The C language interface to SQLite Version 2

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The C language interface to SQLite Version 2

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.

1.0 The Core API

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:

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
);

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.

1.1 Opening a database

Use the sqlite_open 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 sqlite_open 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.

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.

The return value of the sqlite_open function is a pointer to an opaque sqlite 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.

1.2 Closing the database

To close an SQLite database, call the sqlite_close function passing it the sqlite structure pointer that was obtained from a prior call to sqlite_open. If a transaction is active when the database is closed, the transaction is rolled back.

1.3 Executing SQL statements

The sqlite_exec function is used to process SQL statements and queries. This function requires 5 parameters as follows:

A pointer to the sqlite structure obtained from a prior call to sqlite_open.
A null-terminated string containing the text of one or more SQL statements and/or queries to be processed.
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.
A pointer that is forwarded to become the first argument to the callback function.
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.

The callback function is used to receive the results of a query. A prototype for the callback function is as follows:

int Callback(void *pArg, int argc, char **argv, char **columnNames){
  return 0;
}

The first argument to the callback is just a copy of the fourth argument to sqlite_exec 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:

argv[i][0] == 0

But if the i-th parameter is NULL we will get:

argv[i] == 0

The names of the columns are contained in first argc entries of the fourth argument. If the SHOW_DATATYPES pragma is on (it is off by default) then the second argc entries in the 4th argument are the datatypes for the corresponding columns.

If the EMPTY_RESULT_CALLBACKS 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

argv == 0

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.

The callback function should normally return 0. If the callback function returns non-zero, the query is immediately aborted and sqlite_exec will return SQLITE_ABORT.

1.4 Error Codes

The sqlite_exec 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:

#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 constraint 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 */

The meanings of these various return values are as follows:

SQLITE_OK

This value is returned if everything worked and there were no errors.

SQLITE_INTERNAL

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 sqlite_exec call, please report the problem on the SQLite mailing list.

SQLITE_ERROR

This return value indicates that there was an error in the SQL that was passed into the sqlite_exec.

SQLITE_PERM

This return value says that the access permissions on the database file are such that the file cannot be opened.

SQLITE_ABORT

This value is returned if the callback function returns non-zero.

SQLITE_BUSY

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.

SQLITE_LOCKED

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 sqlite_exec. 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.

SQLITE_NOMEM

This value is returned if a call to malloc fails.

SQLITE_READONLY

This return code indicates that an attempt was made to write to a database file that is opened for reading only.

SQLITE_INTERRUPT

This value is returned if a call to sqlite_interrupt interrupts a database operation in progress.

SQLITE_IOERR

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.

SQLITE_CORRUPT

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 a previously 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.

SQLITE_FULL

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.

SQLITE_CANTOPEN

This value is returned if the database file could not be opened for some reason.

SQLITE_PROTOCOL

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.

SQLITE_SCHEMA

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.

SQLITE_TOOBIG

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.

SQLITE_CONSTRAINT

This constant is returned if the SQL statement would have violated a database constraint.

SQLITE_MISMATCH

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.

SQLITE_MISUSE

This error might occur if one or more of the SQLite API routines is used incorrectly. Examples of incorrect usage include calling sqlite_exec after the database has been closed using sqlite_close or calling sqlite_exec with the same database pointer simultaneously from two separate threads.

SQLITE_NOLFS

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.

SQLITE_AUTH

This error indicates that the authorizer callback has disallowed the SQL you are attempting to execute.

SQLITE_ROW

This is one of the return codes from the sqlite_step routine which is part of the non-callback API. It indicates that another row of result data is available.

SQLITE_DONE

This is one of the return codes from the sqlite_step routine which is part of the non-callback API. It indicates that the SQL statement has been completely executed and the sqlite_finalize routine is ready to be called.

2.0 Accessing Data Without Using A Callback Function

The sqlite_exec 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.

The new interface uses three separate functions to replace the single sqlite_exec function.

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 */
);

The strategy is to compile a single SQL statement using sqlite_compile then invoke sqlite_step multiple times, once for each row of output, and finally call sqlite_finalize to clean up after the SQL has finished execution.

2.1 Compiling An SQL Statement Into A Virtual Machine

The sqlite_compile "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 sqlite_open.

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 sqlite_finalize 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.

If any errors are encountered during compilation, an error message is written into memory obtained from malloc 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 sqlite_freemem.

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 sqlite_exec which executes all SQL statements in its input string.) The 3rd parameter to sqlite_compile 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.

On success, sqlite_compile returns SQLITE_OK. Otherwise and error code is returned.

