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SQLite C Interface

Memory Allocation Subsystem

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*);

The SQLite core uses these three routines for all of its own internal memory allocation needs. "Core" in the previous sentence does not include operating-system specific VFS implementation. The Windows VFS uses native malloc() and free() for some operations.

R-08220-00712:[The sqlite3_malloc() routine returns a pointer to a block of memory at least N bytes in length, where N is the parameter. ] R-56692-55226:[If sqlite3_malloc() is unable to obtain sufficient free memory, it returns a NULL pointer. ] R-65312-04917:[If the parameter N to sqlite3_malloc() is zero or negative then sqlite3_malloc() returns a NULL pointer. ]

R-23595-60076:[The sqlite3_malloc64(N) routine works just like sqlite3_malloc(N) except that N is an unsigned 64-bit integer instead of a signed 32-bit integer. ]

R-17203-10061:[Calling sqlite3_free() with a pointer previously returned by sqlite3_malloc() or sqlite3_realloc() releases that memory so that it might be reused. ] R-49053-54554:[The sqlite3_free() routine is a no-op if is called with a NULL pointer. ] Passing a NULL pointer to sqlite3_free() is harmless. After being freed, memory should neither be read nor written. Even reading previously freed memory might result in a segmentation fault or other severe error. Memory corruption, a segmentation fault, or other severe error might result if sqlite3_free() is called with a non-NULL pointer that was not obtained from sqlite3_malloc() or sqlite3_realloc().

R-50848-12841:[The sqlite3_realloc(X,N) interface attempts to resize a prior memory allocation X to be at least N bytes. ] R-04300-56712:[If the X parameter to sqlite3_realloc(X,N) is a NULL pointer then its behavior is identical to calling sqlite3_malloc(N). ] R-26507-47431:[If the N parameter to sqlite3_realloc(X,N) is zero or negative then the behavior is exactly the same as calling sqlite3_free(X). ] R-25722-54578:[sqlite3_realloc(X,N) returns a pointer to a memory allocation of at least N bytes in size or NULL if insufficient memory is available. ] R-59903-02736:[If M is the size of the prior allocation, then min(N,M) bytes of the prior allocation are copied into the beginning of buffer returned by sqlite3_realloc(X,N) and the prior allocation is freed. ] R-18701-28385:[If sqlite3_realloc(X,N) returns NULL and N is positive, then the prior allocation is not freed. ]

R-36007-42565:[The sqlite3_realloc64(X,N) interfaces works the same as sqlite3_realloc(X,N) except that N is a 64-bit unsigned integer instead of a 32-bit signed integer. ]

R-64601-24454:[If X is a memory allocation previously obtained from sqlite3_malloc(), sqlite3_malloc64(), sqlite3_realloc(), or sqlite3_realloc64(), then sqlite3_msize(X) returns the size of that memory allocation in bytes. ] R-19846-48503:[The value returned by sqlite3_msize(X) might be larger than the number of bytes requested when X was allocated. ] R-01244-21922:[If X is a NULL pointer then sqlite3_msize(X) returns zero. ] If X points to something that is not the beginning of memory allocation, or if it points to a formerly valid memory allocation that has now been freed, then the behavior of sqlite3_msize(X) is undefined and possibly harmful.

R-11148-40995:[The memory returned by sqlite3_malloc(), sqlite3_realloc(), sqlite3_malloc64(), and sqlite3_realloc64() is always aligned to at least an 8 byte boundary, or to a 4 byte boundary if the SQLITE_4_BYTE_ALIGNED_MALLOC compile-time option is used. ]

In SQLite version 3.5.0 and 3.5.1, it was possible to define the SQLITE_OMIT_MEMORY_ALLOCATION which would cause the built-in implementation of these routines to be omitted. That capability is no longer provided. Only built-in memory allocators can be used.

Prior to SQLite version 3.7.10, the Windows OS interface layer called the system malloc() and free() directly when converting filenames between the UTF-8 encoding used by SQLite and whatever filename encoding is used by the particular Windows installation. Memory allocation errors were detected, but they were reported back as SQLITE_CANTOPEN or SQLITE_IOERR rather than SQLITE_NOMEM.

The pointer arguments to sqlite3_free() and sqlite3_realloc() must be either NULL or else pointers obtained from a prior invocation of sqlite3_malloc() or sqlite3_realloc() that have not yet been released.

The application must not read or write any part of a block of memory after it has been released using sqlite3_free() or sqlite3_realloc().

See also lists of Objects, Constants, and Functions.