SQLite Memory Management and Configuration API Reference

This document covers SQLite's memory management, configuration options, and runtime limits. This is a machine-generated summary of documentation found on sqlite.org used as a reference during development.

Memory Management
Global Configuration
Database Configuration
Runtime Limits
Compile-Time Options
Status and Statistics

Memory Management

Memory Allocation Functions

void *sqlite3_malloc(int);
void *sqlite3_malloc64(sqlite3_uint64);
void *sqlite3_realloc(void*, int);
void *sqlite3_realloc64(void*, sqlite3_uint64);
void sqlite3_free(void*);
sqlite3_uint64 sqlite3_msize(void*);

Description: SQLite memory allocation routines that track memory usage.

Note: These functions are thread-safe and include internal bookkeeping.

Reference: https://sqlite.org/c3ref/free.html

Memory Statistics

sqlite3_int64 sqlite3_memory_used(void);
sqlite3_int64 sqlite3_memory_highwater(int resetFlag);

Description: Query global memory usage.

Parameters:

resetFlag: If true, reset high-water mark after reading

Soft Heap Limit

sqlite3_int64 sqlite3_soft_heap_limit64(sqlite3_int64 N);

Description: Set a soft limit on heap size. SQLite tries to keep heap usage below this limit.

Parameters:

N: New limit in bytes (-1 to query current limit)

Reference: https://sqlite.org/c3ref/soft_heap_limit64.html

Memory Debugging

void sqlite3_mem_debug(int);
void sqlite3_mem_trace(int);

Description: Enable memory debugging and tracing (requires special build).

Global Configuration

sqlite3_config()

int sqlite3_config(int, ...);

Must be called: Before any other SQLite functions (except sqlite3_initialize).

Memory Configuration Options

Custom Memory Allocator

sqlite3_mem_methods mem = {
    myMalloc,     /* xMalloc */
    myFree,       /* xFree */
    myRealloc,    /* xRealloc */
    mySize,       /* xSize */
    myRoundup,    /* xRoundup */
    myInit,       /* xInit */
    myShutdown,   /* xShutdown */
    0             /* pAppData */
};
sqlite3_config(SQLITE_CONFIG_MALLOC, &mem);

Static Memory

static char heap[8192000];
sqlite3_config(SQLITE_CONFIG_HEAP, heap, sizeof(heap), 64);

Parameters:

Memory buffer
Buffer size
Minimum allocation size

Lookaside Memory

sqlite3_config(SQLITE_CONFIG_LOOKASIDE, 512, 128);

Parameters:

Slot size
Number of slots

Threading Configuration

sqlite3_config(SQLITE_CONFIG_SINGLETHREAD);
sqlite3_config(SQLITE_CONFIG_MULTITHREAD);
sqlite3_config(SQLITE_CONFIG_SERIALIZED);

Modes:

SINGLETHREAD: No mutexes, not thread-safe
MULTITHREAD: Thread-safe for separate connections
SERIALIZED: Fully thread-safe

Other Global Options

// Enable/disable memory status tracking
sqlite3_config(SQLITE_CONFIG_MEMSTATUS, 0);  // 0=disable, 1=enable

// Set page cache
sqlite3_config(SQLITE_CONFIG_PAGECACHE, pBuf, sz, N);

// Enable URI filenames
sqlite3_config(SQLITE_CONFIG_URI, 1);

// Set error log callback
sqlite3_config(SQLITE_CONFIG_LOG, errorLogCallback, pArg);

// Configure mmap size
sqlite3_config(SQLITE_CONFIG_MMAP_SIZE, defaultSize, maxSize);

// Set mutex implementation
sqlite3_config(SQLITE_CONFIG_MUTEX, &myMutexMethods);

// Configure scratch memory (deprecated)
sqlite3_config(SQLITE_CONFIG_SCRATCH, 0, 0, 0);

// Set SQL log callback
sqlite3_config(SQLITE_CONFIG_SQLLOG, sqlLogCallback, pArg);

// Configure covering index scan
sqlite3_config(SQLITE_CONFIG_COVERING_INDEX_SCAN, 1);

// Set statement journal spill threshold
sqlite3_config(SQLITE_CONFIG_STMTJRNL_SPILL, nByte);

// Optimize for small malloc
sqlite3_config(SQLITE_CONFIG_SMALL_MALLOC, 1);

// Set sorter reference size
sqlite3_config(SQLITE_CONFIG_SORTERREF_SIZE, nByte);

Reference: https://sqlite.org/c3ref/config.html

Database Configuration

sqlite3_db_config()

int sqlite3_db_config(sqlite3*, int op, ...);

Can be called: On open database connections.

