sqlite/man/SQLITE_CONFIG_SINGLETHREAD.3

.Dd March 11, 2017
.Dt SQLITE_CONFIG_SINGLETHREAD 3
.Os
.Sh NAME
.Nm SQLITE_CONFIG_SINGLETHREAD ,
.Nm SQLITE_CONFIG_MULTITHREAD ,
.Nm SQLITE_CONFIG_SERIALIZED ,
.Nm SQLITE_CONFIG_MALLOC ,
.Nm SQLITE_CONFIG_GETMALLOC ,
.Nm SQLITE_CONFIG_SCRATCH ,
.Nm SQLITE_CONFIG_PAGECACHE ,
.Nm SQLITE_CONFIG_HEAP ,
.Nm SQLITE_CONFIG_MEMSTATUS ,
.Nm SQLITE_CONFIG_MUTEX ,
.Nm SQLITE_CONFIG_GETMUTEX ,
.Nm SQLITE_CONFIG_LOOKASIDE ,
.Nm SQLITE_CONFIG_PCACHE ,
.Nm SQLITE_CONFIG_GETPCACHE ,
.Nm SQLITE_CONFIG_LOG ,
.Nm SQLITE_CONFIG_URI ,
.Nm SQLITE_CONFIG_PCACHE2 ,
.Nm SQLITE_CONFIG_GETPCACHE2 ,
.Nm SQLITE_CONFIG_COVERING_INDEX_SCAN ,
.Nm SQLITE_CONFIG_SQLLOG ,
.Nm SQLITE_CONFIG_MMAP_SIZE ,
.Nm SQLITE_CONFIG_WIN32_HEAPSIZE ,
.Nm SQLITE_CONFIG_PCACHE_HDRSZ ,
.Nm SQLITE_CONFIG_PMASZ ,
.Nm SQLITE_CONFIG_STMTJRNL_SPILL
.Nd Configuration Options
.Sh SYNOPSIS
.Fd #define SQLITE_CONFIG_SINGLETHREAD
.Fd #define SQLITE_CONFIG_MULTITHREAD
.Fd #define SQLITE_CONFIG_SERIALIZED
.Fd #define SQLITE_CONFIG_MALLOC
.Fd #define SQLITE_CONFIG_GETMALLOC
.Fd #define SQLITE_CONFIG_SCRATCH
.Fd #define SQLITE_CONFIG_PAGECACHE
.Fd #define SQLITE_CONFIG_HEAP
.Fd #define SQLITE_CONFIG_MEMSTATUS
.Fd #define SQLITE_CONFIG_MUTEX
.Fd #define SQLITE_CONFIG_GETMUTEX
.Fd #define SQLITE_CONFIG_LOOKASIDE
.Fd #define SQLITE_CONFIG_PCACHE
.Fd #define SQLITE_CONFIG_GETPCACHE
.Fd #define SQLITE_CONFIG_LOG
.Fd #define SQLITE_CONFIG_URI
.Fd #define SQLITE_CONFIG_PCACHE2
.Fd #define SQLITE_CONFIG_GETPCACHE2
.Fd #define SQLITE_CONFIG_COVERING_INDEX_SCAN
.Fd #define SQLITE_CONFIG_SQLLOG
.Fd #define SQLITE_CONFIG_MMAP_SIZE
.Fd #define SQLITE_CONFIG_WIN32_HEAPSIZE
.Fd #define SQLITE_CONFIG_PCACHE_HDRSZ
.Fd #define SQLITE_CONFIG_PMASZ
.Fd #define SQLITE_CONFIG_STMTJRNL_SPILL
.Sh DESCRIPTION
These constants are the available integer configuration options that
can be passed as the first argument to the sqlite3_config()
interface.
.Pp
New configuration options may be added in future releases of SQLite.
Existing configuration options might be discontinued.
Applications should check the return code from sqlite3_config()
to make sure that the call worked.
The sqlite3_config() interface will return a non-zero
error code if a discontinued or unsupported configuration
option is invoked.
.Bl -tag -width Ds
.It SQLITE_CONFIG_SINGLETHREAD
There are no arguments to this option.
This option sets the threading mode to Single-thread.
