.\" Automatically generated by Pandoc 2.1.3
.\"
.TH "LIBPMEM" "7" "2018-10-17" "PMDK - pmem API version 1.1" "PMDK Programmer's Manual"
.hy
.\" Copyright 2014-2018, Intel Corporation
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\"     * Redistributions of source code must retain the above copyright
.\"       notice, this list of conditions and the following disclaimer.
.\"
.\"     * Redistributions in binary form must reproduce the above copyright
.\"       notice, this list of conditions and the following disclaimer in
.\"       the documentation and/or other materials provided with the
.\"       distribution.
.\"
.\"     * Neither the name of the copyright holder nor the names of its
.\"       contributors may be used to endorse or promote products derived
.\"       from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
.\" "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
.\" LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
.\" A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
.\" OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
.\" OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.SH NAME
.PP
\f[B]libpmem\f[] \- persistent memory support library
.SH SYNOPSIS
.IP
.nf
\f[C]
#include\ <libpmem.h>
cc\ ...\ \-lpmem
\f[]
.fi
.SS Library API versioning:
.IP
.nf
\f[C]
const\ char\ *pmem_check_version(
\ \ \ \ unsigned\ major_required,
\ \ \ \ unsigned\ minor_required);
\f[]
.fi
.SS Error handling:
.IP
.nf
\f[C]
const\ char\ *pmem_errormsg(void);
\f[]
.fi
.SS Other library functions:
.PP
A description of other \f[B]libpmem\f[] functions can be found on the
following manual pages:
.IP \[bu] 2
most commonly used functions: \f[B]pmem_is_pmem\f[](3)
.IP \[bu] 2
partial flushing operations: \f[B]pmem_flush\f[](3)
.IP \[bu] 2
copying to persistent memory: \f[B]pmem_memmove_persist\f[](3)
.SH DESCRIPTION
.PP
\f[B]libpmem\f[] provides low\-level \f[I]persistent memory\f[] (pmem)
support 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 \f[I]non\-volatile memory DIMMs\f[] (NVDIMMs) provide this
type of byte addressable access to storage.
A \f[I]persistent memory aware file system\f[] 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.
.PP
This library is for applications that use persistent memory directly,
without the help of any library\-supplied transactions or memory
allocation.
Higher\-level libraries that build on \f[B]libpmem\f[] are available and
are recommended for most applications, see:
.IP \[bu] 2
\f[B]libpmemobj\f[](7), a general use persistent memory API, providing
memory allocation and transactional operations on variable\-sized
objects.
.IP \[bu] 2
\f[B]libpmemblk\f[](7), providing pmem\-resident arrays of fixed\-sized
blocks with atomic updates.
.IP \[bu] 2
\f[B]libpmemlog\f[](7), providing a pmem\-resident log file.
.PP
Under normal usage, \f[B]libpmem\f[] 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 \f[B]DEBUGGING AND ERROR HANDLING\f[] below.
.SH CAVEATS
.PP
\f[B]libpmem\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 LIBRARY API VERSIONING
.PP
This section describes how the library API is versioned, allowing
applications to work with an evolving API.
.PP
The \f[B]pmem_check_version\f[]() function is used to determine whether
the installed \f[B]libpmem\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]<libpmem.h>\f[], like this:
.IP
.nf
\f[C]
reason\ =\ pmem_check_version(PMEM_MAJOR_VERSION,
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ PMEM_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 performed by \f[B]pmem_check_version\f[]() 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 \f[B]pmem_check_version\f[]() must not be
modified or freed.
.SH ENVIRONMENT
.PP
\f[B]libpmem\f[] can change its default behavior based on the following
environment variables.
These are largely intended for testing and are not normally required.
.IP \[bu] 2
\f[B]PMEM_IS_PMEM_FORCE\f[]=\f[I]val\f[]
.PP
If \f[I]val\f[] is 0 (zero), then \f[B]pmem_is_pmem\f[](3) will always
return false.
Setting \f[I]val\f[] to 1 causes \f[B]pmem_is_pmem\f[](3) to always
return true.
This variable is mostly used for testing but can be used to force pmem
behavior on a system where a range of pmem is not detectable as pmem for
some reason.
.RS
.PP
NOTE: Unlike the other variables, the value of
\f[B]PMEM_IS_PMEM_FORCE\f[] is not queried (and cached) at library
initialization time, but on the first call to \f[B]pmem_is_pmem\f[](3).
This means that in case of \f[B]libpmemlog\f[](7),
\f[B]libpmemblk\f[](7), and \f[B]libpmemobj\f[](7),
\f[B]PMEM_IS_PMEM_FORCE\f[] may still be set or modified by the program
until the first attempt to create or open the persistent memory pool.
.RE
.IP \[bu] 2
\f[B]PMEM_NO_CLWB\f[]=1
.PP
Setting this environment variable to 1 forces \f[B]libpmem\f[] to never
issue the \f[B]CLWB\f[] instruction on Intel hardware, falling back to
other cache flush instructions instead (\f[B]CLFLUSHOPT\f[] or
\f[B]CLFLUSH\f[] on Intel hardware).
Without this environment variable, \f[B]libpmem\f[] will always use the
\f[B]CLWB\f[] instruction for flushing processor caches on platforms
that support the instruction.
This variable is intended for use during library testing but may be
required for some rare cases where using \f[B]CLWB\f[] has a negative
impact on performance.
.IP \[bu] 2
