void *sqlite3_malloc(int); void *sqlite3_realloc(void*, int); void sqlite3_free(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.
The sqlite3_malloc() routine returns a pointer to a block of memory at least N bytes in length, where N is the parameter. If sqlite3_malloc() is unable to obtain sufficient free memory, it returns a NULL pointer. If the parameter N to sqlite3_malloc() is zero or negative then sqlite3_malloc() returns a NULL pointer.
Calling sqlite3_free() with a pointer previously returned by sqlite3_malloc() or sqlite3_realloc() releases that memory so that it might be reused. 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().
The sqlite3_realloc() interface attempts to resize a prior memory allocation to be at least N bytes, where N is the second parameter. The memory allocation to be resized is the first parameter. If the first parameter to sqlite3_realloc() is a NULL pointer then its behavior is identical to calling sqlite3_malloc(N) where N is the second parameter to sqlite3_realloc(). If the second parameter to sqlite3_realloc() is zero or negative then the behavior is exactly the same as calling sqlite3_free(P) where P is the first parameter to sqlite3_realloc(). sqlite3_realloc() returns a pointer to a memory allocation of at least N bytes in size or NULL if sufficient memory is unavailable. 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() and the prior allocation is freed. If sqlite3_realloc() returns NULL, then the prior allocation is not freed.
The memory returned by sqlite3_malloc() and sqlite3_realloc() 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.