1117 lines
43 KiB
Tcl
1117 lines
43 KiB
Tcl
#
|
|
# Run this Tcl script to generate the sqlite.html file.
|
|
#
|
|
set rcsid {$Id: c_interface.tcl,v 1.43 2004/11/19 11:59:24 danielk1977 Exp $}
|
|
source common.tcl
|
|
header {The C language interface to the SQLite library}
|
|
puts {
|
|
<h2>The C language interface to the SQLite library</h2>
|
|
|
|
<p>The SQLite library is designed to be very easy to use from
|
|
a C or C++ program. This document gives an overview of the C/C++
|
|
programming interface.</p>
|
|
|
|
<h3>1.0 The Core API</h3>
|
|
|
|
<p>The interface to the SQLite library consists of three core functions,
|
|
one opaque data structure, and some constants used as return values.
|
|
The core interface is as follows:</p>
|
|
|
|
<blockquote><pre>
|
|
typedef struct sqlite sqlite;
|
|
#define SQLITE_OK 0 /* Successful result */
|
|
|
|
sqlite *sqlite_open(const char *dbname, int mode, char **errmsg);
|
|
|
|
void sqlite_close(sqlite *db);
|
|
|
|
int sqlite_exec(
|
|
sqlite *db,
|
|
char *sql,
|
|
int (*xCallback)(void*,int,char**,char**),
|
|
void *pArg,
|
|
char **errmsg
|
|
);
|
|
</pre></blockquote>
|
|
|
|
<p>
|
|
The above is all you really need to know in order to use SQLite
|
|
in your C or C++ programs. There are other interface functions
|
|
available (and described below) but we will begin by describing
|
|
the core functions shown above.
|
|
</p>
|
|
|
|
<a name="sqlite_open">
|
|
<h4>1.1 Opening a database</h4>
|
|
|
|
<p>Use the <b>sqlite_open</b> function to open an existing SQLite
|
|
database or to create a new SQLite database. The first argument
|
|
is the database name. The second argument is intended to signal
|
|
whether the database is going to be used for reading and writing
|
|
or just for reading. But in the current implementation, the
|
|
second argument to <b>sqlite_open</b> is ignored.
|
|
The third argument is a pointer to a string pointer.
|
|
If the third argument is not NULL and an error occurs
|
|
while trying to open the database, then an error message will be
|
|
written to memory obtained from malloc() and *errmsg will be made
|
|
to point to this error message. The calling function is responsible
|
|
for freeing the memory when it has finished with it.</p>
|
|
|
|
<p>The name of an SQLite database is the name of a file that will
|
|
contain the database. If the file does not exist, SQLite attempts
|
|
to create and initialize it. If the file is read-only (due to
|
|
permission bits or because it is located on read-only media like
|
|
a CD-ROM) then SQLite opens the database for reading only. The
|
|
entire SQL database is stored in a single file on the disk. But
|
|
additional temporary files may be created during the execution of
|
|
an SQL command in order to store the database rollback journal or
|
|
temporary and intermediate results of a query.</p>
|
|
|
|
<p>The return value of the <b>sqlite_open</b> function is a
|
|
pointer to an opaque <b>sqlite</b> structure. This pointer will
|
|
be the first argument to all subsequent SQLite function calls that
|
|
deal with the same database. NULL is returned if the open fails
|
|
for any reason.</p>
|
|
|
|
<a name="sqlite_close">
|
|
<h4>1.2 Closing the database</h4>
|
|
|
|
<p>To close an SQLite database, call the <b>sqlite_close</b>
|
|
function passing it the sqlite structure pointer that was obtained
|
|
from a prior call to <b>sqlite_open</b>.
|
|
If a transaction is active when the database is closed, the transaction
|
|
is rolled back.</p>
|
|
|
|
<a name="sqlite_exec">
|
|
<h4>1.3 Executing SQL statements</h4>
|
|
|
|
<p>The <b>sqlite_exec</b> function is used to process SQL statements
|
|
and queries. This function requires 5 parameters as follows:</p>
|
|
|
|
<ol>
|
|
<li><p>A pointer to the sqlite structure obtained from a prior call
|
|
to <b>sqlite_open</b>.</p></li>
|
|
<li><p>A null-terminated string containing the text of one or more
|
|
SQL statements and/or queries to be processed.</p></li>
|
|
<li><p>A pointer to a callback function which is invoked once for each
|
|
row in the result of a query. This argument may be NULL, in which
|
|
case no callbacks will ever be invoked.</p></li>
|
|
<li><p>A pointer that is forwarded to become the first argument
|
|
to the callback function.</p></li>
|
|
<li><p>A pointer to an error string. Error messages are written to space
|
|
obtained from malloc() and the error string is made to point to
|
|
the malloced space. The calling function is responsible for freeing
|
|
this space when it has finished with it.
|
|
This argument may be NULL, in which case error messages are not
|
|
reported back to the calling function.</p></li>
|
|
</ol>
|
|
|
|
<p>
|
|
The callback function is used to receive the results of a query. A
|
|
prototype for the callback function is as follows:</p>
|
|
|
|
<blockquote><pre>
|
|
int Callback(void *pArg, int argc, char **argv, char **columnNames){
|
|
return 0;
|
|
}
|
|
</pre></blockquote>
|
|
|
|
<a name="callback_row_data">
|
|
<p>The first argument to the callback is just a copy of the fourth argument
|
|
to <b>sqlite_exec</b> This parameter can be used to pass arbitrary
|
|
information through to the callback function from client code.
|
|
The second argument is the number of columns in the query result.
|
|
The third argument is an array of pointers to strings where each string
|
|
is a single column of the result for that record. Note that the
|
|
callback function reports a NULL value in the database as a NULL pointer,
|
|
which is very different from an empty string. If the i-th parameter
|
|
is an empty string, we will get:</p>
|
|
<blockquote><pre>
|
|
argv[i][0] == 0
|
|
</pre></blockquote>
|
|
<p>But if the i-th parameter is NULL we will get:</p>
|
|
<blockquote><pre>
|
|
argv[i] == 0
|
|
</pre></blockquote>
|
|
|
|
<p>The names of the columns are contained in first <i>argc</i>
|
|
entries of the fourth argument.
|
|
If the <a href="pragma.html#pragma_show_datatypes">SHOW_DATATYPES</a> pragma
|
|
is on (it is off by default) then
|
|
the second <i>argc</i> entries in the 4th argument are the datatypes
|
|
for the corresponding columns.
