Provided by: libpmemblk-dev_1.6.1-1ubuntu1_amd64 bug

NAME

       libpmemblk - persistent memory resident array of blocks

SYNOPSIS

              #include <libpmemblk.h>
              cc ... -lpmemblk -lpmem

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

   Managing library behavior:
              void pmemblk_set_funcs(
                  void *(*malloc_func)(size_t size),
                  void (*free_func)(void *ptr),
                  void *(*realloc_func)(void *ptr, size_t size),
                  char *(*strdup_func)(const char *s));

   Error handling:
              const char *pmemblk_errormsg(void);

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

       pmemblk_bsize(3),      pmemblk_create(3),     pmemblk_ctl_exec(3),     pmemblk_ctl_get(3),
       pmemblk_ctl_set(3), pmemblk_read(3), pmemblk_set_zero(3),

DESCRIPTION

       libpmemblk provides an array of blocks in persistent memory (pmem) such that updates to  a
       single  block  are  atomic.  This library is intended 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.   libpmemblk
       builds on this type of memory mapped file.

       This  library  is  for applications that need a potentially large array of blocks, all the
       same size, where any given block is updated atomically  (the  update  cannot  be  torn  by
       program  interruption  such as power failures).  This library builds on the low-level pmem
       support provided by libpmem(3), handling the transactional update of the blocks,  flushing
       to  persistence,  and  recovery for the application.  libpmemblk is one of a collection of
       persistent memory libraries available, the others are:

       · libpmemobj(7), a general use persistent memory  API,  providing  memory  allocation  and
         transactional operations on variable-sized objects.

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

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

       Under  normal  usage,  libpmemblk  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.

       To  use  the  atomic  block  arrays supplied by libpmemblk, a memory pool is first created
       using the pmemblk_create() function described in pmemblk_create(3).  The other  libpmemblk
       functions  operate  on  the  resulting  block memory pool using the opaque handle, of type
       PMEMblkpool*,  that  is  returned  by  pmemblk_create()  or  pmemblk_open().   Internally,
       libpmemblk  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 block  memory  API  provided  by
       libpmemblk.

CAVEATS

       libpmemblk  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 pmemblk_check_version() function is used to determine whether the installed libpmemblk
       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 <ibpmemblk.h>, like this:

              reason = pmemblk_check_version(PMEMBLK_MAJOR_VERSION,
                                             PMEMBLK_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  pmemblk_check_version()  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  pmemblk_check_version()  must  not  be
       modified or freed.

MANAGING LIBRARY BEHAVIOR

       The pmemblk_set_funcs() function allows an application to override memory allocation calls
       used internally by libpmemblk.  Passing in NULL for any of the  handlers  will  cause  the
       libpmemblk 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.

DEBUGGING AND ERROR HANDLING

       The  pmemblk_errormsg()  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
       libpmemblk  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  libpmemblk are typically available on a development system.  The normal
       version, accessed when a program is linked using the -lpmemblk option,  is  optimized  for
       performance.   That  version skips checks that impact performance and never logs any trace
       information or performs any run-time assertions.  If an error is detected  in  a  call  to
       libpmemblk,   the   error   message   describing   the   failure  may  be  retrieved  with
       pmemblk_errormsg() as described above.

       A second version  of  libpmemblk,  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  LD_LIBRARY_PATH  environment  variable   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.

       · PMEMBLK_LOG_LEVEL

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

       · 0 - This is the default level when PMEMBLK_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
         pmemblk_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 libpmemblk developers.

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

       · PMEMBLK_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 PMEMBLK_LOG_FILE is not set, the logging output  is
       written to stderr.

       See  also  libpmem(7)  for  information  on  other  environment  variables that may affect
       libpmemblk behavior.

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

EXAMPLE

       The following example illustrates how the libpmemblk API is used.

              #include <fcntl.h>
              #include <errno.h>
              #include <stdlib.h>
              #include <unistd.h>
              #include <string.h>
              #include <libpmemblk.h>

              /* size of the pmemblk pool -- 1 GB */
              #define POOL_SIZE ((size_t)(1 << 30))

              /* size of each element in the pmem pool */
              #define ELEMENT_SIZE 1024

              int
              main(int argc, char *argv[])
              {
                  const char path[] = "/pmem-fs/myfile";
                  PMEMblkpool *pbp;
                  size_t nelements;
                  char buf[ELEMENT_SIZE];

                  /* create the pmemblk pool or open it if it already exists */
                  pbp = pmemblk_create(path, ELEMENT_SIZE, POOL_SIZE, 0666);

                  if (pbp == NULL)
                      pbp = pmemblk_open(path, ELEMENT_SIZE);

                  if (pbp == NULL) {
                      perror(path);
                      exit(1);
                  }

                  /* how many elements fit into the file? */
                  nelements = pmemblk_nblock(pbp);
                  printf("file holds %zu elements", nelements);

                  /* store a block at index 5 */
                  strcpy(buf, "hello, world");
                  if (pmemblk_write(pbp, buf, 5) < 0) {
                      perror("pmemblk_write");
                      exit(1);
                  }

                  /* read the block at index 10 (reads as zeros initially) */
                  if (pmemblk_read(pbp, buf, 10) < 0) {
                      perror("pmemblk_read");
                      exit(1);
                  }

                  /* zero out the block at index 5 */
                  if (pmemblk_set_zero(pbp, 5) < 0) {
                      perror("pmemblk_set_zero");
                      exit(1);
                  }

                  /* ... */

                  pmemblk_close(pbp);
              }

       See <http://pmem.io/pmdk/libpmemblk> for more examples using the libpmemblk API.

BUGS

       Unlike libpmemobj(7), data replication is not supported in libpmemblk.   Thus,  specifying
       replica sections in pool set files is not allowed.

ACKNOWLEDGEMENTS

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

SEE ALSO

       msync(2),   dlclose(3),    pmemblk_bsize(3),    pmemblk_create(3),    pmemblk_ctl_exec(3),
       pmemblk_ctl_get(3),      pmemblk_ctl_set(3),     pmemblk_read(3),     pmemblk_set_zero(3),
       pmem_is_pmem(3), pmem_persist(3), strerror(3),  libpmem(7),  libpmemlog(7),  libpmemobj(7)
       and <http://pmem.io>