Security and Safety Options

// Enable/disable foreign key constraints
sqlite3_db_config(db, SQLITE_DBCONFIG_ENABLE_FKEY, 1, &oldVal);

// Enable/disable triggers
sqlite3_db_config(db, SQLITE_DBCONFIG_ENABLE_TRIGGER, 1, &oldVal);

// Enable/disable views
sqlite3_db_config(db, SQLITE_DBCONFIG_ENABLE_VIEW, 1, &oldVal);

// Enable defensive mode (prevents corruption from app bugs)
sqlite3_db_config(db, SQLITE_DBCONFIG_DEFENSIVE, 1, &oldVal);

// Enable writable schema
sqlite3_db_config(db, SQLITE_DBCONFIG_WRITABLE_SCHEMA, 1, &oldVal);

// Trust schema (skip some safety checks)
sqlite3_db_config(db, SQLITE_DBCONFIG_TRUSTED_SCHEMA, 1, &oldVal);

Performance Options

// Configure lookaside memory for this connection
sqlite3_db_config(db, SQLITE_DBCONFIG_LOOKASIDE, pBuf, sz, cnt);

// Disable checkpoint on close
sqlite3_db_config(db, SQLITE_DBCONFIG_NO_CKPT_ON_CLOSE, 1, &oldVal);

// Enable query planner stability guarantee
sqlite3_db_config(db, SQLITE_DBCONFIG_ENABLE_QPSG, 1, &oldVal);

// Trigger explain query plan
sqlite3_db_config(db, SQLITE_DBCONFIG_TRIGGER_EQP, 1, &oldVal);

// Enable statement scan status
sqlite3_db_config(db, SQLITE_DBCONFIG_STMT_SCANSTATUS, 1, &oldVal);

// Reverse scan order
sqlite3_db_config(db, SQLITE_DBCONFIG_REVERSE_SCANORDER, 1, &oldVal);

Compatibility Options

// Legacy ALTER TABLE behavior
sqlite3_db_config(db, SQLITE_DBCONFIG_LEGACY_ALTER_TABLE, 1, &oldVal);

// Double-quoted string literals in DML
sqlite3_db_config(db, SQLITE_DBCONFIG_DQS_DML, 1, &oldVal);

// Double-quoted string literals in DDL
sqlite3_db_config(db, SQLITE_DBCONFIG_DQS_DDL, 1, &oldVal);

// Legacy file format
sqlite3_db_config(db, SQLITE_DBCONFIG_LEGACY_FILE_FORMAT, 1, &oldVal);

// Enable FTS3 tokenizer
sqlite3_db_config(db, SQLITE_DBCONFIG_ENABLE_FTS3_TOKENIZER, 1, &oldVal);

// Enable load extension
sqlite3_db_config(db, SQLITE_DBCONFIG_ENABLE_LOAD_EXTENSION, 1, &oldVal);

Database Name

// Set main database name
sqlite3_db_config(db, SQLITE_DBCONFIG_MAINDBNAME, "main");

Reset Database

// Reset database file (dangerous!)
sqlite3_db_config(db, SQLITE_DBCONFIG_RESET_DATABASE, 1, 0);

Reference: https://sqlite.org/c3ref/db_config.html

Runtime Limits

sqlite3_limit()

int sqlite3_limit(sqlite3*, int id, int newVal);

Description: Set or query per-connection limits.