\f[B]PMEM_NO_CLFLUSHOPT\f[]=1
.PP
Setting this environment variable to 1 forces \f[B]libpmem\f[] to never
issue the \f[B]CLFLUSHOPT\f[] instruction on Intel hardware, falling
back to the \f[B]CLFLUSH\f[] instructions instead.
Without this environment variable, \f[B]libpmem\f[] will always use the
\f[B]CLFLUSHOPT\f[] instruction for flushing processor caches on
platforms that support the instruction, but where \f[B]CLWB\f[] is not
available.
This variable is intended for use during library testing.
.IP \[bu] 2
\f[B]PMEM_NO_FLUSH\f[]=1
.PP
Setting this environment variable to 1 forces most \f[B]libpmem\f[]
functions to never issue any of \f[B]CLFLUSH\f[], \f[B]CLFLUSHOPT\f[] or
\f[B]CLWB\f[] instructions on Intel hardware.
The only exceptions are \f[B]pmem_deep_flush\f[](3) and
\f[B]pmem_deep_persist\f[](3) functions.
.IP \[bu] 2
\f[B]PMEM_NO_FLUSH\f[]=0
.PP
Setting this environment variable to 0 forces to always flush CPU caches
using one of \f[B]CLFLUSH\f[], \f[B]CLFLUSHOPT\f[] or \f[B]CLWB\f[]
instructions even if \f[B]pmem_has_auto_flush\f[](3) function returns
true and the platform supports flushing the processor caches on power
loss or system crash.
.IP \[bu] 2
\f[B]PMEM_NO_MOVNT\f[]=1
.PP
Setting this environment variable to 1 forces \f[B]libpmem\f[] to never
use the \f[I]non\-temporal\f[] move instructions on Intel hardware.
Without this environment variable, \f[B]libpmem\f[] will use the
non\-temporal instructions for copying larger ranges to persistent
memory on platforms that support the instructions.
This variable is intended for use during library testing.
.IP \[bu] 2
\f[B]PMEM_MOVNT_THRESHOLD\f[]=\f[I]val\f[]
.PP
This environment variable allows overriding the minimum length of the
\f[B]pmem_memmove_persist\f[](3) operations, for which \f[B]libpmem\f[]
uses \f[I]non\-temporal\f[] move instructions.
Setting this environment variable to 0 forces \f[B]libpmem\f[] to always
use the \f[I]non\-temporal\f[] move instructions if available.
It has no effect if \f[B]PMEM_NO_MOVNT\f[] is set to 1.
This variable is intended for use during library testing.
.IP \[bu] 2
\f[B]PMEM_MMAP_HINT\f[]=\f[I]val\f[]
.PP
This environment variable allows overriding the hint address used by
\f[B]pmem_map_file\f[]().
If set, it also disables mapping address randomization.
This variable is intended for use during library testing and debugging.
Setting it to some fairly large value (i.e.\ 0x10000000000) will very
likely result in mapping the file at the specified address (if not used)
or at the first unused region above given address, without adding any
random offset.
When debugging, this makes it easier to calculate the actual address of
the persistent memory block, based on its offset in the file.
In case of \f[B]libpmemobj\f[] it simplifies conversion of a persistent
object identifier (OID) into a direct pointer to the object.
.RS
.PP
NOTE: \f[B]Setting this environment variable affects all the PMDK
libraries,\f[] disabling mapping address randomization and causing the
specified address to be used as a hint about where to place the mapping.
.RE
.SH DEBUGGING AND ERROR HANDLING
.PP
If an error is detected during the call to a \f[B]libpmem\f[] function,
the application may retrieve an error message describing the reason for
the failure from \f[B]pmem_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]libpmem\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]libpmem\f[] are typically available on a
development system.
The normal version, accessed when a program is linked using the
\f[B]\-lpmem\f[] option, is optimized for performance.
That version skips checks that impact performance and never logs any
trace information or performs any run\-time assertions.
.PP
A second version of \f[B]libpmem\f[], accessed when a program uses the
libraries under \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 environment
variable \f[B]LD_LIBRARY_PATH\f[] 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]PMEM_LOG_LEVEL\f[]
.PP
The value of \f[B]PMEM_LOG_LEVEL\f[] enables trace points in the debug
version of the library, as follows:
.IP \[bu] 2
\f[B]0\f[] \- This is the default level when \f[B]PMEM_LOG_LEVEL\f[] is
not set.
No log messages are emitted at this level.
.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.
The same information may be retrieved using \f[B]pmem_errormsg\f[]().
.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 and fairly obscure tracing information
that is likely only useful to the \f[B]libpmem\f[] developers.
.PP
Unless \f[B]PMEM_LOG_FILE\f[] is set, debugging output is written to
\f[I]stderr\f[].
.IP \[bu] 2
\f[B]PMEM_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]PMEM_LOG_FILE\f[] is not set, output is written to
\f[I]stderr\f[].
.SH EXAMPLE
.PP
The following example uses \f[B]libpmem\f[] to flush changes made to
raw, memory\-mapped persistent memory.
.RS
.PP
WARNING: There is nothing transactional about the
\f[B]pmem_persist\f[](3) or \f[B]pmem_msync\f[](3) calls in this
example.
Interrupting the program may result in a partial write to pmem.
Use a transactional library such as \f[B]libpmemobj\f[](7) to avoid torn
updates.
.RE
.IP
.nf
\f[C]
#include\ <sys/types.h>
#include\ <sys/stat.h>
#include\ <fcntl.h>
#include\ <stdio.h>
#include\ <errno.h>
#include\ <stdlib.h>
#include\ <unistd.h>
#include\ <string.h>
#include\ <libpmem.h>

