Install Now
pmemkv - Key/Value Datastore for Persistent Memory
Contents
Description
Keys and values stored in a pmemkv database can be arbitrary binary data and can contain multiple null
characters. Every function which accepts key expects const char *k pointer to data and its size as
size_t.
Some of the functions (mainly range-query API) are not guaranteed to be implemented by all engines. If
an engine does not support a certain function, it will return PMEMKV_STATUS_NOT_SUPPORTED.
Note: There are no explicit upper_bound/lower_bound functions. If you want to obtain an element(s) above
or below the selected key, you can use pmemkv_get_above() or pmemkv_get_below(). See descriptions of
these functions for details.
intpmemkv_open(constchar*engine,pmemkv_config*config,pmemkv_db**db);
Opens the pmemkv database and stores a pointer to a pmemkv_db instance in *db. The engine parame‐
ter specifies the engine name (see libpmemkv(7) for the list of available engines). The config
parameter specifies configuration (see libpmemkv_config(3) for details). Pmemkv takes ownership
of the config parameter - this means that pmemkv_config_delete() must NOT be called after open
(successful or failed).
voidpmemkv_close(pmemkv_db*kv);
Closes pmemkv database.
intpmemkv_count_all(pmemkv_db*db,size_t*cnt);
Stores in *cnt the number of records in db.
intpmemkv_count_above(pmemkv_db*db,constchar*k,size_tkb,size_t*cnt);
Stores in *cnt the number of records in db whose keys are greater than the key k of length kb.
Order of the elements is specified by a comparator (see libpmemkv(7)).
intpmemkv_count_below(pmemkv_db*db,constchar*k,size_tkb,size_t*cnt);
Stores in *cnt the number of records in db whose keys are less than the key k of length kb. Order
of the elements is specified by a comparator (see libpmemkv(7)).
intpmemkv_count_between(pmemkv_db*db,constchar*k1,size_tkb1,constchar*k2,size_tkb2,size_t*cnt);
Stores in *cnt the number of records in db whose keys are greater than key k1 (of length kb1) and
less than key k2 (of length kb2). Order of the elements is specified by a comparator (see libp‐memkv(7)).
intpmemkv_get_all(pmemkv_db*db,pmemkv_get_kv_callback*c,void*arg);
Executes function c for every record stored in db. Arguments passed to the function are: pointer
to a key, size of the key, pointer to a value, size of the value and arg specified by the user.
Function c can stop iteration by returning non-zero value. In that case pmemkv_get_all() returns
PMEMKV_STATUS_STOPPED_BY_CB. Returning 0 continues iteration. Order of the elements is specified
by a comparator (see libpmemkv(7)).
intpmemkv_get_above(pmemkv_db*db,constchar*k,size_tkb,pmemkv_get_kv_callback*c,void*arg);
Executes function c for every record stored in db whose keys are greater than key k (of length
kb). Arguments passed to c are: pointer to a key, size of the key, pointer to a value, size of
the value and arg specified by the user. Function c can stop iteration by returning non-zero val‐
ue. In that case pmemkv_get_above() returns PMEMKV_STATUS_STOPPED_BY_CB. Returning 0 continues
iteration. Order of the elements is specified by a comparator (see libpmemkv(7)).
intpmemkv_get_below(pmemkv_db*db,constchar*k,size_tkb,pmemkv_get_kv_callback*c,void*arg);
Executes function c for every record stored in db whose keys are less than key k (of length kb).
