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PMEMOBJ_OPEN(3) PMDK Programmer's Manual PMEMOBJ_OPEN(3)


pmemobj_open(), pmemobj_create(), pmemobj_close(), pmemobj_check() pmemobj_set_user_data(), pmemobj_get_user_data() - create, open, close and validate persistent memory transactional object store


#include <libpmemobj.h>
PMEMobjpool *pmemobj_open(const char *path, const char *layout);
PMEMobjpool *pmemobj_create(const char *path, const char *layout,
    size_t poolsize, mode_t mode);
void pmemobj_close(PMEMobjpool *pop);
int pmemobj_check(const char *path, const char *layout);
void pmemobj_set_user_data(PMEMobjpool *pop, void *data);
void *pmemobj_get_user_data(PMEMobjpool *pop);


To use the pmem-resident transactional object store provided by libpmemobj(7), a memory pool must first be created with the pmemobj_create() function described below. Existing pools may be opened with the pmemobj_open() function.

As of libpmemobj 1.11, these functions are thread-safe; be careful if you have to use earlier versions of the library.

Once created, the memory pool is represented by an opaque handle, of type PMEMobjpool*, which is passed to most of the other libpmemobj(7) functions. Internally, libpmemobj(7) will use either pmem_persist(3) or msync(2) when it needs to flush changes, depending on whether the memory pool appears to be persistent memory or a regular file (see the pmem_is_pmem(3) function in libpmem(7) for more information). There is no need for applications to flush changes directly when using the object memory API provided by libpmemobj(7).

The pmemobj_create() function creates a transactional object store with the given total poolsize. path specifies the name of the memory pool file to be created. layout specifies the application's layout type in the form of a string. The layout name is not interpreted by libpmemobj(7), but may be used as a check when pmemobj_open() is called. The layout name, including the terminating null byte (`\0'), cannot be longer than PMEMOBJ_MAX_LAYOUT as defined in <libpmemobj.h>. A NULL layout is equivalent to using an empty string as a layout name. mode specifies the permissions to use when creating the file, as described by creat(2). The memory pool file is fully allocated to the size poolsize using posix_fallocate(3). The caller may choose to take responsibility for creating the memory pool file by creating it before calling pmemobj_create(), and then specifying poolsize as zero. In this case pmemobj_create() will take the pool size from the size of the existing file and will verify that the file appears to be empty by searching for any non-zero data in the pool header at the beginning of the file. The minimum net pool size allowed by the library for a local transactional object store is defined in <libpmemobj.h> as PMEMOBJ_MIN_POOL. For remote replicas the minimum file size is defined in <librpmem.h> as RPMEM_MIN_PART.

Depending on the configuration of the system, the available non-volatile memory space may be divided into multiple memory devices. In such case, the maximum size of the pmemobj memory pool could be limited by the capacity of a single memory device. libpmemobj(7) allows building persistent memory resident object store spanning multiple memory devices by creation of persistent memory pools consisting of multiple files, where each part of such a pool set may be stored on a different memory device or pmem-aware filesystem.

Creation of all the parts of the pool set can be done with pmemobj_create(); however, the recommended method for creating pool sets is with the pmempool(1) utility.

When creating a pool set consisting of multiple files, the path argument passed to pmemobj_create() must point to the special set file that defines the pool layout and the location of all the parts of the pool set. The poolsize argument must be 0. The meaning of the layout and mode arguments does not change, except that the same mode is used for creation of all the parts of the pool set.

The set file is a plain text file, the structure of which is described in poolset(5).

The pmemobj_open() function opens an existing object store memory pool. Similar to pmemobj_create(), path must identify either an existing obj memory pool file, or the set file used to create a pool set. If layout is non-NULL, it is compared to the layout name provided to pmemobj_create() when the pool was first created. This can be used to verify that the layout of the pool matches what was expected. The application must have permission to open the file and memory map it with read/write permissions.

Be aware that if the pool contains bad blocks inside, opening can be aborted by the SIGBUS signal, because currently the pool is not checked against bad blocks during opening. It can be turned on by setting the CHECK_BAD_BLOCKS compat feature. For details see description of this feature in pmempool-feature(1).

The pmemobj_close() function closes the memory pool indicated by pop and deletes the memory pool handle. The object store itself lives on in the file that contains it and may be re-opened at a later time using pmemobj_open() as described above.

The pmemobj_check() function performs a consistency check of the file indicated by path. pmemobj_check() opens the given path read-only so it never makes any changes to the file. This function is not supported on Device DAX.

The pmemobj_set_user_data() function associates custom volatile state, represented by pointer data, with the given pool pop. This state can later be retrieved using pmemobj_get_user_data() function. This state does not survive pool close. If pmemobj_set_user_data() was not called for a given pool, pmemobj_get_user_data() will return NULL.


The pmemobj_create() function returns a memory pool handle to be used with most of the functions in libpmemobj(7). On error it returns NULL and sets errno appropriately.

The pmemobj_open() function returns a memory pool handle to be used with most of the functions in libpmemobj(7). If an error prevents the pool from being opened, or if the given layout does not match the pool's layout, pmemobj_open() returns NULL and sets errno appropriately.

The pmemobj_close() function returns no value.

The pmemobj_check() function returns 1 if the memory pool is found to be consistent. Any inconsistencies found will cause pmemobj_check() to return 0, in which case the use of the file with libpmemobj(7) will result in undefined behavior. The debug version of libpmemobj(7) will provide additional details on inconsistencies when PMEMOBJ_LOG_LEVEL is at least 1, as described in the DEBUGGING AND ERROR HANDLING section in libpmemobj(7). pmemobj_check() returns -1 and sets errno if it cannot perform the consistency check due to other errors.


Not all file systems support posix_fallocate(3). pmemobj_create() will fail if the underlying file system does not support posix_fallocate(3).


creat(2), msync(2), pmem_is_pmem(3), pmem_persist(3), posix_fallocate(3), libpmem(7), libpmemobj(7) and <>

2022-08-25 PMDK - pmemobj API version 2.3