/*
** Given an allocation, find the MemBlockHdr for that allocation.
**
** This routine checks the guards at either end of the allocation and
** if they are incorrect it asserts.
*/
static struct MemBlockHdr *sqlite3MemsysGetHeader(void *pAllocation){
struct MemBlockHdr *p;
int *pInt;
u8 *pU8;
int nReserve;
p = (struct MemBlockHdr*)pAllocation;
p--;
assert( p->iForeGuard==(int)FOREGUARD );
nReserve = ROUND8(p->iSize);
pInt = (int*)pAllocation;
pU8 = (u8*)pAllocation;
assert( pInt[nReserve/sizeof(int)]==(int)REARGUARD );
/* This checks any of the "extra" bytes allocated due
** to rounding up to an 8 byte boundary to ensure
** they haven't been overwritten.
*/
while( nReserve-- > p->iSize ) assert( pU8[nReserve]==0x65 );
return p;
}
通过在分配器的末尾设置一个哨兵值来检查这些哨兵值以确保SQLite内核没有超出缓冲区的两端。