|
|
</p>
|
|
|
|
<p>If the <a href="pragma.html#pragma_empty_result_callbacks">
|
|
EMPTY_RESULT_CALLBACKS</a> pragma is set to ON and the result of
|
|
a query is an empty set, then the callback is invoked once with the
|
|
third parameter (argv) set to 0. In other words
|
|
<blockquote><pre>
|
|
argv == 0
|
|
</pre></blockquote>
|
|
The second parameter (argc)
|
|
and the fourth parameter (columnNames) are still valid
|
|
and can be used to determine the number and names of the result
|
|
columns if there had been a result.
|
|
The default behavior is not to invoke the callback at all if the
|
|
result set is empty.</p>
|
|
|
|
<a name="callback_returns_nonzero">
|
|
<p>The callback function should normally return 0. If the callback
|
|
function returns non-zero, the query is immediately aborted and
|
|
<b>sqlite_exec</b> will return SQLITE_ABORT.</p>
|
|
|
|
<h4>1.4 Error Codes</h4>
|
|
|
|
<p>
|
|
The <b>sqlite_exec</b> function normally returns SQLITE_OK. But
|
|
if something goes wrong it can return a different value to indicate
|
|
the type of error. Here is a complete list of the return codes:
|
|
</p>
|
|
|
|
<blockquote><pre>
|
|
#define SQLITE_OK 0 /* Successful result */
|
|
#define SQLITE_ERROR 1 /* SQL error or missing database */
|
|
#define SQLITE_INTERNAL 2 /* An internal logic error in SQLite */
|
|
#define SQLITE_PERM 3 /* Access permission denied */
|
|
#define SQLITE_ABORT 4 /* Callback routine requested an abort */
|
|
#define SQLITE_BUSY 5 /* The database file is locked */
|
|
#define SQLITE_LOCKED 6 /* A table in the database is locked */
|
|
#define SQLITE_NOMEM 7 /* A malloc() failed */
|
|
#define SQLITE_READONLY 8 /* Attempt to write a readonly database */
|
|
#define SQLITE_INTERRUPT 9 /* Operation terminated by sqlite_interrupt() */
|
|
#define SQLITE_IOERR 10 /* Some kind of disk I/O error occurred */
|
|
#define SQLITE_CORRUPT 11 /* The database disk image is malformed */
|
|
#define SQLITE_NOTFOUND 12 /* (Internal Only) Table or record not found */
|
|
#define SQLITE_FULL 13 /* Insertion failed because database is full */
|
|
#define SQLITE_CANTOPEN 14 /* Unable to open the database file */
|
|
#define SQLITE_PROTOCOL 15 /* Database lock protocol error */
|
|
#define SQLITE_EMPTY 16 /* (Internal Only) Database table is empty */
|
|
#define SQLITE_SCHEMA 17 /* The database schema changed */
|
|
#define SQLITE_TOOBIG 18 /* Too much data for one row of a table */
|
|
#define SQLITE_CONSTRAINT 19 /* Abort due to contraint violation */
|
|
#define SQLITE_MISMATCH 20 /* Data type mismatch */
|
|
#define SQLITE_MISUSE 21 /* Library used incorrectly */
|
|
#define SQLITE_NOLFS 22 /* Uses OS features not supported on host */
|
|
#define SQLITE_AUTH 23 /* Authorization denied */
|
|
#define SQLITE_ROW 100 /* sqlite_step() has another row ready */
|
|
#define SQLITE_DONE 101 /* sqlite_step() has finished executing */
|
|
</pre></blockquote>
|
|
|
|
<p>
|
|
The meanings of these various return values are as follows:
|
|
</p>
|
|
|
|
<blockquote>
|
|
<dl>
|
|
<dt>SQLITE_OK</dt>
|
|
<dd><p>This value is returned if everything worked and there were no errors.
|
|
</p></dd>
|
|
<dt>SQLITE_INTERNAL</dt>
|
|
<dd><p>This value indicates that an internal consistency check within
|
|
the SQLite library failed. This can only happen if there is a bug in
|
|
the SQLite library. If you ever get an SQLITE_INTERNAL reply from
|
|
an <b>sqlite_exec</b> call, please report the problem on the SQLite
|
|
mailing list.
|
|
</p></dd>
|
|
<dt>SQLITE_ERROR</dt>
|
|
<dd><p>This return value indicates that there was an error in the SQL
|
|
that was passed into the <b>sqlite_exec</b>.
|
|
</p></dd>
|
|
<dt>SQLITE_PERM</dt>
|
|
<dd><p>This return value says that the access permissions on the database
|
|
file are such that the file cannot be opened.
|
|
</p></dd>
|
|
<dt>SQLITE_ABORT</dt>
|
|
<dd><p>This value is returned if the callback function returns non-zero.
|
|
</p></dd>
|
|
<dt>SQLITE_BUSY</dt>
|
|
<dd><p>This return code indicates that another program or thread has
|
|
the database locked. SQLite allows two or more threads to read the
|
|
database at the same time, but only one thread can have the database
|
|
open for writing at the same time. Locking in SQLite is on the
|
|
entire database.</p>
|
|
</p></dd>
|
|
<dt>SQLITE_LOCKED</dt>
|
|
<dd><p>This return code is similar to SQLITE_BUSY in that it indicates
|
|
that the database is locked. But the source of the lock is a recursive
|
|
call to <b>sqlite_exec</b>. This return can only occur if you attempt
|
|
to invoke sqlite_exec from within a callback routine of a query
|
|
from a prior invocation of sqlite_exec. Recursive calls to
|
|
sqlite_exec are allowed as long as they do
|
|
not attempt to write the same table.
|
|
</p></dd>
|
|
<dt>SQLITE_NOMEM</dt>
|
|
<dd><p>This value is returned if a call to <b>malloc</b> fails.
|
|
</p></dd>
|
|
<dt>SQLITE_READONLY</dt>
|
|
<dd><p>This return code indicates that an attempt was made to write to
|
|
a database file that is opened for reading only.
|
|
</p></dd>
|
|
<dt>SQLITE_INTERRUPT</dt>
|
|
<dd><p>This value is returned if a call to <b>sqlite_interrupt</b>
|
|
interrupts a database operation in progress.
|
|
</p></dd>
|
|
<dt>SQLITE_IOERR</dt>
|
|
<dd><p>This value is returned if the operating system informs SQLite
|
|
that it is unable to perform some disk I/O operation. This could mean
|
|
that there is no more space left on the disk.
|
|
</p></dd>
|
|
<dt>SQLITE_CORRUPT</dt>
|
|
<dd><p>This value is returned if SQLite detects that the database it is
|
|
working on has become corrupted. Corruption might occur due to a rogue
|
|
process writing to the database file or it might happen due to an
|
|
perviously undetected logic error in of SQLite. This value is also
|
|
returned if a disk I/O error occurs in such a way that SQLite is forced
|
|
to leave the database file in a corrupted state. The latter should only
|
|
happen due to a hardware or operating system malfunction.
