1LIBPMEMLOG(7) PMDK Programmer's Manual LIBPMEMLOG(7)
2
6 libpmemlog - persistent memory resident log file
7
9 #include <libpmemlog.h>
10 cc ... -lpmemlog -lpmem
11 Library API versioning:
13 const char *pmemlog_check_version(
14 unsigned major_required,
15 unsigned minor_required);
16 Managing library behavior:
18 void pmemlog_set_funcs(
19 void *(*malloc_func)(size_t size),
20 void (*free_func)(void *ptr),
21 void *(*realloc_func)(void *ptr, size_t size),
22 char *(*strdup_func)(const char *s));
23 Error handling:
25 int pmemlog_check(const char *path);
26 Other library functions:
28 A description of other libpmemlog functions can be found on the follow‐
29 ing manual pages:
30 pmemlog_append(3), pmemlog_create(3), pmemlog_ctl_exec(3), pmem‐
32 log_ctl_get(3), pmemlog_ctl_set(3), pmemlog_nbyte(3), pmemlog_tell(3)
33
35 libpmemlog provides a log file in persistent memory (pmem) such that
36 additions to the log are appended atomically. This library is intended
37 for applications using direct access storage (DAX), which is storage
38 that supports load/store access without paging blocks from a block
39 storage device. Some types of non-volatile memory DIMMs (NVDIMMs) pro‐
40 vide this type of byte addressable access to storage. A persistent
41 memory aware file system is typically used to expose the direct access
42 to applications. Memory mapping a file from this type of file system
43 results in the load/store, non-paged access to pmem. libpmemlog builds
44 on thistype of memory mapped file.
45 This library is for applications that need a persistent log file updat‐
47 ed atomically (the updates cannot be torn by program interruption such
48 as power failures). This library builds on the low-level pmem support
49 provided by libpmem(7), handling the transactional update of the log,
50 flushing to persistence, and recovery for the application.
51 libpmemlog is one of a collection of persistent memory libraries avail‐
53 able. The others are:
54 • libpmemobj(7), a general use persistent memory API, providing memory
56 allocation and transactional operations on variable-sized objects.
57 • libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks
59 with atomic updates.
60 • libpmem(7), low-level persistent memory support.
62 Under normal usage, libpmemlog will never print messages or intention‐
64 ally cause the process to exit. The only exception to this is the de‐
65 bugging information, when enabled, as described under DEBUGGING AND ER‐
66 ROR HANDLING below.
67 To use the pmem-resident log file provided by libpmemlog, a memory pool
69 is first created. This is done with the pmemlog_create(3) function.
70 The other functions mentioned above in SYNOPSIS section then operate on
71 the resulting log memory pool.
72 Once created, the memory pool is represented by an opaque handle, of
74 type PMEMlogpool*, which is passed to most of the other functions from
75 libpmemlog. Internally, libpmemlog will use either pmem_persist(3) or
76 msync(2) when it needs to flush changes, depending on whether the memo‐
77 ry pool appears to be persistent memory or a regular file (see the
78 pmem_is_pmem(3) function in libpmem(7) for more information). There is
79 no need for applications to flush changes directly when using the log
80 memory API provided by libpmemlog.
81
83 libpmemlog relies on the library destructor being called from the main
84 thread. For this reason, all functions that might trigger destruction
85 (e.g. dlclose(3)) should be called in the main thread. Otherwise some
86 of the resources associated with that thread might not be cleaned up
87 properly.
88
90 This section describes how the library API is versioned, allowing ap‐
91 plications to work with an evolving API.
92 The pmemlog_check_version() function is used to determine whether the
94 installed libpmemlog supports the version of the library API required
95 by an application. The easiest way to do this is for the application
96 to supply the compile-time version information provided by defines in
97 <libpmemlog.h>, like this:
98 reason = pmemlog_check_version(PMEMLOG_MAJOR_VERSION,
100 PMEMLOG_MINOR_VERSION);
101 if (reason != NULL) {
102 /* version check failed, reason string tells you why */
103 }
104 Any mismatch in the major version number is considered a failure, but a
106 library with a newer minor version number will pass this check since
107 increasing minor versions imply backwards compatibility.
108 An application can also check specifically for the existence of an in‐
110 terface by checking for the version where that interface was intro‐
111 duced. These versions are documented in this man page as follows: un‐
112 less otherwise specified, all interfaces described here are available
113 in version 1.0 of the library. Interfaces added after version 1.0 will
114 contain the text introduced in version x.y in the section of this manu‐
115 al describing the feature.
116 On success, pmemlog_check_version() returns NULL. Otherwise, the re‐
118 turn value is a static string describing the reason the version check
119 failed. The string returned by pmemlog_check_version() must not be
120 modified or freed.
121
123 The pmemlog_set_funcs() function allows an application to override mem‐
124 ory allocation calls used internally by libpmemlog. Passing in NULL
125 for any of the handlers will cause the libpmemlog default function to
126 be used. The library does not make heavy use of the system malloc
127 functions, but it does allocate approximately 4-8 kilobytes for each
128 memory pool in use.
