---
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description: ""
disclaimer: "The contents of this web site and the associated <a href=\"https://github.com/pmem\">GitHub repositories</a> are BSD-licensed open source."
aliases: ["libpmem.7.html"]
title: "libpmem | PMDK"
header: "pmem API version 1.1"
---

[comment]: <> (SPDX-License-Identifier: BSD-3-Clause)
[comment]: <> (Copyright 2016-2018, Intel Corporation)

[comment]: <> (libpmem.7 -- man page for libpmem)

[NAME](#name)<br />
[SYNOPSIS](#synopsis)<br />
[DESCRIPTION](#description)<br />
[CAVEATS](#caveats)<br />
[LIBRARY API VERSIONING](#library-api-versioning-1)<br />
[ENVIRONMENT](#environment)<br />
[ERROR HANDLING](#error-handling)<br />
[DEBUGGING](#debugging)<br />
[LOGGING (DEPRECATED)](#logging-deprecated)<br />
[EXAMPLE](#example)<br />
[ACKNOWLEDGEMENTS](#acknowledgements)<br />
[SEE ALSO](#see-also)

# NAME #

**libpmem** - persistent memory support library

# SYNOPSIS #

```c
#include <libpmem.h>
cc ... -lpmem
```

##### Library API versioning: #####

```c
const char *pmem_check_version(
	unsigned major_required,
	unsigned minor_required);
```

##### Error handling: #####

```c
const char *pmem_errormsg(void);
```

##### Other library functions: #####

A description of other **libpmem** functions can be found on the following
manual pages:

+ most commonly used functions: **pmem_is_pmem**(3)

+ partial flushing operations: **pmem_flush**(3)

+ copying to persistent memory: **pmem_memmove_persist**(3)

# DESCRIPTION #

**libpmem** provides low-level *persistent memory* (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 *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.

This library is for applications that use persistent memory directly,
without the help of any library-supplied transactions or memory
allocation. A higher-level library that relies on **libpmem** is available
and recommended for most applications, see:

+ **libpmemobj**(7), a general use persistent memory API, providing memory
  allocation and transactional operations on variable-sized objects.

Under normal usage, **libpmem** 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.

