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libvmem.7 12.40 KiB 
.\" Automatically generated by Pandoc 2.0.6
.\"
.TH "LIBVMEM" "7" "2019-09-26" "PMDK - vmem API version 1.1" "PMDK Programmer's Manual"
.hy
.\" Copyright 2014-2019, Intel Corporation
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.SH NAME
.PP
\f[B]libvmem\f[] \- volatile memory allocation library
.SH SYNOPSIS
.IP
.nf
\f[C]
#include\ <libvmem.h>
cc\ ...\ \-lvmem
\f[]
.fi
.SS Managing overall library behavior:
.IP
.nf
\f[C]
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));
\f[]
.fi
.SS Error handling:
.IP
.nf
\f[C]
const\ char\ *vmem_errormsg(void);
\f[]
.fi
.SS Other library functions:
.PP
A description of other \f[B]libvmem\f[] functions can be found on the
following manual pages:
.IP \[bu] 2
memory pool management: \f[B]vmem_create\f[](3)
.IP \[bu] 2
memory allocation related functions: \f[B]vmem_malloc\f[](3)
.SH DESCRIPTION
.PP
\f[B]libvmem\f[] provides common \f[I]malloc\f[]\-like interfaces to
memory pools built on memory\-mapped files.
These interfaces are for traditional \f[B]volatile\f[] memory allocation
but, unlike the functions described in \f[B]malloc\f[](3), the memory
managed by \f[B]libvmem\f[] may have different attributes, depending on
the file system containing the memory\-mapped files.
In particular, \f[B]libvmem\f[] is part of the \f[I]Persistent Memory
Development Kit\f[] because it is sometimes useful to use non\-volatile
memory as a volatile memory pool, leveraging its capacity, cost, or
performance characteristics.
.PP
It is recommended that new code uses \f[B]memkind\f[](3) instead of
\f[B]libvmem\f[], as this library is no longer actively developed and
lacks certain features of \f[B]memkind\f[] such as NUMA awareness.
Nevertheless, it is mature, and is expected to be maintained for
foreseable future.
.PP
\f[B]libvmem\f[] uses the \f[B]mmap\f[](2) system call to create a pool
of volatile memory.
The library is most useful when used with \f[I]Direct Access\f[] 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 \f[I]malloc\f[]\-like interfaces on top of those pools.
.PP
Under normal usage, \f[B]libvmem\f[] will never print messages or
intentionally cause the process to exit.
Exceptions to this are prints caused by calls to
\f[B]vmem_stats_print\f[](3), or by enabling debugging as described
under \f[B]DEBUGGING AND ERROR HANDLING\f[] below.
The library uses \f[B]pthreads\f[] 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 \f[B]select\f[](2) or \f[B]poll\f[](2).
The system memory allocation routines like \f[B]malloc\f[](3) and
\f[B]free\f[](3) are used by \f[B]libvmem\f[] for managing a small
amount of run\-time state, but applications are allowed to override
these calls if necessary (see the description of
\f[B]vmem_set_funcs\f[]() below).
.PP
\f[B]libvmem\f[] 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.
.SH MANAGING LIBRARY BEHAVIOR
.PP
The \f[B]vmem_check_version\f[]() function is used to see if the
installed \f[B]libvmem\f[] 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
\f[B]<libvmem.h>\f[], like this:
.IP
.nf
\f[C]
reason\ =\ vmem_check_version(VMEM_MAJOR_VERSION,
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ VMEM_MINOR_VERSION);
if\ (reason\ !=\ NULL)\ {
\ \ \ \ /*\ version\ check\ failed,\ reason\ string\ tells\ you\ why\ */
}
\f[]
.fi
.PP
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.
.PP
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 \f[I]introduced
in version x.y\f[] in the section of this manual describing the feature.
.PP
When the version check is successful, \f[B]vmem_check_version\f[]()
returns NULL.
Otherwise, \f[B]vmem_check_version\f[]() returns a static string
describing the reason for failing the version check.
The returned string must not be modified or freed.
.PP
The \f[B]vmem_set_funcs\f[]() function allows an application to override
some interfaces used internally by \f[B]libvmem\f[].
Passing NULL for any of the handlers will cause the \f[B]libvmem\f[]
default function to be used.
The only functions in the malloc family used by the library are
represented by the first four arguments to \f[B]vmem_set_funcs\f[]().
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 \f[I]print_func\f[] function is called by \f[B]libvmem\f[] when the
\f[B]vmem_stats_print\f[]() entry point is used, or when additional
tracing is enabled in the debug version of the library as described in
\f[B]DEBUGGING AND ERROR HANDLING\f[], below.