129
131 The pmemlog_errormsg() function returns a pointer to a static buffer
132 containing the last error message logged for the current thread. If
133 errno was set, the error message may include a description of the cor‐
134 responding error code as returned by strerror(3). The error message
135 buffer is thread-local; errors encountered in one thread do not affect
136 its value in other threads. The buffer is never cleared by any library
137 function; its content is significant only when the return value of the
138 immediately preceding call to a libpmemlog function indicated an error,
139 or if errno was set. The application must not modify or free the error
140 message string, but it may be modified by subsequent calls to other li‐
141 brary functions.
142 Two versions of libpmemlog are typically available on a development
144 system. The normal version, accessed when a program is linked using
145 the -lpmemlog option, is optimized for performance. That version skips
146 checks that impact performance and never logs any trace information or
147 performs any run-time assertions.
148 A second version of libpmemlog, accessed when a program uses the li‐
150 braries under /usr/lib/pmdk_debug, contains run-time assertions and
151 trace points. The typical way to access the debug version is to set
152 the environment variable LD_LIBRARY_PATH to /usr/lib/pmdk_debug or
153 /usr/lib64/pmdk_debug, as appropriate. Debugging output is controlled
154 using the following environment variables. These variables have no ef‐
155 fect on the non-debug version of the library.
156 • PMEMLOG_LOG_LEVEL
158 The value of PMEMLOG_LOG_LEVEL enables trace points in the debug ver‐
160 sion of the library, as follows:
161 • 0 - This is the default level when PMEMLOG_LOG_LEVEL is not set. No
163 log messages are emitted at this level.
164 • 1 - Additional details on any errors detected are logged, in addition
166 to returning the errno-based errors as usual. The same information
167 may be retrieved using pmemlog_errormsg().
168 • 2 - A trace of basic operations is logged.
170 • 3 - Enables a very verbose amount of function call tracing in the li‐
172 brary.
173 • 4 - Enables voluminous and fairly obscure tracing information that is
175 likely only useful to the libpmemlog developers.
176 Unless PMEMLOG_LOG_FILE is set, debugging output is written to stderr.
178 • PMEMLOG_LOG_FILE
180 Specifies the name of a file name where all logging information should
182 be written. If the last character in the name is “-”, the PID of the
183 current process will be appended to the file name when the log file is
184 created. If PMEMLOG_LOG_FILE is not set, logging output is written to
185 stderr.
186 See also libpmem(7) for information about other environment variables
188 affecting libpmemlog behavior.
189
191 The following example illustrates how the libpmemlog API is used.
192 #include <stdio.h>
194 #include <fcntl.h>
195 #include <errno.h>
196 #include <stdlib.h>
197 #include <unistd.h>
198 #include <string.h>
199 #include <libpmemlog.h>
200 /* size of the pmemlog pool -- 1 GB */
202 #define POOL_SIZE ((size_t)(1 << 30))
203 /*
205 * printit -- log processing callback for use with pmemlog_walk()
206 */
207 int
208 printit(const void *buf, size_t len, void *arg)
209 {
210 fwrite(buf, len, 1, stdout);
211 return 0;
212 }
213 int
215 main(int argc, char *argv[])
216 {
217 const char path[] = "/pmem-fs/myfile";
218 PMEMlogpool *plp;
219 size_t nbyte;
220 char *str;
221 /* create the pmemlog pool or open it if it already exists */
223 plp = pmemlog_create(path, POOL_SIZE, 0666);
224 if (plp == NULL)
226 plp = pmemlog_open(path);
227 if (plp == NULL) {
229 perror(path);
230 exit(1);
231 }
232 /* how many bytes does the log hold? */
234 nbyte = pmemlog_nbyte(plp);
235 printf("log holds %zu bytes", nbyte);
236 /* append to the log... */
238 str = "This is the first string appended";
239 if (pmemlog_append(plp, str, strlen(str)) < 0) {
240 perror("pmemlog_append");
241 exit(1);
242 }
243 str = "This is the second string appended";
244 if (pmemlog_append(plp, str, strlen(str)) < 0) {
245 perror("pmemlog_append");
246 exit(1);
247 }
248 /* print the log contents */
250 printf("log contains:");
251 pmemlog_walk(plp, 0, printit, NULL);
252 pmemlog_close(plp);
254 }
255 See <https://pmem.io/pmdk/libpmemlog> for more examples using the libp‐
257 memlog API.
258
260 Unlike libpmemobj(7), data replication is not supported in libpmemlog.
261 Thus, specifying replica sections in pool set files is not allowed.
262
264 libpmemlog builds on the persistent memory programming model recommend‐
265 ed by the SNIA NVM Programming Technical Work Group:
266 <https://snia.org/nvmp>
267
269 msync(2), pmemlog_append(3), pmemlog_create(3), pmemlog_ctl_exec(3),
270 pmemlog_ctl_get(3), pmemlog_ctl_set(3), pmemlog_nbyte(3), pmem‐
271 log_tell(3), strerror(3), libpmem(7), libpmemblk(7), libpmemobj(7) and
272 <https://pmem.io>
273PMDK - pmemlog API version 1.1 2022-08-25 LIBPMEMLOG(7)