|
|
</p></dd>
|
|
<dt>SQLITE_FULL</dt>
|
|
<dd><p>This value is returned if an insertion failed because there is
|
|
no space left on the disk, or the database is too big to hold any
|
|
more information. The latter case should only occur for databases
|
|
that are larger than 2GB in size.
|
|
</p></dd>
|
|
<dt>SQLITE_CANTOPEN</dt>
|
|
<dd><p>This value is returned if the database file could not be opened
|
|
for some reason.
|
|
</p></dd>
|
|
<dt>SQLITE_PROTOCOL</dt>
|
|
<dd><p>This value is returned if some other process is messing with
|
|
file locks and has violated the file locking protocol that SQLite uses
|
|
on its rollback journal files.
|
|
</p></dd>
|
|
<dt>SQLITE_SCHEMA</dt>
|
|
<dd><p>When the database first opened, SQLite reads the database schema
|
|
into memory and uses that schema to parse new SQL statements. If another
|
|
process changes the schema, the command currently being processed will
|
|
abort because the virtual machine code generated assumed the old
|
|
schema. This is the return code for such cases. Retrying the
|
|
command usually will clear the problem.
|
|
</p></dd>
|
|
<dt>SQLITE_TOOBIG</dt>
|
|
<dd><p>SQLite will not store more than about 1 megabyte of data in a single
|
|
row of a single table. If you attempt to store more than 1 megabyte
|
|
in a single row, this is the return code you get.
|
|
</p></dd>
|
|
<dt>SQLITE_CONSTRAINT</dt>
|
|
<dd><p>This constant is returned if the SQL statement would have violated
|
|
a database constraint.
|
|
</p></dd>
|
|
<dt>SQLITE_MISMATCH</dt>
|
|
<dd><p>This error occurs when there is an attempt to insert non-integer
|
|
data into a column labeled INTEGER PRIMARY KEY. For most columns, SQLite
|
|
ignores the data type and allows any kind of data to be stored. But
|
|
an INTEGER PRIMARY KEY column is only allowed to store integer data.
|
|
</p></dd>
|
|
<dt>SQLITE_MISUSE</dt>
|
|
<dd><p>This error might occur if one or more of the SQLite API routines
|
|
is used incorrectly. Examples of incorrect usage include calling
|
|
<b>sqlite_exec</b> after the database has been closed using
|
|
<b>sqlite_close</b> or
|
|
calling <b>sqlite_exec</b> with the same
|
|
database pointer simultaneously from two separate threads.
|
|
</p></dd>
|
|
<dt>SQLITE_NOLFS</dt>
|
|
<dd><p>This error means that you have attempts to create or access a file
|
|
database file that is larger that 2GB on a legacy Unix machine that
|
|
lacks large file support.
|
|
</p></dd>
|
|
<dt>SQLITE_AUTH</dt>
|
|
<dd><p>This error indicates that the authorizer callback
|
|
has disallowed the SQL you are attempting to execute.
|
|
</p></dd>
|
|
<dt>SQLITE_ROW</dt>
|
|
<dd><p>This is one of the return codes from the
|
|
<b>sqlite_step</b> routine which is part of the non-callback API.
|
|
It indicates that another row of result data is available.
|
|
</p></dd>
|
|
<dt>SQLITE_DONE</dt>
|
|
<dd><p>This is one of the return codes from the
|
|
<b>sqlite_step</b> routine which is part of the non-callback API.
|
|
It indicates that the SQL statement has been completely executed and
|
|
the <b>sqlite_finalize</b> routine is ready to be called.
|
|
</p></dd>
|
|
</dl>
|
|
</blockquote>
|
|
|
|
<h3>2.0 Accessing Data Without Using A Callback Function</h3>
|
|
|
|
<p>
|
|
The <b>sqlite_exec</b> routine described above used to be the only
|
|
way to retrieve data from an SQLite database. But many programmers found
|
|
it inconvenient to use a callback function to obtain results. So beginning
|
|
with SQLite version 2.7.7, a second access interface is available that
|
|
does not use callbacks.
|
|
</p>
|
|
|
|
<p>
|
|
The new interface uses three separate functions to replace the single
|
|
<b>sqlite_exec</b> function.
|
|
</p>
|
|
|
|
<blockquote><pre>
|
|
typedef struct sqlite_vm sqlite_vm;
|
|
|
|
int sqlite_compile(
|
|
sqlite *db, /* The open database */
|
|
const char *zSql, /* SQL statement to be compiled */
|
|
const char **pzTail, /* OUT: uncompiled tail of zSql */
|
|
sqlite_vm **ppVm, /* OUT: the virtual machine to execute zSql */
|
|
char **pzErrmsg /* OUT: Error message. */
|
|
);
|
|
|
|
int sqlite_step(
|
|
sqlite_vm *pVm, /* The virtual machine to execute */
|
|
int *pN, /* OUT: Number of columns in result */
|
|
const char ***pazValue, /* OUT: Column data */
|
|
const char ***pazColName /* OUT: Column names and datatypes */
|
|
);
|
|
|
|
int sqlite_finalize(
|
|
sqlite_vm *pVm, /* The virtual machine to be finalized */
|
|
char **pzErrMsg /* OUT: Error message */
|
|
);
|
|
</pre></blockquote>
|
|
|
|
<p>
|
|
The strategy is to compile a single SQL statement using
|
|
<b>sqlite_compile</b> then invoke <b>sqlite_step</b> multiple times,
|
|
once for each row of output, and finally call <b>sqlite_finalize</b>
|
|
to clean up after the SQL has finished execution.
|
|
</p>
|
|
|
|
<h4>2.1 Compiling An SQL Statement Into A Virtual Machine</h4>
|
|
|
|
<p>
|
|
The <b>sqlite_compile</b> "compiles" a single SQL statement (specified
|
|
by the second parameter) and generates a virtual machine that is able
|
|
to execute that statement.
|
|
As with must interface routines, the first parameter must be a pointer
|
|
to an sqlite structure that was obtained from a prior call to
|
|
<b>sqlite_open</b>.
|
|
|
|
<p>
|
|
A pointer to the virtual machine is stored in a pointer which is passed
|
|
in as the 4th parameter.
|
|
Space to hold the virtual machine is dynamically allocated. To avoid
|
|
a memory leak, the calling function must invoke
|
|
<b>sqlite_finalize</b> on the virtual machine after it has finished
|
|
with it.
|
|
The 4th parameter may be set to NULL if an error is encountered during
|
|
compilation.
