NAME

       pmemobj_root(), pmemobj_root_construct() POBJ_ROOT(), pmemobj_root_size() - root object management

SYNOPSIS

              #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);

DESCRIPTION

       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 persis‐
       tent memory pool pop.

RETURN VALUE

       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  persis‐
       tent  memory  pool  pop.   The  returned size is the largest value requested by any of the earlier pmemo‐
       bj_root() calls.  If the root object has not been allocated yet, pmemobj_root_size() returns 0.

SEE ALSO

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