In other words, it disables all mutexing and puts SQLite into a mode
where it can only be used by a single thread.
If SQLite is compiled with the  SQLITE_THREADSAFE=0
compile-time option then it is not possible to change the threading mode
from its default value of Single-thread and so sqlite3_config()
will return SQLITE_ERROR if called with the SQLITE_CONFIG_SINGLETHREAD
configuration option.
.It SQLITE_CONFIG_MULTITHREAD
There are no arguments to this option.
This option sets the threading mode to Multi-thread.
In other words, it disables mutexing on database connection
and prepared statement objects.
The application is responsible for serializing access to database connections
and prepared statements.
But other mutexes are enabled so that SQLite will be safe to use in
a multi-threaded environment as long as no two threads attempt to use
the same database connection at the same time.
If SQLite is compiled with the  SQLITE_THREADSAFE=0
compile-time option then it is not possible to set the Multi-thread
threading mode and sqlite3_config() will
return SQLITE_ERROR if called with the SQLITE_CONFIG_MULTITHREAD
configuration option.
.It SQLITE_CONFIG_SERIALIZED
There are no arguments to this option.
This option sets the threading mode to Serialized.
In other words, this option enables all mutexes including the recursive
mutexes on database connection and prepared statement
objects.
In this mode (which is the default when SQLite is compiled with SQLITE_THREADSAFE=1)
the SQLite library will itself serialize access to database connections
and prepared statements so that the application
is free to use the same database connection or the
same prepared statement in different threads at the
same time.
If SQLite is compiled with the  SQLITE_THREADSAFE=0
compile-time option then it is not possible to set the Serialized threading mode
and sqlite3_config() will return SQLITE_ERROR
if called with the SQLITE_CONFIG_SERIALIZED configuration option.
.It SQLITE_CONFIG_MALLOC
The SQLITE_CONFIG_MALLOC option takes a single argument which is a
pointer to an instance of the sqlite3_mem_methods
structure.
The argument specifies alternative low-level memory allocation routines
to be used in place of the memory allocation routines built into SQLite.
SQLite makes its own private copy of the content of the sqlite3_mem_methods
structure before the sqlite3_config() call returns.
.It SQLITE_CONFIG_GETMALLOC
The SQLITE_CONFIG_GETMALLOC option takes a single argument which is
a pointer to an instance of the sqlite3_mem_methods
structure.
The sqlite3_mem_methods structure is filled with
the currently defined memory allocation routines.
This option can be used to overload the default memory allocation routines
with a wrapper that simulations memory allocation failure or tracks
memory usage, for example.
.It SQLITE_CONFIG_MEMSTATUS
The SQLITE_CONFIG_MEMSTATUS option takes single argument of type int,
interpreted as a boolean, which enables or disables the collection
of memory allocation statistics.
When memory allocation statistics are disabled, the following SQLite
interfaces become non-operational:
.Bl -bullet
.It
sqlite3_memory_used()
.It
sqlite3_memory_highwater()
.It
sqlite3_soft_heap_limit64()
.It
sqlite3_status64()
.El
.Pp
Memory allocation statistics are enabled by default unless SQLite is
compiled with SQLITE_DEFAULT_MEMSTATUS=0 in
which case memory allocation statistics are disabled by default.
.It SQLITE_CONFIG_SCRATCH
The SQLITE_CONFIG_SCRATCH option specifies a static memory buffer that
SQLite can use for scratch memory.
There are three arguments to SQLITE_CONFIG_SCRATCH:  A pointer an 8-byte
aligned memory buffer from which the scratch allocations will be drawn,
the size of each scratch allocation (sz), and the maximum number of
scratch allocations (N).
The first argument must be a pointer to an 8-byte aligned buffer of
at least sz*N bytes of memory.
SQLite will not use more than one scratch buffers per thread.
SQLite will never request a scratch buffer that is more than 6 times
the database page size.
If SQLite needs needs additional scratch memory beyond what is provided
by this configuration option, then sqlite3_malloc()
will be used to obtain the memory needed.<p>  When the application
provides any amount of scratch memory using SQLITE_CONFIG_SCRATCH,
SQLite avoids unnecessary large heap allocations.