Arguments passed to c are: pointer to a key, size of the key, pointer to a value, size of the val‐
ue and arg specified by the user. Function c can stop iteration by returning non-zero value. In
that case pmemkv_get_below() returns PMEMKV_STATUS_STOPPED_BY_CB. Returning 0 continues itera‐
tion. Order of the elements is specified by a comparator (see libpmemkv(7)).
intpmemkv_get_between(pmemkv_db*db,constchar*k1,size_tkb1,constchar*k2,size_tkb2,pmemkv_get_kv_callback*c,void*arg);
Executes function c for every record stored in db whose keys are greater than key k1 (of length
kb1) and less than key k2 (of length kb2). Arguments passed to c are: pointer to a key, size of
the key, pointer to a value, size of the value and arg specified by the user. Function c can stop
iteration by returning non-zero value. In that case pmemkv_get_between() returns PMEMKV_STA‐
TUS_STOPPED_BY_CB. Returning 0 continues iteration. Order of the elements is specified by a com‐
parator (see libpmemkv(7)).
intpmemkv_exists(pmemkv_db*db,constchar*k,size_tkb);
Checks existence of record with key k of length kb. If record is present PMEMKV_STATUS_OK is re‐
turned, otherwise PMEMKV_STATUS_NOT_FOUND is returned. Other possible return values are described
in the ERRORS section.
intpmemkv_get(pmemkv_db*db,constchar*k,size_tkb,pmemkv_get_v_callback*c,void*arg);
Executes function c on record with key k (of length kb). If record is present and no error oc‐
curred the function returns PMEMKV_STATUS_OK. If record does not exist PMEMKV_STATUS_NOT_FOUND is
returned. Other possible return values are described in the ERRORS section. Function c is called
with the following parameters: pointer to a value, size of the value and arg specified by the
user. Value points to the location where data is actually stored (no copy occurs). This function
is guaranteed to be implemented by all engines.
intpmemkv_get_copy(pmemkv_db*db,constchar*k,size_tkb,char*buffer,size_tbuffer_size,size_t*value_size);
Copies value of record with key k of length kb to user provided buffer. buffer points to the val‐
ue buffer, buffer_size specifies its size and *value_size is filled in by this function. If the
value doesn’t fit in the provided buffer then this function returns PMEMKV_STATUS_UNKNOWN_ERROR.
Otherwise, in absence of any errors, PMEMKV_STATUS_OK is returned. Other possible return values
are described in the ERRORS section. This function is guaranteed to be implemented by all en‐
gines.
intpmemkv_put(pmemkv_db*db,constchar*k,size_tkb,constchar*v,size_tvb);
Inserts a key-value pair into pmemkv database. kb is the length of key k and vb is the length of
value v. When this function returns, caller is free to reuse both buffers. This function is
guaranteed to be implemented by all engines.
intpmemkv_remove(pmemkv_db*db,constchar*k,size_tkb);
Removes record with key k of length kb. This function is guaranteed to be implemented by all en‐
gines.
intpmemkv_defrag(pmemkv_db*db,doublestart_percent,doubleamount_percent);
Defragments approximately `amount_percent' percent of elements in the database starting from
`start_percent' percent of elements.
constchar*pmemkv_errormsg(void);
Returns a human readable string describing the last error.
ERRORS
Each function, except for pmemkv_close() and pmemkv_errormsg(), returns one of the following status
codes:
• PMEMKV_STATUS_OK – no error
• PMEMKV_STATUS_UNKNOWN_ERROR – unknown error
• PMEMKV_STATUS_NOT_FOUND – record not found
• PMEMKV_STATUS_NOT_SUPPORTED – function is not implemented by current engine
• PMEMKV_STATUS_INVALID_ARGUMENT – argument to function has wrong value
• PMEMKV_STATUS_CONFIG_PARSING_ERROR – parsing data to config failed
• PMEMKV_STATUS_CONFIG_TYPE_ERROR – config item has different type than expected
• PMEMKV_STATUS_STOPPED_BY_CB – iteration was stopped by user’s callback
• PMEMKV_STATUS_OUT_OF_MEMORY – operation failed because there is not enough memory (or space on the de‐
vice)
• PMEMKV_STATUS_WRONG_ENGINE_NAME – engine name does not match any available engine
• PMEMKV_STATUS_TRANSACTION_SCOPE_ERROR – an error with the scope of the libpmemobj transaction
• PMEMKV_STATUS_DEFRAG_ERROR – the defragmentation process failed (possibly in the middle of a run)
Status returned from a function can change in a future version of a library to a more specific one. For
example, if a function returns PMEMKV_STATUS_UNKNOWN_ERROR, it is possible that in future versions it
will return PMEMKV_STATUS_INVALID_ARGUMENT. Recommended way to check for an error is to compare status
with PMEMKV_STATUS_OK.
