="S30200"></a>
<p>
  Safety-critical systems typically disallow the use of malloc() and
  free() because one never knows when they might fail due to memory
  fragmentation.  However, SQLite makes extensive use of dynamic objects
  and so it must be able to allocate and deallocate memory
  to hold those objects.</p>
  
  <p>In order to be acceptable for use in safety critical systems,
  SQLite can be configured to use its own internal memory allocator
  which, subject to proper usage by the application, guarantees that
  memory allocation will never fail either due to memory fragmentation
  or any other cause.  The proof of correctness is due to J. M. Robson:
  "Bounds for Some Functions Concerning Dynamic Storage Allocations",
  Journal of the ACM, Volume 21, Number 3, July 1974.</p>
  
  <p>The internal memory allocator is seeded with a large contiguous
  block of memory at application start.  SQLite makes all of its
  internal memory allocations from this initial seed.  
  The Robson proof depends on SQLite being coupled to a well-behaved
  application.  The application must not try to use more than a
  precomputed fraction of the available memory - that fraction depending
  on the size ratio between the largest and smallest memory allocations.
  Additional details are provided elsewhere.
  For the purposes of this document, it is sufficient
  to state:
</p><blockquote><b>S30200:</b>
  The SQLite library shall be configurable so that it is guaranteed
  to never fail a memory allocation as long as the application does
  not request resources in excess of reasonable and published limits.
</blockquote><a name="S30210"></a>
<p>
  To help insure that an
  application never fails a memory allocation call, SQLite provides
  interfaces that can inform the application if its memory usage
  is growing close to or has exceeded the critical Robson limits.  
  In practice, the memory used by an application can exceed the 
  limits of the Robson proof by a wide margin with no harmful effect.  
  There is plenty of safety margin.  But the Robson proof does break 
  down once the limits are exceeded  
  and the guarantee that no memory allocation will fail is lost.  Hence
  it is important to be able to track how close an application has come
  to reaching critical limits.
</p><blockquote><b>S30210:</b>
  The SQLite library shall be provide instrumentation that can alert
  the application when its resource usages nears or exceeds the limits
  of the memory breakdown guarantee.
</blockquote><a name="S30220"></a>
<p>
  When SQLite comes under memory pressure, it can be configured to
  recycle memory from one use to another, thus helping to reduce the
  pressure.  "Memory pressure" means that memory available for 
  allocation is becoming less plentiful.  In a safety-critical application,
  memory pressure might mean that the amount of allocated memory is
  getting close to the point where the Robson proof
  breaks down.  On a workstation, memory pressure might mean that
  available virtual memory is running low.
</p><blockquote><b>S30220:</b>
  The SQLite library shall be provide facilities to automatically
  recycle memory when usage nears preset limits.
</blockquote><a name="S30230"></a>
<p>
  SQLite provides the ability to read and write megabyte
  or gigabyte blobs and text strings without having to allocate
  enough memory to hold the entire blob and string in memory all
  at once.  This enables SQLite to read and write BLOBs that
  are actually larger than the available memory on the device.
  It also helps reduce the size of the maximum memory allocation
  which helps keep memory usage below Robson limits and thus helps
  to guarantee failure-free memory allocation.
</p><blockquote><b>S30230:</b>
  The SQLite library shall be permit BLOB and CLOB objects to be
  read and written incrementally using small memory buffers.
</blockquote><a name="S30300"></a>
<p>
  Memory allocation problems do not cause SQLite to fail
  catastrophically.
  SQLite recognizes all memory allocation failures and either works 
  around them, or
  cleanly aborts what it is doing and returns to the application
  with an error that indicates insufficient memory was available.
  Assuming new memory becomes available, SQLite is able to continue 
  operating normally after a memory allocation failure.
</p><blockquote><b>S30300:</b>
  When a memory allocation fails, SQLite shall either silently make
  due without the requested memory or else it shall report the error
  back to the application.