This can help prevent memory allocation failures
due to heap fragmentation in low-memory embedded systems.
.It SQLITE_CONFIG_PAGECACHE
The SQLITE_CONFIG_PAGECACHE option specifies a memory pool that SQLite
can use for the database page cache with the default page cache implementation.
This configuration option is a no-op if an application-define page
cache implementation is loaded using the SQLITE_CONFIG_PCACHE2.
There are three arguments to SQLITE_CONFIG_PAGECACHE: A pointer to
8-byte aligned memory (pMem), the size of each page cache line (sz),
and the number of cache lines (N).
The sz argument should be the size of the largest database page (a
power of two between 512 and 65536) plus some extra bytes for each
page header.
The number of extra bytes needed by the page header can be determined
using SQLITE_CONFIG_PCACHE_HDRSZ.
It is harmless, apart from the wasted memory, for the sz parameter
to be larger than necessary.
The pMem argument must be either a NULL pointer or a pointer to an
8-byte aligned block of memory of at least sz*N bytes, otherwise subsequent
behavior is undefined.
When pMem is not NULL, SQLite will strive to use the memory provided
to satisfy page cache needs, falling back to sqlite3_malloc()
if a page cache line is larger than sz bytes or if all of the pMem
buffer is exhausted.
If pMem is NULL and N is non-zero, then each database connection does
an initial bulk allocation for page cache memory from sqlite3_malloc()
sufficient for N cache lines if N is positive or of -1024*N bytes if
N is negative, .
If additional page cache memory is needed beyond what is provided by
the initial allocation, then SQLite goes to sqlite3_malloc()
separately for each additional cache line.
.It SQLITE_CONFIG_HEAP
The SQLITE_CONFIG_HEAP option specifies a static memory buffer that
SQLite will use for all of its dynamic memory allocation needs beyond
those provided for by SQLITE_CONFIG_SCRATCH and
SQLITE_CONFIG_PAGECACHE.
The SQLITE_CONFIG_HEAP option is only available if SQLite is compiled
with either SQLITE_ENABLE_MEMSYS3 or SQLITE_ENABLE_MEMSYS5
and returns SQLITE_ERROR if invoked otherwise.
There are three arguments to SQLITE_CONFIG_HEAP: An 8-byte aligned
pointer to the memory, the number of bytes in the memory buffer, and
the minimum allocation size.
If the first pointer (the memory pointer) is NULL, then SQLite reverts
to using its default memory allocator (the system malloc() implementation),
undoing any prior invocation of SQLITE_CONFIG_MALLOC.
If the memory pointer is not NULL then the alternative memory allocator
is engaged to handle all of SQLites memory allocation needs.
The first pointer (the memory pointer) must be aligned to an 8-byte
boundary or subsequent behavior of SQLite will be undefined.
The minimum allocation size is capped at 2**12.
Reasonable values for the minimum allocation size are 2**5 through
2**8.
.It SQLITE_CONFIG_MUTEX
The SQLITE_CONFIG_MUTEX option takes a single argument which is a pointer
to an instance of the sqlite3_mutex_methods structure.
The argument specifies alternative low-level mutex routines to be used
in place the mutex routines built into SQLite.
SQLite makes a copy of the content of the sqlite3_mutex_methods
structure before the call to sqlite3_config() returns.
If SQLite is compiled with the  SQLITE_THREADSAFE=0
compile-time option then the entire mutexing subsystem is omitted from
the build and hence calls to sqlite3_config() with
the SQLITE_CONFIG_MUTEX configuration option will return SQLITE_ERROR.
.It SQLITE_CONFIG_GETMUTEX
The SQLITE_CONFIG_GETMUTEX option takes a single argument which is
a pointer to an instance of the sqlite3_mutex_methods
structure.
The sqlite3_mutex_methods structure is filled
with the currently defined mutex routines.
This option can be used to overload the default mutex allocation routines
with a wrapper used to track mutex usage for performance profiling
or testing, for example.
If SQLite is compiled with the  SQLITE_THREADSAFE=0
compile-time option then the entire mutexing subsystem is omitted from
the build and hence calls to sqlite3_config() with
the SQLITE_CONFIG_GETMUTEX configuration option will return SQLITE_ERROR.