Parameters:

id: Limit identifier
newVal: New limit (-1 to query only)

Returns: Previous limit value

Limit Identifiers

// Maximum string/blob length (default: 1000000000)
sqlite3_limit(db, SQLITE_LIMIT_LENGTH, 1000000);

// Maximum SQL statement length (default: 1000000000)
sqlite3_limit(db, SQLITE_LIMIT_SQL_LENGTH, 100000);

// Maximum columns (default: 2000)
sqlite3_limit(db, SQLITE_LIMIT_COLUMN, 100);

// Maximum expression tree depth (default: 1000)
sqlite3_limit(db, SQLITE_LIMIT_EXPR_DEPTH, 500);

// Maximum compound SELECT terms (default: 500)
sqlite3_limit(db, SQLITE_LIMIT_COMPOUND_SELECT, 50);

// Maximum VDBE instructions (default: 1000000000)
sqlite3_limit(db, SQLITE_LIMIT_VDBE_OP, 25000);

// Maximum function arguments (default: 127)
sqlite3_limit(db, SQLITE_LIMIT_FUNCTION_ARG, 8);

// Maximum attached databases (default: 125)
sqlite3_limit(db, SQLITE_LIMIT_ATTACHED, 10);

// Maximum LIKE pattern length (default: 50000)
sqlite3_limit(db, SQLITE_LIMIT_LIKE_PATTERN_LENGTH, 1000);

// Maximum variable number (default: 999)
sqlite3_limit(db, SQLITE_LIMIT_VARIABLE_NUMBER, 99);

// Maximum trigger recursion depth (default: 1000)
sqlite3_limit(db, SQLITE_LIMIT_TRIGGER_DEPTH, 10);

// Maximum auxiliary worker threads (default: 0)
sqlite3_limit(db, SQLITE_LIMIT_WORKER_THREADS, 4);

Reference: https://sqlite.org/c3ref/limit.html

Compile-Time Options

Important compile-time options that affect API behavior:

Feature Toggles

#define SQLITE_ENABLE_FTS5              // Full-text search v5
#define SQLITE_ENABLE_JSON1             // JSON functions
#define SQLITE_ENABLE_RTREE             // R-tree indexes
#define SQLITE_ENABLE_GEOPOLY           // Geopoly extension
#define SQLITE_ENABLE_SESSION           // Session extension
#define SQLITE_ENABLE_PREUPDATE_HOOK    // Pre-update hook
#define SQLITE_ENABLE_COLUMN_METADATA   // Column metadata APIs
#define SQLITE_ENABLE_STAT4             // Advanced query planner stats
#define SQLITE_ENABLE_UPDATE_DELETE_LIMIT // LIMIT on UPDATE/DELETE

Security Options

#define SQLITE_SECURE_DELETE            // Overwrite deleted content
#define SQLITE_ENABLE_CRYPTO            // Encryption support
#define SQLITE_ENABLE_SEE               // SQLite Encryption Extension

Performance Options

#define SQLITE_DEFAULT_CACHE_SIZE=-2000 // 2MB page cache
#define SQLITE_DEFAULT_PAGE_SIZE=4096   // 4KB pages
#define SQLITE_MAX_PAGE_SIZE=65536      // 64KB max page size
#define SQLITE_DEFAULT_WAL_AUTOCHECKPOINT=1000
#define SQLITE_ENABLE_SORTER_REFERENCES // Sorter optimization
#define SQLITE_MAX_WORKER_THREADS=8     // Worker thread limit

Memory Options

#define SQLITE_DEFAULT_MEMSTATUS=0      // Disable memory tracking
#define SQLITE_DEFAULT_LOOKASIDE=1200,100 // Lookaside configuration
#define SQLITE_ENABLE_MEMSYS3           // Alternative memory allocator
#define SQLITE_ENABLE_MEMSYS5           // Alternative memory allocator
#define SQLITE_ZERO_MALLOC              // Omit memory allocator

Status and Statistics

Global Status

int sqlite3_status(int op, int *pCurrent, int *pHighwater, int resetFlag);
int sqlite3_status64(int op, sqlite3_int64 *pCurrent,
                     sqlite3_int64 *pHighwater, int resetFlag);

Status operations:

