1() PMDK Programmer's Manual ()
2
6 libpmemlog - persistent memory resident log file (DEPRECATED)
7 NOTE: Support for Windows and FreeBSD deprecated since PMDK
9 1.13.0 release and will be removed in the PMDK 1.14.0 release.
10
12 #include <libpmemlog.h>
13 cc ... -lpmemlog -lpmem
14 Library API versioning:
16 const char *pmemlog_check_version(
17 unsigned major_required,
18 unsigned minor_required);
19 Managing library behavior:
21 void pmemlog_set_funcs(
22 void *(*malloc_func)(size_t size),
23 void (*free_func)(void *ptr),
24 void *(*realloc_func)(void *ptr, size_t size),
25 char *(*strdup_func)(const char *s));
26 Error handling:
28 int pmemlog_check(const char *path);
29 Other library functions:
31 A description of other libpmemlog functions can be found on the follow‐
32 ing manual pages:
33 pmemlog_append(3), pmemlog_create(3), pmemlog_ctl_exec(3), pmem‐
35 log_ctl_get(3), pmemlog_ctl_set(3), pmemlog_nbyte(3), pmemlog_tell(3)
36
38 libpmemlog provides a log file in persistent memory (pmem) such that
39 additions to the log are appended atomically. This library is intended
40 for applications using direct access storage (DAX), which is storage
41 that supports load/store access without paging blocks from a block
42 storage device. Some types of non-volatile memory DIMMs (NVDIMMs) pro‐
43 vide this type of byte addressable access to storage. A persistent
44 memory aware file system is typically used to expose the direct access
45 to applications. Memory mapping a file from this type of file system
46 results in the load/store, non-paged access to pmem. libpmemlog builds
47 on thistype of memory mapped file.
48 This library is for applications that need a persistent log file updat‐
50 ed atomically (the updates cannot be torn by program interruption such
51 as power failures). This library builds on the low-level pmem support
52 provided by libpmem(7), handling the transactional update of the log,
53 flushing to persistence, and recovery for the application.
54 libpmemlog is one of a collection of persistent memory libraries avail‐
56 able. The others are:
57 • libpmemobj(7), a general use persistent memory API, providing memory
59 allocation and transactional operations on variable-sized objects.
60 • libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks
62 with atomic updates.
63 • libpmem(7), low-level persistent memory support.
65 Under normal usage, libpmemlog will never print messages or intention‐
67 ally cause the process to exit. The only exception to this is the de‐
68 bugging information, when enabled, as described under DEBUGGING AND ER‐
69 ROR HANDLING below.
70 To use the pmem-resident log file provided by libpmemlog, a memory pool
72 is first created. This is done with the pmemlog_create(3) function.
73 The other functions mentioned above in SYNOPSIS section then operate on
74 the resulting log memory pool.
75 Once created, the memory pool is represented by an opaque handle, of
77 type PMEMlogpool*, which is passed to most of the other functions from
78 libpmemlog. Internally, libpmemlog will use either pmem_persist(3) or
79 msync(2) when it needs to flush changes, depending on whether the memo‐
80 ry pool appears to be persistent memory or a regular file (see the
81 pmem_is_pmem(3) function in libpmem(7) for more information). There is
82 no need for applications to flush changes directly when using the log
83 memory API provided by libpmemlog.
84
86 libpmemlog relies on the library destructor being called from the main
87 thread. For this reason, all functions that might trigger destruction
88 (e.g. dlclose(3)) should be called in the main thread. Otherwise some
89 of the resources associated with that thread might not be cleaned up
90 properly.
91
93 This section describes how the library API is versioned, allowing ap‐
94 plications to work with an evolving API.
95 The pmemlog_check_version() function is used to determine whether the
97 installed libpmemlog supports the version of the library API required
98 by an application. The easiest way to do this is for the application
99 to supply the compile-time version information provided by defines in
100 <libpmemlog.h>, like this:
101 reason = pmemlog_check_version(PMEMLOG_MAJOR_VERSION,
103 PMEMLOG_MINOR_VERSION);
104 if (reason != NULL) {
105 /* version check failed, reason string tells you why */
106 }
107 Any mismatch in the major version number is considered a failure, but a
109 library with a newer minor version number will pass this check since
110 increasing minor versions imply backwards compatibility.
111 An application can also check specifically for the existence of an in‐
113 terface by checking for the version where that interface was intro‐
114 duced. These versions are documented in this man page as follows: un‐
115 less otherwise specified, all interfaces described here are available
116 in version 1.0 of the library. Interfaces added after version 1.0 will
117 contain the text introduced in version x.y in the section of this manu‐
118 al describing the feature.
119 On success, pmemlog_check_version() returns NULL. Otherwise, the re‐
121 turn value is a static string describing the reason the version check
122 failed. The string returned by pmemlog_check_version() must not be
123 modified or freed.
124
126 The pmemlog_set_funcs() function allows an application to override mem‐
127 ory allocation calls used internally by libpmemlog. Passing in NULL
128 for any of the handlers will cause the libpmemlog default function to
129 be used. The library does not make heavy use of the system malloc
130 functions, but it does allocate approximately 4-8 kilobytes for each
131 memory pool in use.
