PMDK man page

libpmem

NAME
SYNOPSIS
DESCRIPTION
CAVEATS
LIBRARY API VERSIONING
ENVIRONMENT
DEBUGGING AND ERROR HANDLING
EXAMPLE
ACKNOWLEDGEMENTS
SEE ALSO

NAME

libpmem – persistent memory support library

SYNOPSIS

#include <libpmem.h>
cc ... -lpmem

NOTE: The PMDK API supports UNICODE. If the PMDK_UTF8_API macro is defined, basic API functions are expanded to the UTF-8 API with postfix U. Otherwise they are expanded to the UNICODE API with postfix W.

Library API versioning:
const char *pmem_check_versionU(
	unsigned major_required,
	unsigned minor_required);
const wchar_t *pmem_check_versionW(
	unsigned major_required,
	unsigned minor_required);
Error handling:
const char *pmem_errormsgU(void);
const wchar_t *pmem_errormsgW(void);
Other library functions:

A description of other libpmem functions can be found on the following manual pages:

DESCRIPTION

libpmem provides low-level persistent memory (pmem) support for applications 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 the load/store, non-paged access to pmem.

This library is for applications that use persistent memory directly, without the help of any library-supplied transactions or memory allocation. Higher-level libraries that build on libpmem are available and are recommended for most applications, see:

Under normal usage, libpmem 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.

CAVEATS

libpmem relies on the library destructor being called from the main thread. For this reason, all functions that might trigger destruction (e.g. dlclose(3)) should be called in the main thread. Otherwise some of the resources associated with that thread might not be cleaned up properly.

LIBRARY API VERSIONING

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

The pmem_check_versionU()/pmem_check_versionW() function is used to determine whether the installed libpmem 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 <libpmem.h>, like this:

reason = pmem_check_versionU(PMEM_MAJOR_VERSION,
                            PMEM_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.

When the version check performed by pmem_check_versionU()/pmem_check_versionW() is successful, the return value is NULL. Otherwise the return value is a static string describing the reason for failing the version check. The string returned by pmem_check_versionU()/pmem_check_versionW() must not be modified or freed.

ENVIRONMENT

libpmem can change its default behavior based on the following environment variables. These are largely intended for testing and are not normally required.

If val is 0 (zero), then pmem_is_pmem(3) will always return false. Setting val to 1 causes pmem_is_pmem(3) to always return true. This variable is mostly used for testing but can be used to force pmem behavior on a system where a range of pmem is not detectable as pmem for some reason.

NOTE: Unlike the other variables, the value of PMEM_IS_PMEM_FORCE is not queried (and cached) at library initialization time, but on the first call to pmem_is_pmem(3). This means that in case of libpmemlog(7), libpmemblk(7), and libpmemobj(7), PMEM_IS_PMEM_FORCE may still be set or modified by the program until the first attempt to create or open the persistent memory pool.

Setting this environment variable to 1 forces libpmem to never issue the CLWB instruction on Intel hardware, falling back to other cache flush instructions instead (CLFLUSHOPT or CLFLUSH on Intel hardware). Without this environment variable, libpmem will always use the CLWB instruction for flushing processor caches on platforms that support the instruction. This variable is intended for use during library testing but may be required for some rare cases where using CLWB has a negative impact on performance.

Setting this environment variable to 1 forces libpmem to never issue the CLFLUSHOPT instruction on Intel hardware, falling back to the CLFLUSH instructions instead. Without this environment variable, libpmem will always use the CLFLUSHOPT instruction for flushing processor caches on platforms that support the instruction, but where CLWB is not available. This variable is intended for use during library testing.

Setting this environment variable to 1 forces libpmem to never issue any of CLFLUSH, CLFLUSHOPT or CLWB instructions on Intel hardware. This variable is intended for use during library testing.

Setting this environment variable to 1 forces libpmem to never use the non-temporal move instructions on Intel hardware. Without this environment variable, libpmem will use the non-temporal instructions for copying larger ranges to persistent memory on platforms that support the instructions. This variable is intended for use during library testing.

This environment variable allows overriding the minimum length of the pmem_memmove_persist(3) operations, for which libpmem uses non-temporal move instructions. Setting this environment variable to 0 forces libpmem to always use the non-temporal move instructions if available. It has no effect if PMEM_NO_MOVNT is set to 1. This variable is intended for use during library testing.

This environment variable allows overriding the hint address used by pmem_map_fileU()/pmem_map_fileW(). If set, it also disables mapping address randomization. This variable is intended for use during library testing and debugging. Setting it to some fairly large value (i.e. 0x10000000000) will very likely result in mapping the file at the specified address (if not used) or at the first unused region above given address, without adding any random offset. When debugging, this makes it easier to calculate the actual address of the persistent memory block, based on its offset in the file. In case of libpmemobj it simplifies conversion of a persistent object identifier (OID) into a direct pointer to the object.

NOTE: Setting this environment variable affects all the PMDK libraries, disabling mapping address randomization and causing the specified address to be used as a hint about where to place the mapping.

DEBUGGING AND ERROR HANDLING

If an error is detected during the call to a libpmem function, the application may retrieve an error message describing the reason for the failure from pmem_errormsgU()/pmem_errormsgW(). 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 libpmem 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 libpmem are typically available on a development system. The normal version, accessed when a program is linked using the -lpmem 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 libpmem, accessed when a program uses the libraries under /pmdk/src/x64/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 /pmdk/src/x64/Debug. Debugging output is controlled using the following environment variables. These variables have no effect on the non-debug version of the library.

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

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

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 PMEM_LOG_FILE is not set, output is written to stderr.

EXAMPLE

The following example uses libpmem to flush changes made to raw, memory-mapped persistent memory.

WARNING: There is nothing transactional about the pmem_persist(3) or pmem_msync(3) calls in this example. Interrupting the program may result in a partial write to pmem. Use a transactional library such as libpmemobj(7) to avoid torn updates.

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libpmem.h>

/* using 4k of pmem for this example */
#define PMEM_LEN 4096

#define PATH "/pmem-fs/myfile"

int
main(int argc, char *argv[])
{
	char *pmemaddr;
	size_t mapped_len;
	int is_pmem;

	/* create a pmem file and memory map it */

	if ((pmemaddr = pmem_map_fileU(PATH, PMEM_LEN, PMEM_FILE_CREATE,
			0666, &mapped_len, &is_pmem)) == NULL) {
		perror("pmem_map_fileU");
		exit(1);
	}

	/* store a string to the persistent memory */
	strcpy(pmemaddr, "hello, persistent memory");

	/* flush above strcpy to persistence */
	if (is_pmem)
		pmem_persist(pmemaddr, mapped_len);
	else
		pmem_msync(pmemaddr, mapped_len);

	/*
	 * Delete the mappings. The region is also
	 * automatically unmapped when the process is
	 * terminated.
	 */
	pmem_unmap(pmemaddr, mapped_len);
}

See http://pmem.io/pmdk/libpmem for more examples using the libpmem API.

ACKNOWLEDGEMENTS

libpmem builds on the persistent memory programming model recommended by the SNIA NVM Programming Technical Work Group: http://snia.org/nvmp

SEE ALSO

dlclose(3), pmem_flush(3), pmem_is_pmem(3), pmem_memmove_persist(3), pmem_msync(3), pmem_persist(3), strerror(3), libpmemblk(7), libpmemcto(7), libpmemlog(7), libpmemobj(7) and http://pmem.io