.It SQLITE_CONFIG_LOOKASIDE
The SQLITE_CONFIG_LOOKASIDE option takes two arguments that determine
the default size of lookaside memory on each database connection.
The first argument is the size of each lookaside buffer slot and the
second is the number of slots allocated to each database connection.
SQLITE_CONFIG_LOOKASIDE sets the <i>default</i> lookaside size.
The SQLITE_DBCONFIG_LOOKASIDE option to sqlite3_db_config()
can be used to change the lookaside configuration on individual connections.
.It SQLITE_CONFIG_PCACHE2
The SQLITE_CONFIG_PCACHE2 option takes a single argument which is a
pointer to an sqlite3_pcache_methods2 object.
This object specifies the interface to a custom page cache implementation.
SQLite makes a copy of the sqlite3_pcache_methods2
object.
.It SQLITE_CONFIG_GETPCACHE2
The SQLITE_CONFIG_GETPCACHE2 option takes a single argument which is
a pointer to an sqlite3_pcache_methods2 object.
SQLite copies of the current page cache implementation into that object.
.It SQLITE_CONFIG_LOG
The SQLITE_CONFIG_LOG option is used to configure the SQLite global
error log.
( The SQLITE_CONFIG_LOG option takes two arguments: a pointer to a
function with a call signature of void(*)(void*,int,const char*), and
a pointer to void.
If the function pointer is not NULL, it is invoked by sqlite3_log()
to process each logging event.
If the function pointer is NULL, the sqlite3_log() interface
becomes a no-op.
The void pointer that is the second argument to SQLITE_CONFIG_LOG is
passed through as the first parameter to the application-defined logger
function whenever that function is invoked.
The second parameter to the logger function is a copy of the first
parameter to the corresponding sqlite3_log() call and
is intended to be a result code or an extended result code.
The third parameter passed to the logger is log message after formatting
via sqlite3_snprintf().
The SQLite logging interface is not reentrant; the logger function
supplied by the application must not invoke any SQLite interface.
In a multi-threaded application, the application-defined logger function
must be threadsafe.
.It SQLITE_CONFIG_URI The SQLITE_CONFIG_URI option takes a single argument
of type int.
If non-zero, then URI handling is globally enabled.
If the parameter is zero, then URI handling is globally disabled.
If URI handling is globally enabled, all filenames passed to sqlite3_open(),
sqlite3_open_v2(), sqlite3_open16()
or specified as part of ATTACH commands are interpreted as URIs,
regardless of whether or not the SQLITE_OPEN_URI flag
is set when the database connection is opened.
If it is globally disabled, filenames are only interpreted as URIs
if the SQLITE_OPEN_URI flag is set when the database connection is
opened.
By default, URI handling is globally disabled.
The default value may be changed by compiling with the SQLITE_USE_URI
symbol defined.
.It SQLITE_CONFIG_COVERING_INDEX_SCAN The SQLITE_CONFIG_COVERING_INDEX_SCAN
option takes a single integer argument which is interpreted as a boolean
in order to enable or disable the use of covering indices for full
table scans in the query optimizer.
The default setting is determined by the SQLITE_ALLOW_COVERING_INDEX_SCAN
compile-time option, or is "on" if that compile-time option is omitted.
The ability to disable the use of covering indices for full table scans
is because some incorrectly coded legacy applications might malfunction
when the optimization is enabled.
Providing the ability to disable the optimization allows the older,
buggy application code to work without change even with newer versions
of SQLite.
.It SQLITE_CONFIG_PCACHE and SQLITE_CONFIG_GETPCACHE These options are
obsolete and should not be used by new code.
They are retained for backwards compatibility but are now no-ops.
.It SQLITE_CONFIG_SQLLOG This option is only available if sqlite is compiled
with the SQLITE_ENABLE_SQLLOG pre-processor macro
defined.
The first argument should be a pointer to a function of type void(*)(void*,sqlite3*,const
char*, int).
The second should be of type (void*).
The callback is invoked by the library in three separate circumstances,
identified by the value passed as the fourth parameter.