|
|
</p>
|
|
|
|
<p>
|
|
If any errors are encountered during compilation, an error message is
|
|
written into memory obtained from <b>malloc</b> and the 5th parameter
|
|
is made to point to that memory. If the 5th parameter is NULL, then
|
|
no error message is generated. If the 5th parameter is not NULL, then
|
|
the calling function should dispose of the memory containing the error
|
|
message by calling <b>sqlite_freemem</b>.
|
|
</p>
|
|
|
|
<p>
|
|
If the 2nd parameter actually contains two or more statements of SQL,
|
|
only the first statement is compiled. (This is different from the
|
|
behavior of <b>sqlite_exec</b> which executes all SQL statements
|
|
in its input string.) The 3rd parameter to <b>sqlite_compile</b>
|
|
is made to point to the first character beyond the end of the first
|
|
statement of SQL in the input. If the 2nd parameter contains only
|
|
a single SQL statement, then the 3rd parameter will be made to point
|
|
to the '\000' terminator at the end of the 2nd parameter.
|
|
</p>
|
|
|
|
<p>
|
|
On success, <b>sqlite_compile</b> returns SQLITE_OK.
|
|
Otherwise and error code is returned.
|
|
</p>
|
|
|
|
<h4>2.2 Step-By-Step Execution Of An SQL Statement</h4>
|
|
|
|
<p>
|
|
After a virtual machine has been generated using <b>sqlite_compile</b>
|
|
it is executed by one or more calls to <b>sqlite_step</b>. Each
|
|
invocation of <b>sqlite_step</b>, except the last one,
|
|
returns a single row of the result.
|
|
The number of columns in the result is stored in the integer that
|
|
the 2nd parameter points to.
|
|
The pointer specified by the 3rd parameter is made to point
|
|
to an array of pointers to column values.
|
|
The pointer in the 4th parameter is made to point to an array
|
|
of pointers to column names and datatypes.
|
|
The 2nd through 4th parameters to <b>sqlite_step</b> convey the
|
|
same information as the 2nd through 4th parameters of the
|
|
<b>callback</b> routine when using
|
|
the <b>sqlite_exec</b> interface. Except, with <b>sqlite_step</b>
|
|
the column datatype information is always included in the in the
|
|
4th parameter regardless of whether or not the
|
|
<a href="pragma.html#pragma_show_datatypes">SHOW_DATATYPES</a> pragma
|
|
is on or off.
|
|
</p>
|
|
|
|
<p>
|
|
Each invocation of <b>sqlite_step</b> returns an integer code that
|
|
indicates what happened during that step. This code may be
|
|
SQLITE_BUSY, SQLITE_ROW, SQLITE_DONE, SQLITE_ERROR, or
|
|
SQLITE_MISUSE.
|
|
</p>
|
|
|
|
<p>
|
|
If the virtual machine is unable to open the database file because
|
|
it is locked by another thread or process, <b>sqlite_step</b>
|
|
will return SQLITE_BUSY. The calling function should do some other
|
|
activity, or sleep, for a short amount of time to give the lock a
|
|
chance to clear, then invoke <b>sqlite_step</b> again. This can
|
|
be repeated as many times as desired.
|
|
</p>
|
|
|
|
<p>
|
|
Whenever another row of result data is available,
|
|
<b>sqlite_step</b> will return SQLITE_ROW. The row data is
|
|
stored in an array of pointers to strings and the 2nd parameter
|
|
is made to point to this array.
|
|
</p>
|
|
|
|
<p>
|
|
When all processing is complete, <b>sqlite_step</b> will return
|
|
either SQLITE_DONE or SQLITE_ERROR. SQLITE_DONE indicates that the
|
|
statement completed successfully and SQLITE_ERROR indicates that there
|
|
was a run-time error. (The details of the error are obtained from
|
|
<b>sqlite_finalize</b>.) It is a misuse of the library to attempt
|
|
to call <b>sqlite_step</b> again after it has returned SQLITE_DONE
|
|
or SQLITE_ERROR.
|
|
</p>
|
|
|
|
<p>
|
|
When <b>sqlite_step</b> returns SQLITE_DONE or SQLITE_ERROR,
|
|
the *pN and *pazColName values are set to the number of columns
|
|
in the result set and to the names of the columns, just as they
|
|
are for an SQLITE_ROW return. This allows the calling code to
|
|
find the number of result columns and the column names and datatypes
|
|
even if the result set is empty. The *pazValue parameter is always
|
|
set to NULL when the return codes is SQLITE_DONE or SQLITE_ERROR.
|
|
If the SQL being executed is a statement that does not
|
|
return a result (such as an INSERT or an UPDATE) then *pN will
|
|
be set to zero and *pazColName will be set to NULL.
|
|
</p>
|
|
|
|
<p>
|
|
If you abuse the library by trying to call <b>sqlite_step</b>
|
|
inappropriately it will attempt return SQLITE_MISUSE.
|
|
This can happen if you call sqlite_step() on the same virtual machine
|
|
at the same
|
|
time from two or more threads or if you call sqlite_step()
|
|
again after it returned SQLITE_DONE or SQLITE_ERROR or if you
|
|
pass in an invalid virtual machine pointer to sqlite_step().
|
|
You should not depend on the SQLITE_MISUSE return code to indicate
|
|
an error. It is possible that a misuse of the interface will go
|
|
undetected and result in a program crash. The SQLITE_MISUSE is
|
|
intended as a debugging aid only - to help you detect incorrect
|
|
usage prior to a mishap. The misuse detection logic is not guaranteed
|
|
to work in every case.
|
|
</p>
|
|
|
|
<h4>2.3 Deleting A Virtual Machine</h4>
|
|
|
|
<p>
|
|
Every virtual machine that <b>sqlite_compile</b> creates should
|
|
eventually be handed to <b>sqlite_finalize</b>. The sqlite_finalize()
|
|
procedure deallocates the memory and other resources that the virtual
|
|
machine uses. Failure to call sqlite_finalize() will result in
|
|
resource leaks in your program.
|
|
</p>
|
|
|
|
<p>
|
|
The <b>sqlite_finalize</b> routine also returns the result code
|
|
that indicates success or failure of the SQL operation that the
|
|
virtual machine carried out.
|
|
The value returned by sqlite_finalize() will be the same as would
|
|
have been returned had the same SQL been executed by <b>sqlite_exec</b>.
|
|
The error message returned will also be the same.