/*\ using\ 4k\ of\ pmem\ for\ this\ example\ */
#define\ PMEM_LEN\ 4096

#define\ PATH\ "/pmem\-fs/myfile"

int
main(int\ argc,\ char\ *argv[])
{
\ \ \ \ char\ *pmemaddr;
\ \ \ \ size_t\ mapped_len;
\ \ \ \ int\ is_pmem;

\ \ \ \ /*\ create\ a\ pmem\ file\ and\ memory\ map\ it\ */

\ \ \ \ if\ ((pmemaddr\ =\ pmem_map_file(PATH,\ PMEM_LEN,\ PMEM_FILE_CREATE,
\ \ \ \ \ \ \ \ \ \ \ \ 0666,\ &mapped_len,\ &is_pmem))\ ==\ NULL)\ {
\ \ \ \ \ \ \ \ perror("pmem_map_file");
\ \ \ \ \ \ \ \ exit(1);
\ \ \ \ }

\ \ \ \ /*\ store\ a\ string\ to\ the\ persistent\ memory\ */
\ \ \ \ strcpy(pmemaddr,\ "hello,\ persistent\ memory");

\ \ \ \ /*\ flush\ above\ strcpy\ to\ persistence\ */
\ \ \ \ if\ (is_pmem)
\ \ \ \ \ \ \ \ pmem_persist(pmemaddr,\ mapped_len);
\ \ \ \ else
\ \ \ \ \ \ \ \ pmem_msync(pmemaddr,\ mapped_len);

\ \ \ \ /*
\ \ \ \ \ *\ Delete\ the\ mappings.\ The\ region\ is\ also
\ \ \ \ \ *\ automatically\ unmapped\ when\ the\ process\ is
\ \ \ \ \ *\ terminated.
\ \ \ \ \ */
\ \ \ \ pmem_unmap(pmemaddr,\ mapped_len);
}
\f[]
.fi
.PP
See <http://pmem.io/pmdk/libpmem> for more examples using the
\f[B]libpmem\f[] API.
.SH ACKNOWLEDGEMENTS
.PP
\f[B]libpmem\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]dlclose\f[](3), \f[B]pmem_flush\f[](3), \f[B]pmem_is_pmem\f[](3),
\f[B]pmem_memmove_persist\f[](3), \f[B]pmem_msync\f[](3),
\f[B]pmem_persist\f[](3), \f[B]strerror\f[](3), \f[B]libpmemblk\f[](7),
\f[B]libpmemlog\f[](7), \f[B]libpmemobj\f[](7) and
\f[B]<http://pmem.io>\f[]