# CAVEATS #

**libpmem** 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 **pmem_check_version**() function is used to determine whether the installed
**libpmem** 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
**\<libpmem.h\>**, like this:

```c
reason = pmem_check_version(PMEM_MAJOR_VERSION, PMEM_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 **pmem_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 **pmem_check_version**() must not be modified or
freed.

# ENVIRONMENT #

**libpmem** can change its default behavior based on the following
environment variables. These are largely intended for testing and are
not normally required.

+ **PMEM_IS_PMEM_FORCE**=*val*

If *val* is 0 (zero), then **pmem_is_pmem**(3) will always return
false. Setting *val* to 1 causes **pmem_is_pmem**(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.

>NOTE:
Unlike the other variables, the value of
**PMEM_IS_PMEM_FORCE** is not queried (and cached) at
library initialization time, but on the first call to
**pmem_is_pmem**(3). This means that in case of **libpmemobj**(7),
**PMEM_IS_PMEM_FORCE** may still be set or modified by the program
until the first attempt to create or open the persistent
memory pool.

+ **PMEM_NO_CLWB**=1

Setting this environment variable to 1 forces **libpmem** to never issue
the **CLWB** instruction on Intel hardware, falling back to other cache
flush instructions instead (**CLFLUSHOPT** or **CLFLUSH** on Intel
hardware). Without this environment variable, **libpmem** will always
use the **CLWB** 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
**CLWB** has a negative impact on performance.

+ **PMEM_NO_CLFLUSHOPT**=1

Setting this environment variable to 1 forces **libpmem** to never issue
the **CLFLUSHOPT** instruction on Intel hardware, falling back to the
**CLFLUSH** instructions instead. Without this environment variable,
**libpmem** will always use the **CLFLUSHOPT** instruction for flushing
processor caches on platforms that support the instruction, but where
**CLWB** is not available. This variable is intended for use during
library testing.

+ **PMEM_NO_FLUSH**=1

Setting this environment variable to 1 forces most **libpmem** functions
to never issue any of **CLFLUSH**, **CLFLUSHOPT** or **CLWB** instructions
on Intel hardware. The only exceptions are **pmem_deep_flush**(3) and
**pmem_deep_persist**(3) functions.

+ **PMEM_NO_FLUSH**=0

Setting this environment variable to 0 forces to always flush CPU caches
using one of **CLFLUSH**, **CLFLUSHOPT** or **CLWB** instructions
even if **pmem_has_auto_flush**(3) function returns true and the platform
supports flushing the processor caches on power loss or system crash.

+ **PMEM_NO_MOVNT**=1

Setting this environment variable to 1 forces **libpmem** to never use
the *non-temporal* move instructions on Intel hardware. Without this
environment variable, **libpmem** 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.

+ **PMEM_MOVNT_THRESHOLD**=*val*

This environment variable allows overriding the minimum length of
the **pmem_memmove_persist**(3) operations, for which **libpmem** uses
*non-temporal* move instructions. Setting this environment variable to 0
forces **libpmem** to always use the *non-temporal* move instructions if
available. It has no effect if **PMEM_NO_MOVNT** is set to 1.
This variable is intended for use during library testing.

+ **PMEM_MMAP_HINT**=*val*

This environment variable allows overriding
the hint address used by **pmem_map_file**(). 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 **libpmemobj** it
simplifies conversion of a persistent object identifier (OID) into a
direct pointer to the object.

>NOTE:
**Setting this environment variable
affects all the PMDK libraries,** disabling mapping address randomization
and causing the specified address to be used as a hint about where to
place the mapping.

# ERROR HANDLING #

If an error is detected during the call to a **libpmem** function, the
application may retrieve an error message describing the reason of the failure
from **pmem_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 **libpmem** function indicated an error.
The application must not modify or free the error message string.
Subsequent calls to other library functions may modify the previous message.

In parallel to the above mechanism, all logging messages are written to
**syslog**(3) and/or **stderr**(3) or passed to the user-provided logging
function. Please see **pmem_log_set_function**(3) for details.
The influx of the reported messages can be controlled by setting a respective
threshold value. Please see **pmem_log_set_threshold**(3) for details.

# DEBUGGING #

Two versions of **libpmem** are typically available on a development
system. The normal version, accessed when a program is linked using the
**-lpmem** option, 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 of **libpmem**, 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 environment variable
**LD_LIBRARY_PATH** to **/usr/lib/pmdk_debug** or **/usr/lib64/pmdk_debug**,
as appropriate.

# LOGGING (DEPRECATED) #

Debugging output is controlled using the following environment
variables. These variables have no effect on the non-debug version of the library.

+ **PMEM_LOG_LEVEL**

The value of the **PMEM_LOG_LEVEL** environment variable enables trace points in
the debug version of the library, as follows:

+ **0** - This is the default level when **PMEM_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 **pmem_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 **libpmem** developers.

Unless the **PMEM_LOG_FILE** environment variable is set, the debugging output
is written to *stderr*.

+ **PMEM_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 **PMEM_LOG_FILE** is not set, output is
written to *stderr*.

Whenever either **PMEM_LOG_LEVEL** or **PMEM_LOG_FILE** environment variable is
set the messages are not passed via the routes described in
the [ERROR HANDLING](#error-handling) section except for the **pmem_errormsg**()
which works unconditionally.

# EXAMPLE #

The following example uses **libpmem** to flush changes made to raw,
memory-mapped persistent memory.

>WARNING:
There is nothing transactional about the **pmem_persist**(3) or
**pmem_msync**(3) calls in this example. Interrupting the program may
result in a partial write to pmem. Use a transactional library such as
**libpmemobj**(7) to avoid torn updates.

```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);
}
```

See <https://pmem.io/pmdk/libpmem>
for more examples using the **libpmem** API.

# ACKNOWLEDGEMENTS #

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

# SEE ALSO #

**dlclose**(3),
**pmem_flush**(3), **pmem_is_pmem**(3), **pmem_memmove_persist**(3),
**pmem_msync**(3), **pmem_persist**(3), **strerror**(3),
**libpmemobj**(7) and **<https://pmem.io>**