2.2 Step-By-Step Execution Of An SQL Statement

After a virtual machine has been generated using sqlite_compile it is executed by one or more calls to sqlite_step. Each invocation of sqlite_step, 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 sqlite_step convey the same information as the 2nd through 4th parameters of the callback routine when using the sqlite_exec interface. Except, with sqlite_step the column datatype information is always included in the in the 4th parameter regardless of whether or not the SHOW_DATATYPES pragma is on or off.

Each invocation of sqlite_step 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.

If the virtual machine is unable to open the database file because it is locked by another thread or process, sqlite_step 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 sqlite_step again. This can be repeated as many times as desired.

Whenever another row of result data is available, sqlite_step 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.

When all processing is complete, sqlite_step 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 sqlite_finalize.) It is a misuse of the library to attempt to call sqlite_step again after it has returned SQLITE_DONE or SQLITE_ERROR.

When sqlite_step 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.

If you abuse the library by trying to call sqlite_step 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.

2.3 Deleting A Virtual Machine

Every virtual machine that sqlite_compile creates should eventually be handed to sqlite_finalize. 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.

The sqlite_finalize 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 sqlite_exec. The error message returned will also be the same.

It is acceptable to call sqlite_finalize on a virtual machine before sqlite_step 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 sqlite_exec had returned non-zero.

It is also acceptable to call sqlite_finalize on a virtual machine that has never been passed to sqlite_step even once.

3.0 The Extended API

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:

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*);

All of the above definitions are included in the "sqlite.h" header file that comes in the source tree.

3.1 The ROWID of the most recent insert

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_".

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 sqlite_last_insert_rowid API function.

3.2 The number of rows that changed

The sqlite_changes 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 sqlite_exec and no VMs created by sqlite_compile that have not been finalized by sqlite_finalize. In common usage, sqlite_changes returns the number of rows inserted, deleted, or modified by the most recent sqlite_exec call or since the most recent sqlite_compile. But if you have nested calls to sqlite_exec (that is, if the callback routine of one sqlite_exec invokes another sqlite_exec) or if you invoke sqlite_compile to create a new VM while there is still another VM in existence, then the meaning of the number returned by sqlite_changes 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 not counted.

SQLite implements the command "DELETE FROM table" (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 sqlite_changes 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 "DELETE FROM table WHERE 1" instead.

3.3 Querying into memory obtained from malloc()

The sqlite_get_table function is a wrapper around sqlite_exec 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.

The main result from sqlite_get_table 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.

As an example, consider the following query:

SELECT employee_name, login, host FROM users WHERE login LIKE 'd%';

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 sqlite_get_table the result might look like this:

nrow = 2
ncolumn = 3
result[0] = "employee_name"
result[1] = "login"
result[2] = "host"
result[3] = "dummy"
result[4] = "No such user"
result[5] = 0
result[6] = "D. Richard Hipp"
result[7] = "drh"
result[8] = "zadok"

Notice that the "host" value for the "dummy" record is NULL so the result[] array contains a NULL pointer at that slot.

If the result set of a query is empty, then by default sqlite_get_table 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:

SELECT employee_name, login, host FROM users WHERE employee_name IS NULL;

The default behavior gives this results:

nrow = 0
ncolumn = 0
result = 0

But if the EMPTY_RESULT_CALLBACKS pragma is ON, then the following is returned:

nrow = 0
ncolumn = 3
result[0] = "employee_name"
result[1] = "login"
result[2] = "host"

Memory to hold the information returned by sqlite_get_table is obtained from malloc(). But the calling function should not try to free this information directly. Instead, pass the complete table to sqlite_free_table when the table is no longer needed. It is safe to call sqlite_free_table with a NULL pointer such as would be returned if the result set is empty.

The sqlite_get_table routine returns the same integer result code as sqlite_exec.

3.4 Interrupting an SQLite operation

The sqlite_interrupt 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 sqlite_exec routine (or the equivalent) that started the database operation will return SQLITE_INTERRUPT.

3.5 Testing for a complete SQL statement

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 sqlite_complete 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 sqlite_complete returns false, then more text is required to complete the SQL statement.

For the purpose of the sqlite_complete function, an SQL statement is complete if it ends in a semicolon.

The sqlite command-line utility uses the sqlite_complete function to know when it needs to call sqlite_exec. After each line of input is received, sqlite calls sqlite_complete on all input in its buffer. If sqlite_complete returns true, then sqlite_exec is called and the input buffer is reset. If sqlite_complete returns false, then the prompt is changed to the continuation prompt and another line of text is read and added to the input buffer.

3.6 Library version string

The SQLite library exports the string constant named sqlite_version 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 sqlite_version string constant to verify that the version number of the header file and the library match.

3.7 Library character encoding

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 sqlite_encoding will be set to either "UTF-8" or "iso8859" to indicate how the library was compiled. In addition, the sqlite.h header file will define one of the macros SQLITE_UTF8 or SQLITE_ISO8859, as appropriate.