SQLITE_STATUS_MEMORY_USED - Current memory in use
SQLITE_STATUS_PAGECACHE_USED - Page cache memory
SQLITE_STATUS_PAGECACHE_OVERFLOW - Page cache overflows
SQLITE_STATUS_SCRATCH_USED - Scratch memory (deprecated)
SQLITE_STATUS_MALLOC_SIZE - Largest malloc request
SQLITE_STATUS_PARSER_STACK - Parser stack depth
SQLITE_STATUS_PAGECACHE_SIZE - Page cache allocation size
SQLITE_STATUS_SCRATCH_SIZE - Scratch allocation size
SQLITE_STATUS_MALLOC_COUNT - Number of mallocs

Database Status

int sqlite3_db_status(sqlite3*, int op, int *pCur, int *pHiwtr, int resetFlg);

Operations (see Advanced Features document for full list)

Statement Status

int sqlite3_stmt_status(sqlite3_stmt*, int op, int resetFlg);

Operations:

SQLITE_STMTSTATUS_FULLSCAN_STEP - Full table scan steps
SQLITE_STMTSTATUS_SORT - Sort operations
SQLITE_STMTSTATUS_AUTOINDEX - Automatic indexes created
SQLITE_STMTSTATUS_VM_STEP - Virtual machine steps
SQLITE_STMTSTATUS_REPREPARE - Statement re-preparations
SQLITE_STMTSTATUS_RUN - Times statement has been run
SQLITE_STMTSTATUS_MEMUSED - Memory used by statement

Best Practices

Configure early: Call sqlite3_config() before any other SQLite functions
Set appropriate limits: Use sqlite3_limit() to prevent resource exhaustion
Monitor memory: Use status functions to track memory usage
Choose threading mode: Select appropriate threading model at startup
Enable only needed features: Use compile-time options to reduce size
Test configuration: Verify settings work correctly in your environment
Document settings: Keep track of non-default configurations

Common Configuration Patterns

Low Memory Environment

// Use static memory allocation
static char heap[2048000];
sqlite3_config(SQLITE_CONFIG_HEAP, heap, sizeof(heap), 64);
sqlite3_config(SQLITE_CONFIG_LOOKASIDE, 128, 32);
sqlite3_config(SQLITE_CONFIG_PAGECACHE, pageCache, 1024, 100);

High Performance

// Increase cache sizes
sqlite3_db_config(db, SQLITE_DBCONFIG_LOOKASIDE, NULL, 512, 200);
sqlite3_exec(db, "PRAGMA cache_size=10000", 0, 0, 0);
sqlite3_exec(db, "PRAGMA temp_store=MEMORY", 0, 0, 0);
sqlite3_limit(db, SQLITE_LIMIT_WORKER_THREADS, 4);

Maximum Safety

// Enable all safety features
sqlite3_db_config(db, SQLITE_DBCONFIG_DEFENSIVE, 1, NULL);
sqlite3_db_config(db, SQLITE_DBCONFIG_TRUSTED_SCHEMA, 0, NULL);
sqlite3_exec(db, "PRAGMA synchronous=FULL", 0, 0, 0);
sqlite3_exec(db, "PRAGMA foreign_keys=ON", 0, 0, 0);

References

SQLite Configuration: https://sqlite.org/c3ref/config.html
SQLite Limits: https://sqlite.org/limits.html
SQLite Compile Options: https://sqlite.org/compile.html
SQLite Memory: https://sqlite.org/malloc.html
SQLite Status: https://sqlite.org/c3ref/status.html

SQLite Memory Management and Configuration API Reference

Table of Contents

Memory Management

Memory Allocation Functions

Memory Statistics

Soft Heap Limit

Memory Debugging

Global Configuration

sqlite3_config()

Memory Configuration Options

Custom Memory Allocator

Static Memory

Lookaside Memory

Threading Configuration

Other Global Options

Database Configuration

sqlite3_db_config()

Security and Safety Options

Performance Options

Compatibility Options

Database Name

Reset Database

Runtime Limits

sqlite3_limit()

Limit Identifiers

Compile-Time Options

Feature Toggles

Security Options

Performance Options

Memory Options

Status and Statistics

Global Status

Database Status

Statement Status

Best Practices

Common Configuration Patterns

Low Memory Environment

High Performance

Maximum Safety

References

Settings

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