If the fourth parameter is 0, then the database connection passed as
the second argument has just been opened.
The third argument points to a buffer containing the name of the main
database file.
If the fourth parameter is 1, then the SQL statement that the third
parameter points to has just been executed.
Or, if the fourth parameter is 2, then the connection being passed
as the second parameter is being closed.
The third parameter is passed NULL In this case.
An example of using this configuration option can be seen in the "test_sqllog.c"
source file in the canonical SQLite source tree.
.It SQLITE_CONFIG_MMAP_SIZE SQLITE_CONFIG_MMAP_SIZE takes two 64-bit integer
(sqlite3_int64) values that are the default mmap size limit (the default
setting for PRAGMA mmap_size) and the maximum allowed
mmap size limit.
The default setting can be overridden by each database connection using
either the PRAGMA mmap_size command, or by using the
SQLITE_FCNTL_MMAP_SIZE file control.
The maximum allowed mmap size will be silently truncated if necessary
so that it does not exceed the compile-time maximum mmap size set by
the SQLITE_MAX_MMAP_SIZE compile-time option.
If either argument to this option is negative, then that argument is
changed to its compile-time default.
.It SQLITE_CONFIG_WIN32_HEAPSIZE The SQLITE_CONFIG_WIN32_HEAPSIZE option
is only available if SQLite is compiled for Windows with the SQLITE_WIN32_MALLOC
pre-processor macro defined.
SQLITE_CONFIG_WIN32_HEAPSIZE takes a 32-bit unsigned integer value
that specifies the maximum size of the created heap.
.It SQLITE_CONFIG_PCACHE_HDRSZ The SQLITE_CONFIG_PCACHE_HDRSZ option takes
a single parameter which is a pointer to an integer and writes into
that integer the number of extra bytes per page required for each page
in SQLITE_CONFIG_PAGECACHE.
The amount of extra space required can change depending on the compiler,
target platform, and SQLite version.
.It SQLITE_CONFIG_PMASZ The SQLITE_CONFIG_PMASZ option takes a single
parameter which is an unsigned integer and sets the "Minimum PMA Size"
for the multithreaded sorter to that integer.
The default minimum PMA Size is set by the SQLITE_SORTER_PMASZ
compile-time option.
New threads are launched to help with sort operations when multithreaded
sorting is enabled (using the PRAGMA threads command)
and the amount of content to be sorted exceeds the page size times
the minimum of the PRAGMA cache_size setting and this
value.
.It SQLITE_CONFIG_STMTJRNL_SPILL The SQLITE_CONFIG_STMTJRNL_SPILL option
takes a single parameter which becomes the statement journal
spill-to-disk threshold.
Statement journals are held in memory until their
size (in bytes) exceeds this threshold, at which point they are written
to disk.
Or if the threshold is -1, statement journals are always held exclusively
in memory.
Since many statement journals never become large, setting the spill
threshold to a value such as 64KiB can greatly reduce the amount of
I/O required to support statement rollback.
The default value for this setting is controlled by the SQLITE_STMTJRNL_SPILL
compile-time option.
.El
.Pp
.Sh SEE ALSO
.Xr sqlite3 3 ,
.Xr sqlite3_stmt 3 ,
.Xr sqlite3_config 3 ,
.Xr sqlite3_db_config 3 ,
.Xr sqlite3_log 3 ,
.Xr sqlite3_malloc 3 ,
.Xr sqlite3_mem_methods 3 ,
.Xr sqlite3_memory_used 3 ,
.Xr sqlite3_mutex_methods 3 ,
.Xr sqlite3_open 3 ,
.Xr sqlite3_pcache_methods2 3 ,
.Xr sqlite3_mprintf 3 ,
.Xr sqlite3_soft_heap_limit64 3 ,
.Xr sqlite3_status 3 ,
.Xr SQLITE_CONFIG_SINGLETHREAD 3 ,
.Xr SQLITE_DBCONFIG_MAINDBNAME 3 ,
.Xr SQLITE_OK 3 ,
.Xr SQLITE_FCNTL_LOCKSTATE 3 ,
.Xr SQLITE_OPEN_READONLY 3