|
|
</p>
|
|
|
|
<p>
|
|
It is acceptable to call <b>sqlite_finalize</b> on a virtual machine
|
|
before <b>sqlite_step</b> has returned SQLITE_DONE. Doing so has
|
|
the effect of interrupting the operation in progress. Partially completed
|
|
changes will be rolled back and the database will be restored to its
|
|
original state (unless an alternative recovery algorithm is selected using
|
|
an ON CONFLICT clause in the SQL being executed.) The effect is the
|
|
same as if a callback function of <b>sqlite_exec</b> had returned
|
|
non-zero.
|
|
</p>
|
|
|
|
<p>
|
|
It is also acceptable to call <b>sqlite_finalize</b> on a virtual machine
|
|
that has never been passed to <b>sqlite_step</b> even once.
|
|
</p>
|
|
|
|
<h3>3.0 The Extended API</h3>
|
|
|
|
<p>Only the three core routines described in section 1.0 are required to use
|
|
SQLite. But there are many other functions that provide
|
|
useful interfaces. These extended routines are as follows:
|
|
</p>
|
|
|
|
<blockquote><pre>
|
|
int sqlite_last_insert_rowid(sqlite*);
|
|
|
|
int sqlite_changes(sqlite*);
|
|
|
|
int sqlite_get_table(
|
|
sqlite*,
|
|
char *sql,
|
|
char ***result,
|
|
int *nrow,
|
|
int *ncolumn,
|
|
char **errmsg
|
|
);
|
|
|
|
void sqlite_free_table(char**);
|
|
|
|
void sqlite_interrupt(sqlite*);
|
|
|
|
int sqlite_complete(const char *sql);
|
|
|
|
void sqlite_busy_handler(sqlite*, int (*)(void*,const char*,int), void*);
|
|
|
|
void sqlite_busy_timeout(sqlite*, int ms);
|
|
|
|
const char sqlite_version[];
|
|
|
|
const char sqlite_encoding[];
|
|
|
|
int sqlite_exec_printf(
|
|
sqlite*,
|
|
char *sql,
|
|
int (*)(void*,int,char**,char**),
|
|
void*,
|
|
char **errmsg,
|
|
...
|
|
);
|
|
|
|
int sqlite_exec_vprintf(
|
|
sqlite*,
|
|
char *sql,
|
|
int (*)(void*,int,char**,char**),
|
|
void*,
|
|
char **errmsg,
|
|
va_list
|
|
);
|
|
|
|
int sqlite_get_table_printf(
|
|
sqlite*,
|
|
char *sql,
|
|
char ***result,
|
|
int *nrow,
|
|
int *ncolumn,
|
|
char **errmsg,
|
|
...
|
|
);
|
|
|
|
int sqlite_get_table_vprintf(
|
|
sqlite*,
|
|
char *sql,
|
|
char ***result,
|
|
int *nrow,
|
|
int *ncolumn,
|
|
char **errmsg,
|
|
va_list
|
|
);
|
|
|
|
char *sqlite_mprintf(const char *zFormat, ...);
|
|
|
|
char *sqlite_vmprintf(const char *zFormat, va_list);
|
|
|
|
void sqlite_freemem(char*);
|
|
|
|
void sqlite_progress_handler(sqlite*, int, int (*)(void*), void*);
|
|
|
|
</pre></blockquote>
|
|
|
|
<p>All of the above definitions are included in the "sqlite.h"
|
|
header file that comes in the source tree.</p>
|
|
|
|
<h4>3.1 The ROWID of the most recent insert</h4>
|
|
|
|
<p>Every row of an SQLite table has a unique integer key. If the
|
|
table has a column labeled INTEGER PRIMARY KEY, then that column
|
|
serves as the key. If there is no INTEGER PRIMARY KEY column then
|
|
the key is a unique integer. The key for a row can be accessed in
|
|
a SELECT statement or used in a WHERE or ORDER BY clause using any
|
|
of the names "ROWID", "OID", or "_ROWID_".</p>
|
|
|
|
<p>When you do an insert into a table that does not have an INTEGER PRIMARY
|
|
KEY column, or if the table does have an INTEGER PRIMARY KEY but the value
|
|
for that column is not specified in the VALUES clause of the insert, then
|
|
the key is automatically generated. You can find the value of the key
|
|
for the most recent INSERT statement using the
|
|
<b>sqlite_last_insert_rowid</b> API function.</p>
|
|
|
|
<h4>3.2 The number of rows that changed</h4>
|
|
|
|
<p>The <b>sqlite_changes</b> API function returns the number of rows
|
|
that have been inserted, deleted, or modified since the database was
|
|
last quiescent. A "quiescent" database is one in which there are
|
|
no outstanding calls to <b>sqlite_exec</b> and no VMs created by
|
|
<b>sqlite_compile</b> that have not been finalized by <b>sqlite_finalize</b>.
|
|
In common usage, <b>sqlite_changes</b> returns the number
|
|
of rows inserted, deleted, or modified by the most recent <b>sqlite_exec</b>
|
|
call or since the most recent <b>sqlite_compile</b>. But if you have
|
|
nested calls to <b>sqlite_exec</b> (that is, if the callback routine
|
|
of one <b>sqlite_exec</b> invokes another <b>sqlite_exec</b>) or if
|
|
you invoke <b>sqlite_compile</b> to create a new VM while there is
|
|
still another VM in existance, then
|
|
the meaning of the number returned by <b>sqlite_changes</b> is more
|
|
complex.
|
|
The number reported includes any changes
|
|
that were later undone by a ROLLBACK or ABORT. But rows that are
|
|
deleted because of a DROP TABLE are <em>not</em> counted.</p>
|
|
|
|
<p>SQLite implements the command "<b>DELETE FROM table</b>" (without
|
|
a WHERE clause) by dropping the table then recreating it.
|
|
This is much faster than deleting the elements of the table individually.