</blockquote><a name="S30400"></a>
<p>
  SQLite responses sanely to disk I/O errors.  If it is unable
  to work around the problem, SQLite might have to report the error
  back up to the application.  In either case, SQLite is able to
  continue functioning, assuming of course that the I/O error was
  transient.
</p><blockquote><b>S30400:</b>
  When a I/O operation fails, SQLite shall either silently 
  recover or else it shall report the error
  back to the application.
</blockquote><a name="S30500"></a>
<p>
  SQLite is able to cleanly abort an operation in progress and
  afterwards continue functioning normally without any memory or
  other resource leaks.  An example of where this functionality is
  used occurs in the command-line interface (CLI) program for SQLite.
  If the user enters a query that has millions of result rows, those
  rows begin pouring out onto the screen.  The operator can then
  hit the interrupt key sequence (which varies from one operating
  system to another but it often Control-C) which causes the query
  to be aborted.
</p><blockquote><b>S30500:</b>
  SQLite shall provide the capability to monitor
  the progress and interrupt the evaluation of a long-running query.
</blockquote><a name="S30600"></a>
<p>
  When information is deleted from an SQLite database, the default
  action is for SQLite to mark the space as unused and then to reuse
  the space at the next opportune INSERT.  On devices where persistent 
  storage is scarce, however, it is sometime desirable to return the
  unused space back to the operating system.  SQLite supports this.
</p><blockquote><b>S30600:</b>
  All unused portions of a well-formed SQLite database file shall
  be available for reuse.
</blockquote><a name="S30700"></a>
<blockquote><b>S30700:</b>
  SQLite shall provide the capability to incrementally decrease the
  size of the persistent storage file as information is removed from
  the database.
</blockquote><a name="S30800"></a>
<p>
  In consumer-grade software, it is often acceptable to run tests
  on an instrumented version of the code.  But for high-reliability
  systems, it is better to test the code exactly as it is deployed.
  The saying at NASA is "test what you fly and fly what you test."
  In support of this goal, SQLite includes interfaces whose only purpose
  is to observe internal state and to place SQLite into internal states
  for the testing.
</p><blockquote><b>S30800:</b>
  SQLite shall provide the interfaces that support testing and
  validation of the library code in an as-delivered configuration.
</blockquote><a name="S30900"></a>
<p>
  On resource-constrained devices, it is desirable to get double-duty
  out of resources where possible.
</p><blockquote><b>S30900:</b>
  SQLite shall provide the ability for separate database connections
  within the same process to share resources.
</blockquote>
<h2>4.0 SQLite is safe to use in multi-thread and multi-process
applications.</h2>
<a name="S40000"></a>
<p>
  In nearly all modern digital systems, there are many things happening
  at once.  And many of those things involve SQLite.
</p><blockquote><b>S40000:</b>
  The SQLite library shall be safe for use in applications that
  make concurrent access to the underlying database from different
  threads and/or processes.
</blockquote><a name="S40100"></a>
<p>
  The developers of SQLite believe that "thread-safe" is a
  self contradiction.  No application that includes multiple threads
  of control within the same address space is every truly "safe".
  And yet it is recognized that many developers want to
  create multithreaded applications and to use SQLite in those
  applications.  Therefore, SQLite is engineered to be "thread-safe".
</p><blockquote><b>S40100:</b>
  The SQLite library shall be configurable to operate correctly in
  a multi-threaded application.
</blockquote><a name="S40200"></a>
<p>
  Multiple database connections can be created and operated
  independently within the same thread or process.
</p><blockquote><b>S40200:</b>
  The SQLite library shall support multiple independent database
  connections per thread and per process.
</blockquote><a name="S40300"></a>
<p>
  SQLite uses both internal mutexes and external file locking to 
  ensure that two or more threads or processes working
  on the same database file play nicely with one another.
</p><blockquote><b>S40300:</b>
  The SQLite library shall automatically control access to common
  databases from different connections in different threads or processes.
</blockquote><a name="S40400"></a>
<blockquote><b>S40400:</b>
  The SQLite library shall notify the application if an operation can
  not be completed due to concurrent access constraints.