Note that the character encoding mechanism used by SQLite cannot be changed at run-time. This is a compile-time option only. The sqlite_encoding character string just tells you how the library was compiled.

3.8 Changing the library's response to locked files

The sqlite_busy_handler 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.

The arguments to sqlite_busy_handler are the opaque structure returned from sqlite_open, 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 sqlite_busy_handler, 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.

For the common case where we want the busy callback to sleep, the SQLite library provides a convenience routine sqlite_busy_timeout. The first argument to sqlite_busy_timeout is a pointer to an open SQLite database and the second argument is a number of milliseconds. After sqlite_busy_timeout 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.

3.9 Using the `_printf()` wrapper functions

The four utility functions

sqlite_exec_printf()
sqlite_exec_vprintf()
sqlite_get_table_printf()
sqlite_get_table_vprintf()

implement the same query functionality as sqlite_exec and sqlite_get_table. But instead of taking a complete SQL statement as their second argument, the four _printf 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.

There are two advantages to using the SQLite printf functions instead of sprintf. First of all, with the SQLite printf routines, there is never a danger of overflowing a static buffer as there is with sprintf. The SQLite printf routines automatically allocate (and later frees) as much memory as is necessary to hold the SQL statements generated.

The second advantage the SQLite printf routines have over sprintf 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.

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:

sqlite_exec_printf(db,
  "INSERT INTO table1 VALUES('%s')",
  0, 0, 0, zString);

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:

INSERT INTO table1 VALUES('Hi y'all')

This is not valid SQL because of the apostrophe in the word "y'all". But if the %q formatting option is used instead of %s, like this:

sqlite_exec_printf(db,
  "INSERT INTO table1 VALUES('%q')",
  0, 0, 0, zString);

Then the generated SQL will look like the following:

INSERT INTO table1 VALUES('Hi y''all')

Here the apostrophe 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 sprintf.

If the %Q formatting option is used instead of %q, like this:

sqlite_exec_printf(db,
  "INSERT INTO table1 VALUES(%Q)",
  0, 0, 0, zString);

Then the generated SQL will look like the following:

INSERT INTO table1 VALUES('Hi y''all')

If the value of the zString variable is NULL, the generated SQL will look like the following:

INSERT INTO table1 VALUES(NULL)

All of the _printf() routines above are built around the following two functions:

char *sqlite_mprintf(const char *zFormat, ...);
char *sqlite_vmprintf(const char *zFormat, va_list);

The sqlite_mprintf() routine works like the standard library sprintf() except that it writes its results into memory obtained from malloc() and returns a pointer to the malloced buffer. sqlite_mprintf() also understands the %q and %Q extensions described above. The sqlite_vmprintf() is a varargs version of the same routine. The string pointer that these routines return should be freed by passing it to sqlite_freemem().

3.10 Performing background jobs during large queries

The sqlite_progress_handler() routine can be used to register a callback routine with an SQLite database to be invoked periodically during long running calls to sqlite_exec(), sqlite_step() and the various wrapper functions.

The callback is invoked every N virtual machine operations, where N is supplied as the second argument to sqlite_progress_handler(). The third and fourth arguments to sqlite_progress_handler() are a pointer to the routine to be invoked and a void pointer to be passed as the first argument to it.

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.

4.0 Adding New SQL Functions

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:

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*);

The sqlite_create_function() interface is used to create regular functions and sqlite_create_aggregate() is used to create new aggregate functions. In both cases, the db parameter is an open SQLite database on which the functions should be registered, zName is the name of the new function, nArg is the number of arguments, and pUserData 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.

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.

For regular functions, the xFunc callback is invoked once for each function call. The implementation of xFunc should call one of the sqlite_set_result_... interfaces to return its result. The sqlite_user_data() routine can be used to retrieve the pUserData pointer that was passed in when the function was registered.

For aggregate functions, the xStep callback is invoked once for each row in the result and then xFinalize is invoked at the end to compute a final answer. The xStep routine can use the sqlite_aggregate_context() 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 sqlite_aggregate_count() 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 sqlite_set_result_... interfaces to set the final result of the aggregate.

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 func.c.

5.0 Multi-Threading And SQLite

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 sqlite* pointer returned from sqlite_open. It is never safe for two or more threads to access the same sqlite* pointer at the same time.

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.

Under Unix, an sqlite* pointer should not be carried across a fork() system call into the child process. The child process should open its own copy of the database after the fork().

6.0 Usage Examples

For examples of how the SQLite C/C++ interface can be used, refer to the source code for the sqlite program in the file src/shell.c of the source tree. Additional information about sqlite is available at sqlite.html. See also the sources to the Tcl interface for SQLite in the source file src/tclsqlite.c.