The default \f[I]print_func\f[] used by the library prints to the file
specified by the \f[B]VMEM_LOG_FILE\f[] environment variable, or to
\f[I]stderr\f[] if that variable is not set.
.SH CAVEATS
.PP
\f[B]libvmem\f[] relies on the library destructor being called from the
main thread.
For this reason, all functions that might trigger destruction (e.g.
\f[B]dlclose\f[](3)) should be called in the main thread.
Otherwise some of the resources associated with that thread might not be
cleaned up properly.
.SH DEBUGGING AND ERROR HANDLING
.PP
If an error is detected during the call to a \f[B]libvmem\f[] function,
the application may retrieve an error message describing the reason for
the failure from \f[B]vmem_errormsg\f[]().
This function returns a pointer to a static buffer containing the last
error message logged for the current thread.
If \f[I]errno\f[] was set, the error message may include a description
of the corresponding error code as returned by \f[B]strerror\f[](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 \f[B]libvmem\f[] function indicated an error, or if \f[I]errno\f[]
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.
.PP
Two versions of \f[B]libvmem\f[] 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
\f[B]/usr/lib/pmdk_debug\f[], contains run\-time assertions and trace
points.
The typical way to access the debug version is to set the
\f[B]LD_LIBRARY_PATH\f[] environment variable to
\f[B]/usr/lib/pmdk_debug\f[] or \f[B]/usr/lib64/pmdk_debug\f[], as
appropriate.
Debugging output is controlled using the following environment
variables.
These variables have no effect on the non\-debug version of the library.
.IP \[bu] 2
\f[B]VMEM_LOG_LEVEL\f[]
.PP
The value of \f[B]VMEM_LOG_LEVEL\f[] enables trace points in the debug
version of the library, as follows:
.IP \[bu] 2
\f[B]0\f[] \- Tracing is disabled.
This is the default level when \f[B]VMEM_LOG_LEVEL\f[] is not set.
Only statistics are logged, and then only in response to a call to
\f[B]vmem_stats_print\f[]().
.IP \[bu] 2
\f[B]1\f[] \- Additional details on any errors detected are logged, in
addition to returning the \f[I]errno\f[]\-based errors as usual.
.IP \[bu] 2
\f[B]2\f[] \- A trace of basic operations is logged.
.IP \[bu] 2
\f[B]3\f[] \- Enables a very verbose amount of function call tracing in
the library.
.IP \[bu] 2
\f[B]4\f[] \- Enables voluminous tracing information about all memory
allocations and deallocations.
.PP
Unless \f[B]VMEM_LOG_FILE\f[] is set, debugging output is written to
\f[I]stderr\f[].
.IP \[bu] 2
\f[B]VMEM_LOG_FILE\f[]
.PP
Specifies the name of a file where all logging information should be
written.
If the last character in the name is \[lq]\-\[rq], the \f[I]PID\f[] of
the current process will be appended to the file name when the log file
is created.
If \f[B]VMEM_LOG_FILE\f[] is not set, output is written to
\f[I]stderr\f[].
.SH EXAMPLE
.PP
The following example creates a memory pool, allocates some memory to
contain the string \[lq]hello, world\[rq], and then frees that memory.
.IP
.nf
\f[C]
#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);
}
\f[]
.fi
.PP
See <http://pmem.io/pmdk/libvmem> for more examples using the
\f[B]libvmem\f[] API.
.SH BUGS
.PP
Unlike the normal \f[B]malloc\f[](3), which asks the system for
additional memory when it runs out, \f[B]libvmem\f[] allocates the size
it is told to and never attempts to grow or shrink that memory pool.
.SH ACKNOWLEDGEMENTS
.PP
\f[B]libvmem\f[] depends on jemalloc, written by Jason Evans, to do the
heavy lifting of managing dynamic memory allocation.
See: <http://www.canonware.com/jemalloc>
.PP
\f[B]libvmem\f[] builds on the persistent memory programming model
recommended by the SNIA NVM Programming Technical Work Group:
<http://snia.org/nvmp>
.SH SEE ALSO
.PP
\f[B]mmap\f[](2), \f[B]dlclose\f[](3), \f[B]malloc\f[](3),
\f[B]strerror\f[](3), \f[B]vmem_create\f[](3), \f[B]vmem_malloc\f[](3),
and \f[B]<http://pmem.io>\f[]
.PP
On Linux:
.PP
\f[B]jemalloc\f[](3), \f[B]pthreads\f[](7)
.PP
On FreeBSD:
.PP
\f[B]pthread\f[](3)