Ubuntu Manpage: libvmem - volatile memory allocation library

NAME

       libvmem - volatile memory allocation library

SYNOPSIS

              #include <libvmem.h>
              cc ... -lvmem

   Managing overall library behavior:
              const char *vmem_check_version(
                  unsigned major_required,
                  unsigned minor_required);

              void vmem_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),
                  void (*print_func)(const char *s));

   Error handling:
              const char *vmem_errormsg(void);

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

       • memory pool management: vmem_create(3)

       • memory allocation related functions: vmem_malloc(3)

DESCRIPTION

libvmem provides common malloc-like interfaces to memory pools built on memory-mapped
files. These interfaces are for traditional volatile memory allocation but, unlike the
functions described in malloc(3), the memory managed by libvmem may have different
attributes, depending on the file system containing the memory-mapped files.

It is recommended that new code uses memkind(3) instead of libvmem, as this library is no
longer actively developed and lacks certain features of memkind such as NUMA awareness.
Nevertheless, it is mature, and is expected to be maintained for foreseable future.

libvmem uses the mmap(2) system call to create a pool of volatile memory. The library is
most useful when used with Direct Access storage (DAX), which is memory-addressable
persistent storage that supports load/store access without being paged via the system page
cache. A Persistent Memory-aware file system is typically used to provide this type of
access. Memory-mapping a file from a Persistent Memory-aware file system provides the raw
memory pools, and this library supplies the more familiar malloc-like interfaces on top of
those pools.

Under normal usage, libvmem will never print messages or intentionally cause the process
to exit. Exceptions to this are prints caused by calls to vmem_stats_print(3), or by
enabling debugging as described under DEBUGGING AND ERROR HANDLING below. The library
uses pthreads to be fully MT-safe, but never creates or destroys threads itself. The
library does not make use of any signals, networking, and never calls select(2) or
poll(2). The system memory allocation routines like malloc(3) and free(3) are used by
libvmem for managing a small amount of run-time state, but applications are allowed to
override these calls if necessary (see the description of vmem_set_funcs() below).

libvmem interfaces are grouped into three categories: those that manage memory pools,
those providing the basic memory allocation functions, and those interfaces less commonly
used for managing the overall library behavior.

MANAGING LIBRARY BEHAVIOR

       The  vmem_check_version()  function  is  used to see if the installed libvmem 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
       <libvmem.h>, like this:

              reason = vmem_check_version(VMEM_MAJOR_VERSION,
                                          VMEM_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  is  successful,  vmem_check_version()  returns  NULL.   Otherwise,
       vmem_check_version() returns a static string describing the reason for failing the version
       check.  The returned string must not be modified or freed.

       The vmem_set_funcs() function allows an  application  to  override  some  interfaces  used
       internally  by  libvmem.   Passing  NULL  for  any  of the handlers will cause the libvmem
       default function to be used.  The only functions in the malloc family used by the  library
       are  represented  by the first four arguments to vmem_set_funcs().  While the library does
       not make heavy use of the system malloc functions,  it  does  allocate  approximately  4-8
       kilobytes  for each memory pool in use.  The print_func function is called by libvmem when
       the vmem_stats_print() entry point is used, or when additional tracing is enabled  in  the
       debug  version  of  the  library as described in DEBUGGING AND ERROR HANDLING, below.  The
       default print_func used by the library prints to the file specified by  the  VMEM_LOG_FILE
       environment variable, or to stderr if that variable is not set.

CAVEATS

       libvmem  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.

DEBUGGING AND ERROR HANDLING

       If an error is detected during the  call  to  a  libvmem  function,  the  application  may
       retrieve  an  error  message  describing  the reason for the failure from vmem_errormsg().
       This 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 libvmem 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 libvmem are typically available  on  a  development  system.   The  normal
       version  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,  accessed  when  using  libraries  from  /usr/lib/vmem_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/vmem_debug or /usr/lib64/vmem_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.

       • VMEM_LOG_LEVEL

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

       • 0 - Tracing is disabled.  This is the default level  when  VMEM_LOG_LEVEL  is  not  set.
         Only statistics are logged, and then only in response to a call to vmem_stats_print().

       • 1  -  Additional details on any errors detected are logged, in addition to returning the
         errno-based errors as usual.

       • 2 - A trace of basic operations is logged.

       • 3 - Enables a very verbose amount of function call tracing in the library.

       • 4  -  Enables  voluminous  tracing  information  about  all   memory   allocations   and
         deallocations.

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

       • VMEM_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 VMEM_LOG_FILE is not set, output is written to
       stderr.

EXAMPLE

       The following example creates a memory pool, allocates some memory to contain  the  string
       “hello, world”, and then frees that memory.

              #include <stdio.h>
              #include <stdlib.h>
              #include <string.h>
              #include <libvmem.h>

              int
              main(int argc, char *argv[])
              {
                  VMEM *vmp;
                  char *ptr;

                  /* create minimum size pool of memory */
                  if ((vmp = vmem_create("/pmem-fs",
                          VMEM_MIN_POOL)) == NULL) {
                      perror("vmem_create");
                      exit(1);
                  }

                  if ((ptr = vmem_malloc(vmp, 100)) == NULL) {
                      perror("vmem_malloc");
                      exit(1);
                  }

                  strcpy(ptr, "hello, world");

                  /* give the memory back */
                  vmem_free(vmp, ptr);

                  /* ... */

                  vmem_delete(vmp);
              }

       See <http://pmem.io/vmem/libvmem> for more examples using the libvmem API.

BUGS

       Unlike the normal malloc(3), which asks the system for additional memory when it runs out,
       libvmem allocates the size it is told to and never attempts to grow or shrink that  memory
       pool.

ACKNOWLEDGEMENTS

       libvmem  depends  on jemalloc, written by Jason Evans, to do the heavy lifting of managing
       dynamic memory allocation.  See: <http://www.canonware.com/jemalloc>

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