focal (7) libpmemblk.7.gz

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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(7),  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 <libpmemblk.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.

              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.

       • 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.

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