Provided by: libpmemlog-dev_1.4.1-0ubuntu1~18.04.1_amd64 
NAME
libpmemlog - persistent memory resident log file
SYNOPSIS
#include <libpmemlog.h>
cc ... -lpmemlog -lpmem
Library API versioning:
const char *pmemlog_check_version(
unsigned major_required,
unsigned minor_required);
Managing library behavior:
void pmemlog_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));
Error handling:
int pmemlog_check(const char *path);
Other library functions:
A description of other libpmemlog functions can be found on the following manual pages:
pmemlog_create(3), pmemlog_nbyte(3), pmemlog_append(3), pmemlog_tell(3)
DESCRIPTION
libpmemlog provides a log file in persistent memory (pmem) such that additions to the log
are appended atomically. This library is intended 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. libpmemlog
builds on thistype of memory mapped file.
This library is for applications that need a persistent log file updated atomically (the
updates cannot be torn by program interruption such as power failures). This library
builds on the low-level pmem support provided by libpmem(7), handling the transactional
update of the log, flushing to persistence, and recovery for the application.
libpmemlog is one of a collection of persistent memory libraries available. The others
are:
• libpmemobj(7), a general use persistent memory API, providing memory allocation and
transactional operations on variable-sized objects.
• libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks with atomic updates.
• libpmemcto(7), providing close-to-open persistence.
• libpmem(7), low-level persistent memory support.
Under normal usage, libpmemlog 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.
To use the pmem-resident log file provided by libpmemlog, a memory pool is first created.
This is done with the pmemlog_create(3) function. The other functions mentioned above in
SYNOPSIS section then operate on the resulting log memory pool.
Once created, the memory pool is represented by an opaque handle, of type PMEMlogpool*,
which is passed to most of the other functions from libpmemlog. Internally, libpmemlog
will use either pmem_persist(3) or msync(2) when it needs to flush changes, depending on
whether the memory pool appears to be persistent memory or a regular file (see the
pmem_is_pmem(3) function in libpmem(7) for more information). There is no need for
applications to flush changes directly when using the log memory API provided by
libpmemlog.
CAVEATS
libpmemlog 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 pmemlog_check_version() function is used to determine whether the installed libpmemlog
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 provided by
defines in <libpmemlog.h>, like this:
reason = pmemlog_check_version(PMEMLOG_MAJOR_VERSION,
PMEMLOG_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.
On success, pmemlog_check_version() returns NULL. Otherwise, the return value is a static
string describing the reason the version check failed. The string returned by
pmemlog_check_version() must not be modified or freed.
MANAGING LIBRARY BEHAVIOR
The pmemlog_set_funcs() function allows an application to override memory allocation calls
used internally by libpmemlog. Passing in NULL for any of the handlers will cause the
libpmemlog default function to be used. The library does not make heavy use of the system
malloc functions, but it does allocate approximately 4-8 kilobytes for each memory pool in
use.
DEBUGGING AND ERROR HANDLING
The pmemlog_errormsg() 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
libpmemlog 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 libpmemlog are typically available on a development system. The normal
version, accessed when a program is linked using the -lpmemlog 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 libpmemlog, 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. Debugging output is
controlled using the following environment variables. These variables have no effect on
the non-debug version of the library.
• PMEMLOG_LOG_LEVEL
The value of PMEMLOG_LOG_LEVEL enables trace points in the debug version of the library,
as follows:
• 0 - This is the default level when PMEMLOG_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
pmemlog_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 libpmemlog developers.
Unless PMEMLOG_LOG_FILE is set, debugging output is written to stderr.
• PMEMLOG_LOG_FILE
Specifies the name of a file name 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 PMEMLOG_LOG_FILE is not set, logging output is
written to stderr.
See also libpmem(7) for information about other environment variables affecting libpmemlog
behavior.
NOTE: on Ubuntu systems, this extra debug version of the library is shipped in the
respective -debug Debian package and placed in the /usr/lib/$ARCH/pmdk_dbg/
directory.
EXAMPLE
The following example illustrates how the libpmemlog API is used.
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libpmemlog.h>
/* size of the pmemlog pool -- 1 GB */
#define POOL_SIZE ((size_t)(1 << 30))
/*
* printit -- log processing callback for use with pmemlog_walk()
*/
int
printit(const void *buf, size_t len, void *arg)
{
fwrite(buf, len, 1, stdout);
return 0;
}
int
main(int argc, char *argv[])
{
const char path[] = "/pmem-fs/myfile";
PMEMlogpool *plp;
size_t nbyte;
char *str;
/* create the pmemlog pool or open it if it already exists */
plp = pmemlog_create(path, POOL_SIZE, 0666);
if (plp == NULL)
plp = pmemlog_open(path);
if (plp == NULL) {
perror(path);
exit(1);
}
/* how many bytes does the log hold? */
nbyte = pmemlog_nbyte(plp);
printf("log holds %zu bytes", nbyte);
/* append to the log... */
str = "This is the first string appended";
if (pmemlog_append(plp, str, strlen(str)) < 0) {
perror("pmemlog_append");
exit(1);
}
str = "This is the second string appended";
if (pmemlog_append(plp, str, strlen(str)) < 0) {
perror("pmemlog_append");
exit(1);
}
/* print the log contents */
printf("log contains:");
pmemlog_walk(plp, 0, printit, NULL);
pmemlog_close(plp);
}
See <http://pmem.io/pmdk/libpmemlog> for more examples using the libpmemlog API.
BUGS
Unlike libpmemobj(7), data replication is not supported in libpmemlog. Thus, specifying
replica sections in pool set files is not allowed.
ACKNOWLEDGEMENTS
libpmemlog builds on the persistent memory programming model recommended by the SNIA NVM
Programming Technical Work Group: <http://snia.org/nvmp>
SEE ALSO
msync(2), pmemlog_append(3), pmemlog_create(3), pmemlog_nbyte(3), pmemlog_tell(3),
strerror(3), libpmem(7), libpmemblk(7), libpmemcto(7), libpmemobj(7) and <http://pmem.io>