|
|
But it also means that the value returned from <b>sqlite_changes</b>
|
|
will be zero regardless of the number of elements that were originally
|
|
in the table. If an accurate count of the number of elements deleted
|
|
is necessary, use "<b>DELETE FROM table WHERE 1</b>" instead.</p>
|
|
|
|
<h4>3.3 Querying into memory obtained from malloc()</h4>
|
|
|
|
<p>The <b>sqlite_get_table</b> function is a wrapper around
|
|
<b>sqlite_exec</b> that collects all the information from successive
|
|
callbacks and writes it into memory obtained from malloc(). This
|
|
is a convenience function that allows the application to get the
|
|
entire result of a database query with a single function call.</p>
|
|
|
|
<p>The main result from <b>sqlite_get_table</b> is an array of pointers
|
|
to strings. There is one element in this array for each column of
|
|
each row in the result. NULL results are represented by a NULL
|
|
pointer. In addition to the regular data, there is an added row at the
|
|
beginning of the array that contains the name of each column of the
|
|
result.</p>
|
|
|
|
<p>As an example, consider the following query:</p>
|
|
|
|
<blockquote>
|
|
SELECT employee_name, login, host FROM users WHERE login LIKE 'd%';
|
|
</blockquote>
|
|
|
|
<p>This query will return the name, login and host computer name
|
|
for every employee whose login begins with the letter "d". If this
|
|
query is submitted to <b>sqlite_get_table</b> the result might
|
|
look like this:</p>
|
|
|
|
<blockquote>
|
|
nrow = 2<br>
|
|
ncolumn = 3<br>
|
|
result[0] = "employee_name"<br>
|
|
result[1] = "login"<br>
|
|
result[2] = "host"<br>
|
|
result[3] = "dummy"<br>
|
|
result[4] = "No such user"<br>
|
|
result[5] = 0<br>
|
|
result[6] = "D. Richard Hipp"<br>
|
|
result[7] = "drh"<br>
|
|
result[8] = "zadok"
|
|
</blockquote>
|
|
|
|
<p>Notice that the "host" value for the "dummy" record is NULL so
|
|
the result[] array contains a NULL pointer at that slot.</p>
|
|
|
|
<p>If the result set of a query is empty, then by default
|
|
<b>sqlite_get_table</b> will set nrow to 0 and leave its
|
|
result parameter is set to NULL. But if the EMPTY_RESULT_CALLBACKS
|
|
pragma is ON then the result parameter is initialized to the names
|
|
of the columns only. For example, consider this query which has
|
|
an empty result set:</p>
|
|
|
|
<blockquote>
|
|
SELECT employee_name, login, host FROM users WHERE employee_name IS NULL;
|
|
</blockquote>
|
|
|
|
<p>
|
|
The default behavior gives this results:
|
|
</p>
|
|
|
|
<blockquote>
|
|
nrow = 0<br>
|
|
ncolumn = 0<br>
|
|
result = 0<br>
|
|
</blockquote>
|
|
|
|
<p>
|
|
But if the EMPTY_RESULT_CALLBACKS pragma is ON, then the following
|
|
is returned:
|
|
</p>
|
|
|
|
<blockquote>
|
|
nrow = 0<br>
|
|
ncolumn = 3<br>
|
|
result[0] = "employee_name"<br>
|
|
result[1] = "login"<br>
|
|
result[2] = "host"<br>
|
|
</blockquote>
|
|
|
|
<p>Memory to hold the information returned by <b>sqlite_get_table</b>
|
|
is obtained from malloc(). But the calling function should not try
|
|
to free this information directly. Instead, pass the complete table
|
|
to <b>sqlite_free_table</b> when the table is no longer needed.
|
|
It is safe to call <b>sqlite_free_table</b> with a NULL pointer such
|
|
as would be returned if the result set is empty.</p>
|
|
|
|
<p>The <b>sqlite_get_table</b> routine returns the same integer
|
|
result code as <b>sqlite_exec</b>.</p>
|
|
|
|
<h4>3.4 Interrupting an SQLite operation</h4>
|
|
|
|
<p>The <b>sqlite_interrupt</b> function can be called from a
|
|
different thread or from a signal handler to cause the current database
|
|
operation to exit at its first opportunity. When this happens,
|
|
the <b>sqlite_exec</b> routine (or the equivalent) that started
|
|
the database operation will return SQLITE_INTERRUPT.</p>
|
|
|
|
<h4>3.5 Testing for a complete SQL statement</h4>
|
|
|
|
<p>The next interface routine to SQLite is a convenience function used
|
|
to test whether or not a string forms a complete SQL statement.
|
|
If the <b>sqlite_complete</b> function returns true when its input
|
|
is a string, then the argument forms a complete SQL statement.
|
|
There are no guarantees that the syntax of that statement is correct,
|
|
but we at least know the statement is complete. If <b>sqlite_complete</b>
|
|
returns false, then more text is required to complete the SQL statement.</p>
|
|
|
|
<p>For the purpose of the <b>sqlite_complete</b> function, an SQL
|
|
statement is complete if it ends in a semicolon.</p>
|
|
|
|
<p>The <b>sqlite</b> command-line utility uses the <b>sqlite_complete</b>
|
|
function to know when it needs to call <b>sqlite_exec</b>. After each
|
|
line of input is received, <b>sqlite</b> calls <b>sqlite_complete</b>
|
|
on all input in its buffer. If <b>sqlite_complete</b> returns true,
|
|
then <b>sqlite_exec</b> is called and the input buffer is reset. If
|
|
<b>sqlite_complete</b> returns false, then the prompt is changed to
|
|
the continuation prompt and another line of text is read and added to
|
|
the input buffer.</p>
|
|
|
|
<h4>3.6 Library version string</h4>
|
|
|
|
<p>The SQLite library exports the string constant named
|
|
<b>sqlite_version</b> which contains the version number of the
|
|
library. The header file contains a macro SQLITE_VERSION
|
|
with the same information. If desired, a program can compare
|
|
the SQLITE_VERSION macro against the <b>sqlite_version</b>
|
|
string constant to verify that the version number of the
|
|
header file and the library match.</p>
|
|
|
|
<h4>3.7 Library character encoding</h4>
|
|
|
|
<p>By default, SQLite assumes that all data uses a fixed-size
|
|
8-bit character (iso8859). But if you give the --enable-utf8 option
|
|
to the configure script, then the library assumes UTF-8 variable
|
|
sized characters. This makes a difference for the LIKE and GLOB
|
|
operators and the LENGTH() and SUBSTR() functions. The static
|
|
string <b>sqlite_encoding</b> will be set to either "UTF-8" or
|
|
"iso8859" to indicate how the library was compiled. In addition,
|
|
the <b>sqlite.h</b> header file will define one of the
|
|
macros <b>SQLITE_UTF8</b> or <b>SQLITE_ISO8859</b>, as appropriate.</p>
|
|
|
|
<p>Note that the character encoding mechanism used by SQLite cannot
|
|
be changed at run-time. This is a compile-time option only. The
|
|
<b>sqlite_encoding</b> character string just tells you how the library
|
|
was compiled.</p>
|
|
|
|
<h4>3.8 Changing the library's response to locked files</h4>
|
|
|
|
<p>The <b>sqlite_busy_handler</b> procedure can be used to register
|
|
a busy callback with an open SQLite database. The busy callback will
|
|
be invoked whenever SQLite tries to access a database that is locked.
