Ubuntu Manpage: libpmemblk - persistent memory resident array of blocks (DEPRECATED)

Provided by: libpmemblk-dev_1.13.1-1.1ubuntu2_amd64

NAME

       libpmemblk - persistent memory resident array of blocks (DEPRECATED)

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

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.

NOTE

              NOTICE: The libpmemblk library is deprecated since PMDK 1.13.0 release and will be removed in  the
              PMDK 1.14.0 release.

ACKNOWLEDGEMENTS

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