NAME¶

libpmemobj - persistent memory transactional object store

SYNOPSIS¶

#include <libpmemobj.h>
cc -std=gnu99 ... -lpmemobj -lpmem
    

DESCRIPTION¶

libpmemobj provides a transactional object store in persistent memory (pmem) for applications that require transactions and persistent memory management using direct access storage (DAX), which is storage that supports load/store access without paging blocks from a block storage device. Some types of non-volatile memory DIMMs (NVDIMMs) provide this type of byte addressable access to storage. A persistent memory aware file system is typically used to expose the direct access to applications. Memory mapping a file from this type of file system results in load/store, non-paged access to pmem. libpmemobj builds on this type of memory mapped file using the low-level pmem support provided by libpmem(7), handling the transactional updates, flushing changes to persistence, and managing recovery for the application.

libpmemobj requires the -std=gnu99 compilation flag to build properly.

libpmemobj is one of a collection of persistent memory libraries available. The others are:

•

libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks with atomic updates.

•

libpmemlog(7), providing a pmem-resident log file.

•

libpmem(7), low-level persistent memory support.

Under normal usage, libpmemobj will never print messages or intentionally cause the process to exit. The only exception to this is the debugging information, when enabled, as described under DEBUGGING AND ERROR HANDLING, below.

LIBRARY API VERSIONING¶

This section describes how the library API is versioned, allowing applications to work with an evolving API.

The pmemobj_check_version() function is used to see if the installed libpmemobj supports the version of the library API required by an application. The easiest way to do this is for the application to supply the compile-time version information, supplied by defines in <libpmemobj.h>, like this:

reason = pmemobj_check_version(PMEMOBJ_MAJOR_VERSION,


                               PMEMOBJ_MINOR_VERSION);
if (reason != NULL) {


    /* version check failed, reason string tells you why */
}
    

Any mismatch in the major version number is considered a failure, but a library with a newer minor version number will pass this check since increasing minor versions imply backwards compatibility.

An application can also check specifically for the existence of an interface by checking for the version where that interface was introduced. These versions are documented in this man page as follows: unless otherwise specified, all interfaces described here are available in version 1.0 of the library. Interfaces added after version 1.0 will contain the text introduced in version x.y in the section of this manual describing the feature.

On success, pmemobj_check_version() returns NULL. Otherwise, the return value is a static string describing the reason the version check failed. The string returned by pmemobj_check_version() must not be modified or freed.

MANAGING LIBRARY BEHAVIOR¶

The pmemobj_set_funcs() function allows an application to override memory allocation calls used internally by libpmemobj. Passing in NULL for any of the handlers will cause the libpmemobj default function to be used. The library does not make heavy use of the system malloc functions, but it does allocate approximately 4-8 kilobytes for each memory pool in use.

By default, libpmemobj supports up to 1024 parallel transactions/allocations. For debugging purposes it is possible to decrease this value by setting the PMEMOBJ_NLANES environment variable to the desired limit.

DEBUGGING AND ERROR HANDLING¶

If an error is detected during the call to a libpmemobj function, the application may retrieve an error message describing the reason for the failure from pmemobj_errormsg(). This function returns a pointer to a static buffer containing the last error message logged for the current thread. If errno was set, the error message may include a description of the corresponding error code as returned by strerror(3). The error message buffer is thread-local; errors encountered in one thread do not affect its value in other threads. The buffer is never cleared by any library function; its content is significant only when the return value of the immediately preceding call to a libpmemobj function indicated an error, or if errno was set. The application must not modify or free the error message string, but it may be modified by subsequent calls to other library functions.

Two versions of libpmemobj are typically available on a development system. The normal version, accessed when a program is linked using the -lpmemobj option, is optimized for performance. That version skips checks that impact performance and never logs any trace information or performs any run-time assertions.

A second version of libpmemobj, accessed when a program uses the libraries under /usr/lib/pmdk_debug, contains run-time assertions and trace points. The typical way to access the debug version is to set the environment variable LD_LIBRARY_PATH to /usr/lib/pmdk_debug or /usr/lib64/pmdk_debug, as appropriate. Debugging output is controlled using the following environment variables. These variables have no effect on the non-debug version of the library.

•

PMEMOBJ_LOG_LEVEL

The value of PMEMOBJ_LOG_LEVEL enables trace points in the debug version of the library, as follows:

•

0 - This is the default level when PMEMOBJ_LOG_LEVEL is not set. No log messages are emitted at this level.

•

1 - Additional details on any errors detected are logged, in addition to returning the errno-based errors as usual. The same information may be retrieved using pmemobj_errormsg().

•

2 - A trace of basic operations is logged.

•

3 - Enables a very verbose amount of function call tracing in the library.

•

4 - Enables voluminous and fairly obscure tracing information that is likely only useful to the libpmemobj developers.

Unless PMEMOBJ_LOG_FILE is set, debugging output is written to stderr.

•

PMEMOBJ_LOG_FILE

Specifies the name of a file where all logging information should be written. If the last character in the name is “-”, the PID of the current process will be appended to the file name when the log file is created. If PMEMOBJ_LOG_FILE is not set, logging output is written to stderr.

See also libpmem(7) to get information about other environment variables affecting libpmemobj behavior.

EXAMPLE¶

See <https://pmem.io/pmdk/libpmemobj> for examples using the libpmemobj API.

ACKNOWLEDGEMENTS¶

libpmemobj builds on the persistent memory programming model recommended by the SNIA NVM Programming Technical Work Group: <https://snia.org/nvmp>

SEE ALSO¶

OID_IS_NULL(3), pmemobj_alloc(3), pmemobj_ctl_exec(3), pmemobj_ctl_get(3), pmemobj_ctl_set(3), pmemobj_first(3), pmemobj_list_insert(3), pmemobj_memcpy_persist(3), pmemobj_mutex_zero(3), pmemobj_open(3), pmemobj_root(3), pmemobj_tx_add_range(3), pmemobj_tx_alloc(3), pmemobj_tx_begin(3), POBJ_LAYOUT_BEGIN(3), POBJ_LIST_HEAD(3), strerror(3), TOID_DECLARE(3), libpmem(7), libpmemblk(7), libpmemlog(7) and <https://pmem.io>