|
|
The callback will typically do some other useful work, or perhaps sleep,
|
|
in order to give the lock a chance to clear. If the callback returns
|
|
non-zero, then SQLite tries again to access the database and the cycle
|
|
repeats. If the callback returns zero, then SQLite aborts the current
|
|
operation and returns SQLITE_BUSY.</p>
|
|
|
|
<p>The arguments to <b>sqlite_busy_handler</b> are the opaque
|
|
structure returned from <b>sqlite_open</b>, a pointer to the busy
|
|
callback function, and a generic pointer that will be passed as
|
|
the first argument to the busy callback. When SQLite invokes the
|
|
busy callback, it sends it three arguments: the generic pointer
|
|
that was passed in as the third argument to <b>sqlite_busy_handler</b>,
|
|
the name of the database table or index that the library is trying
|
|
to access, and the number of times that the library has attempted to
|
|
access the database table or index.</p>
|
|
|
|
<p>For the common case where we want the busy callback to sleep,
|
|
the SQLite library provides a convenience routine <b>sqlite_busy_timeout</b>.
|
|
The first argument to <b>sqlite_busy_timeout</b> is a pointer to
|
|
an open SQLite database and the second argument is a number of milliseconds.
|
|
After <b>sqlite_busy_timeout</b> has been executed, the SQLite library
|
|
will wait for the lock to clear for at least the number of milliseconds
|
|
specified before it returns SQLITE_BUSY. Specifying zero milliseconds for
|
|
the timeout restores the default behavior.</p>
|
|
|
|
<h4>3.9 Using the <tt>_printf()</tt> wrapper functions</h4>
|
|
|
|
<p>The four utility functions</p>
|
|
|
|
<p>
|
|
<ul>
|
|
<li><b>sqlite_exec_printf()</b></li>
|
|
<li><b>sqlite_exec_vprintf()</b></li>
|
|
<li><b>sqlite_get_table_printf()</b></li>
|
|
<li><b>sqlite_get_table_vprintf()</b></li>
|
|
</ul>
|
|
</p>
|
|
|
|
<p>implement the same query functionality as <b>sqlite_exec</b>
|
|
and <b>sqlite_get_table</b>. But instead of taking a complete
|
|
SQL statement as their second argument, the four <b>_printf</b>
|
|
routines take a printf-style format string. The SQL statement to
|
|
be executed is generated from this format string and from whatever
|
|
additional arguments are attached to the end of the function call.</p>
|
|
|
|
<p>There are two advantages to using the SQLite printf
|
|
functions instead of <b>sprintf</b>. First of all, with the
|
|
SQLite printf routines, there is never a danger of overflowing a
|
|
static buffer as there is with <b>sprintf</b>. The SQLite
|
|
printf routines automatically allocate (and later frees)
|
|
as much memory as is
|
|
necessary to hold the SQL statements generated.</p>
|
|
|
|
<p>The second advantage the SQLite printf routines have over
|
|
<b>sprintf</b> are two new formatting options specifically designed
|
|
to support string literals in SQL. Within the format string,
|
|
the %q formatting option works very much like %s in that it
|
|
reads a null-terminated string from the argument list and inserts
|
|
it into the result. But %q translates the inserted string by
|
|
making two copies of every single-quote (') character in the
|
|
substituted string. This has the effect of escaping the end-of-string
|
|
meaning of single-quote within a string literal. The %Q formatting
|
|
option works similar; it translates the single-quotes like %q and
|
|
additionally encloses the resulting string in single-quotes.
|
|
If the argument for the %Q formatting options is a NULL pointer,
|
|
the resulting string is NULL without single quotes.
|
|
</p>
|
|
|
|
<p>Consider an example. Suppose you are trying to insert a string
|
|
value into a database table where the string value was obtained from
|
|
user input. Suppose the string to be inserted is stored in a variable
|
|
named zString. The code to do the insertion might look like this:</p>
|
|
|
|
<blockquote><pre>
|
|
sqlite_exec_printf(db,
|
|
"INSERT INTO table1 VALUES('%s')",
|
|
0, 0, 0, zString);
|
|
</pre></blockquote>
|
|
|
|
<p>If the zString variable holds text like "Hello", then this statement
|
|
will work just fine. But suppose the user enters a string like
|
|
"Hi y'all!". The SQL statement generated reads as follows:
|
|
|
|
<blockquote><pre>
|
|
INSERT INTO table1 VALUES('Hi y'all')
|
|
</pre></blockquote>
|
|
|
|
<p>This is not valid SQL because of the apostrophy in the word "y'all".
|
|
But if the %q formatting option is used instead of %s, like this:</p>
|
|
|
|
<blockquote><pre>
|
|
sqlite_exec_printf(db,
|
|
"INSERT INTO table1 VALUES('%q')",
|
|
0, 0, 0, zString);
|
|
</pre></blockquote>
|
|
|
|
<p>Then the generated SQL will look like the following:</p>
|
|
|
|
<blockquote><pre>
|
|
INSERT INTO table1 VALUES('Hi y''all')
|
|
</pre></blockquote>
|
|
|
|
<p>Here the apostrophy has been escaped and the SQL statement is well-formed.
|
|
When generating SQL on-the-fly from data that might contain a
|
|
single-quote character ('), it is always a good idea to use the
|
|
SQLite printf routines and the %q formatting option instead of <b>sprintf</b>.
|
|
</p>
|
|
|
|
<p>If the %Q formatting option is used instead of %q, like this:</p>
|
|
|
|
<blockquote><pre>
|
|
sqlite_exec_printf(db,
|
|
"INSERT INTO table1 VALUES(%Q)",
|
|
0, 0, 0, zString);
|
|
</pre></blockquote>
|
|
|
|
<p>Then the generated SQL will look like the following:</p>
|
|
|
|
<blockquote><pre>
|
|
INSERT INTO table1 VALUES('Hi y''all')
|
|
</pre></blockquote>
|
|
|
|
<p>If the value of the zString variable is NULL, the generated SQL
|
|
will look like the following:</p>
|
|
|
|
<blockquote><pre>
|
|
INSERT INTO table1 VALUES(NULL)
|
|
</pre></blockquote>
|
|
|
|
<p>All of the _printf() routines above are built around the following
|
|
two functions:</p>
|
|
|
|
<blockquote><pre>
|
|
char *sqlite_mprintf(const char *zFormat, ...);
|
|
char *sqlite_vmprintf(const char *zFormat, va_list);
|
|
</pre></blockquote>
|
|
|
|
<p>The <b>sqlite_mprintf()</b> routine works like the the standard library
|
|
<b>sprintf()</b> except that it writes its results into memory obtained
|
|
from malloc() and returns a pointer to the malloced buffer.
|
|
<b>sqlite_mprintf()</b> also understands the %q and %Q extensions described
|
|
above. The <b>sqlite_vmprintf()</b> is a varargs version of the same
|
|
routine. The string pointer that these routines return should be freed
|
|
by passing it to <b>sqlite_freemem()</b>.