132
134 The pmemlog_errormsg() function returns a pointer to a static buffer
135 containing the last error message logged for the current thread. If
136 errno was set, the error message may include a description of the cor‐
137 responding error code as returned by strerror(3). The error message
138 buffer is thread-local; errors encountered in one thread do not affect
139 its value in other threads. The buffer is never cleared by any library
140 function; its content is significant only when the return value of the
141 immediately preceding call to a libpmemlog function indicated an error,
142 or if errno was set. The application must not modify or free the error
143 message string, but it may be modified by subsequent calls to other li‐
144 brary functions.
145 Two versions of libpmemlog are typically available on a development
147 system. The normal version, accessed when a program is linked using
148 the -lpmemlog option, is optimized for performance. That version skips
149 checks that impact performance and never logs any trace information or
150 performs any run-time assertions.
151 A second version of libpmemlog, accessed when a program uses the li‐
153 braries under /usr/lib/pmdk_debug, contains run-time assertions and
154 trace points. The typical way to access the debug version is to set
155 the environment variable LD_LIBRARY_PATH to /usr/lib/pmdk_debug or
156 /usr/lib64/pmdk_debug, as appropriate. Debugging output is controlled
157 using the following environment variables. These variables have no ef‐
158 fect on the non-debug version of the library.
159 • PMEMLOG_LOG_LEVEL
161 The value of PMEMLOG_LOG_LEVEL enables trace points in the debug ver‐
163 sion of the library, as follows:
164 • 0 - This is the default level when PMEMLOG_LOG_LEVEL is not set. No
166 log messages are emitted at this level.
167 • 1 - Additional details on any errors detected are logged, in addition
169 to returning the errno-based errors as usual. The same information
170 may be retrieved using pmemlog_errormsg().
171 • 2 - A trace of basic operations is logged.
173 • 3 - Enables a very verbose amount of function call tracing in the li‐
175 brary.
176 • 4 - Enables voluminous and fairly obscure tracing information that is
178 likely only useful to the libpmemlog developers.
179 Unless PMEMLOG_LOG_FILE is set, debugging output is written to stderr.
181 • PMEMLOG_LOG_FILE
183 Specifies the name of a file name where all logging information should
185 be written. If the last character in the name is “-”, the PID of the
186 current process will be appended to the file name when the log file is
187 created. If PMEMLOG_LOG_FILE is not set, logging output is written to
188 stderr.
189 See also libpmem(7) for information about other environment variables
191 affecting libpmemlog behavior.
192
194 The following example illustrates how the libpmemlog API is used.
195 #include <stdio.h>
197 #include <fcntl.h>
198 #include <errno.h>
199 #include <stdlib.h>
200 #include <unistd.h>
201 #include <string.h>
202 #include <libpmemlog.h>
203 /* size of the pmemlog pool -- 1 GB */
205 #define POOL_SIZE ((size_t)(1 << 30))
206 /*
208 * printit -- log processing callback for use with pmemlog_walk()
209 */
210 int
211 printit(const void *buf, size_t len, void *arg)
212 {
213 fwrite(buf, len, 1, stdout);
214 return 0;
215 }
216 int
218 main(int argc, char *argv[])
219 {
220 const char path[] = "/pmem-fs/myfile";
221 PMEMlogpool *plp;
222 size_t nbyte;
223 char *str;
224 /* create the pmemlog pool or open it if it already exists */
226 plp = pmemlog_create(path, POOL_SIZE, 0666);
227 if (plp == NULL)
229 plp = pmemlog_open(path);
230 if (plp == NULL) {
232 perror(path);
233 exit(1);
234 }
235 /* how many bytes does the log hold? */
237 nbyte = pmemlog_nbyte(plp);
238 printf("log holds %zu bytes", nbyte);
239 /* append to the log... */
241 str = "This is the first string appended";
242 if (pmemlog_append(plp, str, strlen(str)) < 0) {
243 perror("pmemlog_append");
244 exit(1);
245 }
246 str = "This is the second string appended";
247 if (pmemlog_append(plp, str, strlen(str)) < 0) {
248 perror("pmemlog_append");
249 exit(1);
250 }
251 /* print the log contents */
253 printf("log contains:");
254 pmemlog_walk(plp, 0, printit, NULL);
255 pmemlog_close(plp);
257 }
258 See <https://pmem.io/pmdk/libpmemlog> for more examples using the libp‐
260 memlog API.
261
263 Unlike libpmemobj(7), data replication is not supported in libpmemlog.
264 Thus, specifying replica sections in pool set files is not allowed.
265
267 NOTICE: The libpmemlog library is deprecated since PMDK 1.13.0
268 release and will be removed in the PMDK 1.14.0 release.
269
271 libpmemlog builds on the persistent memory programming model recommend‐
272 ed by the SNIA NVM Programming Technical Work Group:
273 <https://snia.org/nvmp>
274
276 msync(2), pmemlog_append(3), pmemlog_create(3), pmemlog_ctl_exec(3),
277 pmemlog_ctl_get(3), pmemlog_ctl_set(3), pmemlog_nbyte(3), pmem‐
278 log_tell(3), strerror(3), libpmem(7), libpmemblk(7), libpmemobj(7) and
279 <https://pmem.io>
280PMDK - 2023-06-05 ()