Ubuntu Manpage: libpmem - persistent memory support library

Provided by: libpmem-dev_1.13.1-1.1ubuntu2_amd64

NAME

       libpmem - persistent memory support library

              NOTE: Support for Windows and FreeBSD are deprecated since PMDK 1.13.0 release and will be removed
              in the PMDK 1.14.0 release.

SYNOPSIS

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

   Library API versioning:
              const char *pmem_check_version(
                  unsigned major_required,
                  unsigned minor_required);

   Error handling:
              const char *pmem_errormsg(void);

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

       • most commonly used functions: pmem_is_pmem(3)

       • partial flushing operations: pmem_flush(3)

       • copying to persistent memory: pmem_memmove_persist(3)

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  de‐
       vice.   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 applica‐
       tions.  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:

       • libpmemobj(7), a general use persistent memory API, providing memory allocation and transactional oper‐
         ations on variable-sized objects.

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

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

       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  func‐
       tions  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_version() 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 sup‐
       ply the compile-time version information, supplied by defines in <libpmem.h>, like this:

              reason = pmem_check_version(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 ver‐
       sion 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.   Inter‐
       faces added after version 1.0 will contain the text introduced in version x.y in the section of this man‐
       ual describing the feature.

       When the version check performed by pmem_check_version() is successful, the return value is NULL.  Other‐
       wise the return value is a static string describing the reason for failing the version check.  The string
       returned by pmem_check_version() 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.

• PMEM_IS_PMEM_FORCE=val

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.

• PMEM_NO_CLWB=1

Setting this environment variable to 1 forces libpmem to never issue the CLWB instruction on Intel hard‐
ware, 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 test‐
ing but may be required for some rare cases where using CLWB has a negative impact on performance.

• PMEM_NO_CLFLUSHOPT=1

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.

• PMEM_NO_FLUSH=1

Setting this environment variable to 1 forces most libpmem functions to never issue any of CLFLUSH,
CLFLUSHOPT or CLWB instructions on Intel hardware. The only exceptions are pmem_deep_flush(3) and
pmem_deep_persist(3) functions.

• PMEM_NO_FLUSH=0

Setting this environment variable to 0 forces to always flush CPU caches using one of CLFLUSH, CLFLUSHOPT
or CLWB instructions even if pmem_has_auto_flush(3) function returns true and the platform supports
flushing the processor caches on power loss or system crash.

• PMEM_NO_MOVNT=1

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.

• PMEM_MOVNT_THRESHOLD=val

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.

• PMEM_MMAP_HINT=val

This environment variable allows overriding the hint address used by pmem_map_file(). If set, it also
disables mapping address randomization. This variable is intended for use during library testing and de‐
bugging. 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, with‐
out 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 con‐
version 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 map‐
ping.

DEBUGGING AND ERROR HANDLING

       If an error is detected during the call to a libpmem function, the application may retrieve an error mes‐
       sage describing the reason of the failure from pmem_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 er‐
       ror  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.  The application
       must not modify or free the error message string.  Subsequent calls to other library functions may modify
       the previous message.

       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  /usr/lib/pmdk_debug,  con‐
       tains  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.

              NOTE:  On Debian/Ubuntu systems, this extra debug version of the library is shipped in the respec‐
              tive -debug Debian package and placed in the /usr/lib/$ARCH/pmdk_dbg/ directory.

       • PMEM_LOG_LEVEL

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

       • 0 - This is the default level when PMEM_LOG_LEVEL is not set.  No log messages are emitted at this lev‐
         el.

       • 1 - Additional details on any errors detected are logged, in addition to returning the errno-based  er‐
         rors as usual.  The same information may be retrieved using pmem_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 libpmem
         developers.

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

       • PMEM_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 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 li‐
              brary 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_file(PATH, PMEM_LEN, PMEM_FILE_CREATE,
                          0666, &mapped_len, &is_pmem)) == NULL) {
                      perror("pmem_map_file");
                      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 <https://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 Techni‐
       cal Work Group: <https://snia.org/nvmp>