NAME¶

libpmempool - persistent memory pool management library

SYNOPSIS¶

#include <libpmempool.h>
cc -std=gnu99 ... -lpmempool -lpmem

DESCRIPTION¶

libpmempool provides a set of utilities for off-line analysis and manipulation of a pool. A pool in this manpage means a pmemobj pool, pmemblk pool, pmemlog pool or BTT layout, independent of the underlying storage. Some libpmempool functions are required to work without any impact on the pool but some may create a new or modify an existing pool.

libpmempool is for applications that need high reliability or built-in troubleshooting. It may be useful for testing and debugging purposes also.

libpmempool introduces functionality of pool set health check, synchronization, transformation and removal.

CAVEATS¶

libpmempool 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.

libpmempool requires the -std=gnu99 compilation flag to build properly.

LIBRARY API VERSIONING¶

This section describes how the library API is versioned, allowing applications to work with an evolving API.

The pmempool_check_version() function is used to see if the installed libpmempool supports the version of the library API required by an application. The easiest way to do this for the application is to supply the compile-time version information, supplied by defines in <libpmempool.h>, like this:

reason = pmempool_check_version(PMEMPOOL_MAJOR_VERSION,
                                PMEMPOOL_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 pmempool_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 pmempool_check_version() must not be modified or freed.

DEBUGGING AND ERROR HANDLING¶

If an error is detected during the call to a libpmempool function, the application may retrieve an error message describing the reason for the failure from pmempool_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 libpmempool 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 libpmempool are typically available on a development system. The normal version, accessed when a program is linked using the -lpmempool 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 libpmempool, 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.

•

PMEMPOOL_LOG_LEVEL

The value of PMEMPOOL_LOG_LEVEL enables trace points in the debug version of the library, as follows:

•

0 - This is the default level when PMEMPOOL_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 pmempool_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 libpmempool developers.

Unless PMEMPOOL_LOG_FILE is set, debugging output is written to stderr.

•

PMEMPOOL_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 PMEMPOOL_LOG_FILE is not set, output is written to stderr.

EXAMPLE¶

The following example illustrates how the libpmempool API is used. The program detects the type and checks consistency of given pool. If there are any issues detected, the pool is automatically repaired.

#include <stddef.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <libpmempool.h>
#define PATH "./pmem-fs/myfile"
#define CHECK_FLAGS (PMEMPOOL_CHECK_FORMAT_STR|PMEMPOOL_CHECK_REPAIR|\
                     PMEMPOOL_CHECK_VERBOSE)
int
main(int argc, char *argv[])
{
    PMEMpoolcheck *ppc;
    struct pmempool_check_status *status;
    enum pmempool_check_result ret;
    /* arguments for check */
    struct pmempool_check_args args = {
        .path       = PATH,
        .backup_path    = NULL,
        .pool_type  = PMEMPOOL_POOL_TYPE_DETECT,
        .flags      = CHECK_FLAGS
    };
    /* initialize check context */
    if ((ppc = pmempool_check_init(&args, sizeof(args))) == NULL) {
        perror("pmempool_check_init");
        exit(EXIT_FAILURE);
    }
    /* perform check and repair, answer 'yes' for each question */
    while ((status = pmempool_check(ppc)) != NULL) {
        switch (status->type) {
        case PMEMPOOL_CHECK_MSG_TYPE_ERROR:
            printf("%s\n", status->str.msg);
            break;
        case PMEMPOOL_CHECK_MSG_TYPE_INFO:
            printf("%s\n", status->str.msg);
            break;
        case PMEMPOOL_CHECK_MSG_TYPE_QUESTION:
            printf("%s\n", status->str.msg);
            status->str.answer = "yes";
            break;
        default:
            pmempool_check_end(ppc);
            exit(EXIT_FAILURE);
        }
    }
    /* finalize the check and get the result */
    ret = pmempool_check_end(ppc);
    switch (ret) {
        case PMEMPOOL_CHECK_RESULT_CONSISTENT:
        case PMEMPOOL_CHECK_RESULT_REPAIRED:
            return 0;
        default:
            return 1;
    }
}

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

ACKNOWLEDGEMENTS¶

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

SEE ALSO¶

dlclose(3), pmempool_check_init(3), pmempool_feature_query(3), pmempool_rm(3), pmempool_sync(3), strerror(3), libpmem(7), libpmemblk(7), libpmemlog(7), libpmemobj(7) and <https://pmem.io>**