OID_IS_NULL(3)

NAME

OID_IS_NULL(), OID_EQUALS(), pmemobj_direct(), pmemobj_oid(), pmemobj_type_num(), pmemobj_pool_by_oid(), pmemobj_pool_by_ptr() – functions that allow mapping operations between object addresses, object handles, oids or type numbers

SYNOPSIS

    #include <libpmemobj.h>
    OID_IS_NULL(PMEMoid oid)
    OID_EQUALS(PMEMoid lhs, PMEMoid rhs)
    void *pmemobj_direct(PMEMoid oid);
    PMEMoid pmemobj_oid(const void *addr);
    uint64_t pmemobj_type_num(PMEMoid oid);
    PMEMobjpool *pmemobj_pool_by_oid(PMEMoid oid);
    PMEMobjpool *pmemobj_pool_by_ptr(const void *addr);

DESCRIPTION

Each object stored in a persistent memory pool is represented by an object handle of type PMEMoid. In practice, such a handle is a unique Object IDentifier (OID) of global scope, which means that two objects from different pools will never have the same OID. The special OID_NULL macro defines a NULL-like handle that does not represent any object. The size of a single object is limited by PMEMOBJ_MAX_ALLOC_SIZE. Thus an allocation with a requested size greater than this value will fail.

An OID cannot be used as a direct pointer to an object. Each time the program attempts to read or write object data, it must obtain the current memory address of the object by converting its OID into a pointer.

In contrast to the memory address, the OID value for given object does not change during the life of an object (except for realloc), and remains valid after closing and reopening the pool. For this reason, if an object contains a reference to another persistent object, for example, to build some kind of a linked data structure, the reference must be an OID and not a memory address.

pmemobj_direct() returns a pointer to the PMEMoid object with handle oid.

pmemobj_oid() returns a PMEMoid handle to the object pointed to by addr.

pmemobj_type_num() returns the type number of the PMEMoid object with handle oid.

pmemobj_pool_by_oid() returns a PMEMobjpool* handle to the pool containing the PMEMoid object with handle oid.

pmemobj_pool_by_ptr() returns a PMEMobjpool* handle to the pool containing the address addr.

At the time of allocation (or reallocation), each object may be assigned a number representing its type. Such a type number may be used to arrange the persistent objects based on their actual user-defined structure type, thus facilitating implementation of a simple run-time type safety mechanism. This also allows iterating through all the objects of a given type that are stored in the persistent memory pool. See pmemobj_first(3) for more information.

The OID_IS_NULL() macro checks if PMEMoid represents a NULL object.

The OID_EQUALS() macro compares two PMEMoid objects.

RETURN VALUE

The pmemobj_direct() function returns a pointer to the object represented by oid. If oid is OID_NULL, pmemobj_direct() returns NULL.

The pmemobj_oid() function returns a PMEMoid handle to the object pointed to by addr. If addr is not from within a pmemobj pool, OID_NULL is returned. If addr is not the start of an object (does not point to the beginning of a valid allocation), the resulting PMEMoid can be safely used only with:

•   pmemobj_pool_by_oid()

•   pmemobj_direct()

•   pmemobj_tx_add_range(3)

The pmemobj_type_num() function returns the type number of the object represented by oid.

The pmemobj_pool_by_oid() function returns a handle to the pool that contains the object represented by oid. If the the pool is not open or oid is OID_NULL, pmemobj_pool_by_oid() returns NULL.

The pmemobj_pool_by_ptr() function returns a handle to the pool that contains the address, or NULL if the address does not belong to any open pool.

NOTES

For performance reasons, on Linux and FreeBSD the pmemobj_direct() function is inlined by default. To use the non-inlined variant of pmemobj_direct(), define PMEMOBJ_DIRECT_NON_INLINE prior to the #include of <libpmemobj.h>, either with #define or with the -D option to the compiler.

SEE ALSO

libpmemobj(7) and <http://pmem.io>