|
|
</p>
|
|
|
|
<h4>3.10 Performing background jobs during large queries</h3>
|
|
|
|
<p>The <b>sqlite_progress_handler()</b> routine can be used to register a
|
|
callback routine with an SQLite database to be invoked periodically during long
|
|
running calls to <b>sqlite_exec()</b>, <b>sqlite_step()</b> and the various
|
|
wrapper functions.
|
|
</p>
|
|
|
|
<p>The callback is invoked every N virtual machine operations, where N is
|
|
supplied as the second argument to <b>sqlite_progress_handler()</b>. The third
|
|
and fourth arguments to <b>sqlite_progress_handler()</b> are a pointer to the
|
|
routine to be invoked and a void pointer to be passed as the first argument to
|
|
it.
|
|
</p>
|
|
|
|
<p>The time taken to execute each virtual machine operation can vary based on
|
|
many factors. A typical value for a 1 GHz PC is between half and three million
|
|
per second but may be much higher or lower, depending on the query. As such it
|
|
is difficult to schedule background operations based on virtual machine
|
|
operations. Instead, it is recommended that a callback be scheduled relatively
|
|
frequently (say every 1000 instructions) and external timer routines used to
|
|
determine whether or not background jobs need to be run.
|
|
</p>
|
|
|
|
<a name="cfunc">
|
|
<h3>4.0 Adding New SQL Functions</h3>
|
|
|
|
<p>Beginning with version 2.4.0, SQLite allows the SQL language to be
|
|
extended with new functions implemented as C code. The following interface
|
|
is used:
|
|
</p>
|
|
|
|
<blockquote><pre>
|
|
typedef struct sqlite_func sqlite_func;
|
|
|
|
int sqlite_create_function(
|
|
sqlite *db,
|
|
const char *zName,
|
|
int nArg,
|
|
void (*xFunc)(sqlite_func*,int,const char**),
|
|
void *pUserData
|
|
);
|
|
int sqlite_create_aggregate(
|
|
sqlite *db,
|
|
const char *zName,
|
|
int nArg,
|
|
void (*xStep)(sqlite_func*,int,const char**),
|
|
void (*xFinalize)(sqlite_func*),
|
|
void *pUserData
|
|
);
|
|
|
|
char *sqlite_set_result_string(sqlite_func*,const char*,int);
|
|
void sqlite_set_result_int(sqlite_func*,int);
|
|
void sqlite_set_result_double(sqlite_func*,double);
|
|
void sqlite_set_result_error(sqlite_func*,const char*,int);
|
|
|
|
void *sqlite_user_data(sqlite_func*);
|
|
void *sqlite_aggregate_context(sqlite_func*, int nBytes);
|
|
int sqlite_aggregate_count(sqlite_func*);
|
|
</pre></blockquote>
|
|
|
|
<p>
|
|
The <b>sqlite_create_function()</b> interface is used to create
|
|
regular functions and <b>sqlite_create_aggregate()</b> is used to
|
|
create new aggregate functions. In both cases, the <b>db</b>
|
|
parameter is an open SQLite database on which the functions should
|
|
be registered, <b>zName</b> is the name of the new function,
|
|
<b>nArg</b> is the number of arguments, and <b>pUserData</b> is
|
|
a pointer which is passed through unchanged to the C implementation
|
|
of the function. Both routines return 0 on success and non-zero
|
|
if there are any errors.
|
|
</p>
|
|
|
|
<p>
|
|
The length of a function name may not exceed 255 characters.
|
|
Any attempt to create a function whose name exceeds 255 characters
|
|
in length will result in an error.
|
|
</p>
|
|
|
|
<p>
|
|
For regular functions, the <b>xFunc</b> callback is invoked once
|
|
for each function call. The implementation of xFunc should call
|
|
one of the <b>sqlite_set_result_...</b> interfaces to return its
|
|
result. The <b>sqlite_user_data()</b> routine can be used to
|
|
retrieve the <b>pUserData</b> pointer that was passed in when the
|
|
function was registered.
|
|
</p>
|
|
|
|
<p>
|
|
For aggregate functions, the <b>xStep</b> callback is invoked once
|
|
for each row in the result and then <b>xFinalize</b> is invoked at the
|
|
end to compute a final answer. The xStep routine can use the
|
|
<b>sqlite_aggregate_context()</b> interface to allocate memory that
|
|
will be unique to that particular instance of the SQL function.
|
|
This memory will be automatically deleted after xFinalize is called.
|
|
The <b>sqlite_aggregate_count()</b> routine can be used to find out
|
|
how many rows of data were passed to the aggregate. The xFinalize
|
|
callback should invoke one of the <b>sqlite_set_result_...</b>
|
|
interfaces to set the final result of the aggregate.
|
|
</p>
|
|
|
|
<p>
|
|
SQLite now implements all of its built-in functions using this
|
|
interface. For additional information and examples on how to create
|
|
new SQL functions, review the SQLite source code in the file
|
|
<b>func.c</b>.
|
|
</p>
|
|
|
|
<h3>5.0 Multi-Threading And SQLite</h3>
|
|
|
|
<p>
|
|
If SQLite is compiled with the THREADSAFE preprocessor macro set to 1,
|
|
then it is safe to use SQLite from two or more threads of the same process
|
|
at the same time. But each thread should have its own <b>sqlite*</b>
|
|
pointer returned from <b>sqlite_open</b>. It is never safe for two
|
|
or more threads to access the same <b>sqlite*</b> pointer at the same time.
|
|
</p>
|
|
|
|
<p>
|
|
In precompiled SQLite libraries available on the website, the Unix
|
|
versions are compiled with THREADSAFE turned off but the windows
|
|
versions are compiled with THREADSAFE turned on. If you need something
|
|
different that this you will have to recompile.
|
|
</p>
|
|
|
|
<p>
|
|
Under Unix, an <b>sqlite*</b> pointer should not be carried across a
|
|
<b>fork()</b> system call into the child process. The child process
|
|
should open its own copy of the database after the <b>fork()</b>.
|
|
</p>
|
|
|
|
<h3>6.0 Usage Examples</h3>
|
|
|
|
<p>For examples of how the SQLite C/C++ interface can be used,
|
|
refer to the source code for the <b>sqlite</b> program in the
|
|
file <b>src/shell.c</b> of the source tree.
|
|
Additional information about sqlite is available at
|
|
<a href="sqlite.html">sqlite.html</a>.
|
|
See also the sources to the Tcl interface for SQLite in
|
|
the source file <b>src/tclsqlite.c</b>.</p>
|
|
}
|
|
footer $rcsid
|