</blockquote><a name="S40410"></a>
<p>
  If an SQL statement cannot be completed because another process is
  holding a lock on the database, then the application needs to be able
  to take corrective action, such waiting for the lock to clear.
</p><blockquote><b>S40410:</b>
  The SQLite library shall provide interfaces to assist the application
  in responding appropriately when an operation can
  not be completed due to concurrent access constraints.
</blockquote>
<h2>5.0 SQLite is cross-platform</h2>
<a name="S50000"></a>
<p>
  Cross-platform in this context means that the SQLite 
  can be used on a wide variety of operating systems and processors,
  ranging from small, special-purpose embedded systems, to workstations,
  to servers.  Platforms can be 32- or 64-bit, big-endian or little-endian.
  Cross-platform refers to the source code.  Obviously the SQLite would
  need to be recompiled in order to run on processors with different
  instruction sets.
</p><blockquote><b>S50000:</b>
  The SQLite library shall be cross-platform.
</blockquote><a name="S50100"></a>
<p>
  C has been called the "universal assembly language".
  Nearly all computer systems accept code written in C.
  Thus, to help make SQLite cross-platform:
</p><blockquote><b>S50100:</b>
  The SQLite library shall be implemented in ANSI-C.
</blockquote><a name="S50200"></a>
<p>
  SQLite stores text data as unicode.  Three separate unicode 
  representations are allowed:
</p><blockquote><b>S50200:</b>
  The SQLite library shall support text encoded as UTF-8,
  UTF-16le, or UTF-16be.
</blockquote><a name="S50300"></a>
<p>
  An SQLite database file can be freely moved between machine
  with different operating systems, different processors,
  different size integers, and different byte orders.  The same
  database file should work on any machine.
</p><blockquote><b>S50300:</b>
  SQLite database files shall be processor and byte-order independent.
</blockquote>
<h2>6.0 Introspection</h2>
<a name="S60000"></a>
<p>
  Some applications need to be able to discover characteristics of
  their environment at run-time and to make appropriate adjustments to
  their processing to accommodate the environment they find themselves in.
  SQLite attempts to support this need.
</p><blockquote><b>S60000:</b>
  The SQLite library shall provide introspection capabilities to the
  application.
</blockquote><a name="S60100"></a>
<p>
  Some applications are designed to work with different versions
  of SQLite which may or may not enable selected features.  For example,
  SQLite can be compiled to be threadsafe or not.  The threadsafe version
  works in multi-threaded applications.  The non-threadsafe build runs
  faster.  When an application is using an unknown version of SQLite
  it is important that it be able to determine the characteristics of
  the particular SQLite build it is using.
</p><blockquote><b>S60100:</b>
  The SQLite library shall provide interfaces that an application can
  use to discover fixed, compile-time characteristics of the 
  SQLite library.
</blockquote><a name="S60200"></a>
<p>
  In addition to the compile-time characteristics, SQLite allows
  the run-time settings of the library and of the underlying
  database file to be interrogated.
</p><blockquote><b>S60200:</b>
  The SQLite library shall provide interfaces that an application can
  use to find run-time performance characteristics and status of the
  SQLite library.
</blockquote><a name="S60300"></a>
<blockquote><b>S60300:</b>
  The SQLite library shall provide interfaces that permit an application
  to query the schema of a database.
</blockquote><a name="S60400"></a>
<blockquote><b>S60400:</b>
  The SQLite library shall provide interfaces that allow an application
  to monitor sequence of queries and progress of submitted to SQLite.
</blockquote><a name="S60500"></a>
<blockquote><b>S60500:</b>
  The SQLite library shall provide interfaces that allow an application
  to discover the algorithms that SQLite has chosen to implement specific
  SQL statements.
</blockquote><a name="S60600"></a>
<p>
  SQLite objects are often related.  For example, every prepared
  statement is associated with a database connection.  And every
  function context is associated with a prepared statement.  
  Applications and extensions frequently find it useful to be able
  to discover these relationships at runtime.  Hence: