Ubuntu Manpage: libvmem - volatile memory allocation library

Provided by: libvmem-dev_1.4.1-0ubuntu1~18.04.1_amd64

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 in‐
terfaces 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. In particular, libvmem is part of the Persistent Memory Development Kit because it
is sometimes useful to use non-volatile memory as a volatile memory pool, leveraging its capacity, cost,
or performance characteristics.

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 inter‐
faces on top of those pools.

Under normal usage, libvmem will never print messages or intentionally cause the process to exit. Excep‐
tions 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 cre‐
ates 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 li‐
brary behavior.

MANAGING LIBRARY BEHAVIOR

       The vmem_check_version() function is used to see if the installed libvmem supports the version of the li‐
       brary 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 ver‐
       sion 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.   Inter‐
       faces added after version 1.0 will contain the text introduced in version x.y in the section of this man‐
       ual 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  lib‐
       vmem.  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  al‐
       locate  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 de‐
       bug 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  func‐
       tions  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 mes‐
       sage 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 er‐
       ror  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 subse‐
       quent 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/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_de‐
       bug, 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  er‐
         rors 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.

              NOTE: on Ubuntu systems, this extra debug version of the library is shipped in the respective -de‐
              bug Debian package and placed in the /usr/lib/$ARCH/pmdk_dbg/ directory.

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/pmdk/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  allo‐
       cates 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 Techni‐
       cal Work Group: <http://snia.org/nvmp>