focal (7) libpmemlog.7.gz

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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.

              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.

       • 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.

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 <https://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: <https://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 <https://pmem.io>