#include <libpmemobj.h> PMEMoid pmemobj_root(PMEMobjpool *pop, size_t size); PMEMoid pmemobj_root_construct(PMEMobjpool *pop, size_t size, pmemobj_constr constructor, void *arg); POBJ_ROOT(PMEMobjpool *pop, TYPE) size_t pmemobj_root_size(PMEMobjpool *pop);
The root object of a persistent memory pool is an entry point for all other persistent objects allocated using the libpmemobj API. In other words, every object stored in the persistent memory pool has the root object at the end of its reference path. It may be assumed that for each persistent memory pool the root object always exists, and there is exactly one root object in each pool.
The pmemobj_root() function creates or resizes the root object for the persistent memory pool pop. If this is the first call to pmemobj_root(), the requested size is greater than zero and the root object does not exist, it is implicitly allocated in a thread-safe manner, so the function may be called by more than one thread simultaneously (as long as all threads use the identical size value). The size of the root object is guaranteed to be not less than the requested size. If the requested size is larger than the current size, the root object is automatically resized. In such case, the old data is preserved and the extra space is zeroed. If the requested size is equal to or smaller than the current size, the root object remains unchanged. If the requested size is equal to zero, the root object is not allocated.
pmemobj_root_construct() performs the same actions as pmemobj_root(), but instead of zeroing the newly allocated object a constructor function is called to initialize the object. The constructor is also called on reallocations.
The POBJ_ROOT() macro works the same way as the pmemobj_root() function except it returns a typed OID value.
The pmemobj_root_size() function returns the current size of the root object associated with the persistent memory pool pop.
Upon success, pmemobj_root() returns a handle to the root object associated with the persistent memory pool pop. The same root object handle is returned in all the threads. If the requested object size is larger than the maximum allocation size supported for the pool, or if there is not enough free space in the pool to satisfy a reallocation request, pmemobj_root() returns OID_NULL and sets errno to ENOMEM. If the size was equal to zero and the root object has not been allocated, pmemobj_root() returns OID_NULL and sets errno to EINVAL.
If the pmemobj_root_construct() constructor fails, the allocation is canceled, pmemobj_root_construct() returns OID_NULL, and errno is set to ECANCELED. pmemobj_root_size() can be used in the constructor to check whether this is the first call to the constructor.
POBJ_ROOT() returns a typed OID of type TYPE instead of the PMEMoid returned by pmemobj_root().
The pmemobj_root_size() function returns the current size of the root object associated with the persistent memory pool pop. The returned size is the largest value requested by any of the earlier pmemobj_root() calls. If the root object has not been allocated yet, pmemobj_root_size() returns 0.
libpmemobj(7) and <https://pmem.io>
The man pages pmemobj_root_construct(3), pmemobj_root_size(3) and pobj_root(3) are aliases of pmemobj_root(3).