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

NAME

       libpmemlog - persistent memory resident log file

SYNOPSIS

              #include <libpmemlog.h>
              cc ... -lpmemlog -lpmem

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

   Managing library behavior:
              void pmemlog_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:
              int pmemlog_check(const char *path);

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

       pmemlog_append(3),     pmemlog_create(3),     pmemlog_ctl_exec(3),     pmemlog_ctl_get(3),
       pmemlog_ctl_set(3), pmemlog_nbyte(3), pmemlog_tell(3)

DESCRIPTION

       libpmemlog provides a log file in persistent memory (pmem) such that additions to the  log
       are  appended  atomically.   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.   libpmemlog
       builds on thistype of memory mapped file.

       This  library  is for applications that need a persistent log file updated atomically (the
       updates cannot be torn by program interruption such  as  power  failures).   This  library
       builds  on  the  low-level pmem support provided by libpmem(7), handling the transactional
       update of the log, flushing to persistence, and recovery for the application.

       libpmemlog 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.

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

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

       Under normal usage, libpmemlog 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 pmem-resident log file provided by libpmemlog, a memory pool is first  created.
       This  is done with the pmemlog_create(3) function.  The other functions mentioned above in
       SYNOPSIS section then operate on the resulting log memory pool.

       Once created, the memory pool is represented by an opaque handle,  of  type  PMEMlogpool*,
       which  is  passed  to most of the other functions from libpmemlog.  Internally, libpmemlog
       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  log  memory  API provided by
       libpmemlog.

CAVEATS

       libpmemlog 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 pmemlog_check_version() function is used to determine whether the installed libpmemlog
       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  provided  by
       defines in <libpmemlog.h>, like this:

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

MANAGING LIBRARY BEHAVIOR

       The pmemlog_set_funcs() function allows an application to override memory allocation calls
       used internally by libpmemlog.  Passing in NULL for any of the  handlers  will  cause  the
       libpmemlog 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  pmemlog_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
       libpmemlog  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  libpmemlog are typically available on a development system.  The normal
       version, accessed when a program is linked using the -lpmemlog 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  libpmemlog,  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.

       · PMEMLOG_LOG_LEVEL

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

       · 0 - This is the default level when PMEMLOG_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
         pmemlog_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 libpmemlog developers.

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

       · PMEMLOG_LOG_FILE

       Specifies the name of a file name 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 PMEMLOG_LOG_FILE is not set, logging output is
       written to stderr.

       See also libpmem(7) for information about other environment variables affecting libpmemlog
       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 libpmemlog API is used.

              #include <stdio.h>
              #include <fcntl.h>
              #include <errno.h>
              #include <stdlib.h>
              #include <unistd.h>
              #include <string.h>
              #include <libpmemlog.h>

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

              /*
               * printit -- log processing callback for use with pmemlog_walk()
               */
              int
              printit(const void *buf, size_t len, void *arg)
              {
                  fwrite(buf, len, 1, stdout);
                  return 0;
              }

              int
              main(int argc, char *argv[])
              {
                  const char path[] = "/pmem-fs/myfile";
                  PMEMlogpool *plp;
                  size_t nbyte;
                  char *str;

                  /* create the pmemlog pool or open it if it already exists */
                  plp = pmemlog_create(path, POOL_SIZE, 0666);

                  if (plp == NULL)
                      plp = pmemlog_open(path);

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

                  /* how many bytes does the log hold? */
                  nbyte = pmemlog_nbyte(plp);
                  printf("log holds %zu bytes", nbyte);

                  /* append to the log... */
                  str = "This is the first string appended";
                  if (pmemlog_append(plp, str, strlen(str)) < 0) {
                      perror("pmemlog_append");
                      exit(1);
                  }
                  str = "This is the second string appended";
                  if (pmemlog_append(plp, str, strlen(str)) < 0) {
                      perror("pmemlog_append");
                      exit(1);
                  }

                  /* print the log contents */
                  printf("log contains:");
                  pmemlog_walk(plp, 0, printit, NULL);

                  pmemlog_close(plp);
              }

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

BUGS

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

ACKNOWLEDGEMENTS

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

SEE ALSO

       msync(2), pmemlog_append(3), pmemlog_create(3),  pmemlog_ctl_exec(3),  pmemlog_ctl_get(3),
       pmemlog_ctl_set(3),    pmemlog_nbyte(3),    pmemlog_tell(3),    strerror(3),   libpmem(7),
       libpmemblk(7), libpmemobj(7) and <http://pmem.io>