Example
The following example is taken from examples/pmemkv_basic_c directory.
Basic pmemkv usage in C:
#include <assert.h>
#include <libpmemkv.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ASSERT(expr) \
do { \
if (!(expr)) \
puts(pmemkv_errormsg()); \
assert(expr); \
} while (0)
#define LOG(msg) puts(msg)
#define MAX_VAL_LEN 64
static const uint64_t SIZE = 1024UL * 1024UL * 1024UL;
int get_kv_callback(const char *k, size_t kb, const char *value, size_t value_bytes,
void *arg)
{
printf(" visited: %s\n", k);
return 0;
}
int main(int argc, char *argv[])
{
if (argc < 2) {
fprintf(stderr, "Usage: %s file\n", argv[0]);
exit(1);
}
/* See libpmemkv_config(3) for more detailed example of config creation */
LOG("Creating config");
pmemkv_config *cfg = pmemkv_config_new();
ASSERT(cfg != NULL);
int s = pmemkv_config_put_path(cfg, argv[1]);
ASSERT(s == PMEMKV_STATUS_OK);
s = pmemkv_config_put_size(cfg, SIZE);
ASSERT(s == PMEMKV_STATUS_OK);
s = pmemkv_config_put_create_if_missing(cfg, true);
ASSERT(s == PMEMKV_STATUS_OK);
/* Alternatively create_or_error_if_exists flag can be set, to fail if file exists
* For differences between the two flags, see manpage libpmemkv(7). */
/* s = pmemkv_config_put_create_or_error_if_exists(cfg, true); */
LOG("Opening pmemkv database with 'cmap' engine");
pmemkv_db *db = NULL;
s = pmemkv_open("cmap", cfg, &db);
ASSERT(s == PMEMKV_STATUS_OK);
ASSERT(db != NULL);
LOG("Putting new key");
const char *key1 = "key1";
const char *value1 = "value1";
s = pmemkv_put(db, key1, strlen(key1), value1, strlen(value1));
ASSERT(s == PMEMKV_STATUS_OK);
size_t cnt;
s = pmemkv_count_all(db, &cnt);
ASSERT(s == PMEMKV_STATUS_OK);
ASSERT(cnt == 1);
LOG("Reading key back");
char val[MAX_VAL_LEN];
s = pmemkv_get_copy(db, key1, strlen(key1), val, MAX_VAL_LEN, NULL);
ASSERT(s == PMEMKV_STATUS_OK);
ASSERT(!strcmp(val, "value1"));
LOG("Iterating existing keys");
const char *key2 = "key2";
const char *value2 = "value2";
const char *key3 = "key3";
const char *value3 = "value3";
pmemkv_put(db, key2, strlen(key2), value2, strlen(value2));
pmemkv_put(db, key3, strlen(key3), value3, strlen(value3));
pmemkv_get_all(db, &get_kv_callback, NULL);
LOG("Removing existing key");
s = pmemkv_remove(db, key1, strlen(key1));
ASSERT(s == PMEMKV_STATUS_OK);
ASSERT(pmemkv_exists(db, key1, strlen(key1)) == PMEMKV_STATUS_NOT_FOUND);
LOG("Defragmenting the database");
s = pmemkv_defrag(db, 0, 100);
ASSERT(s == PMEMKV_STATUS_OK);
LOG("Closing database");
pmemkv_close(db);
return 0;
}
Commonusagemistake
Common mistake in pmemkv API usage (especially when using C++ API) is to dereference pointer to the data
stored in pmemkv outside of a callback function scope.
std::string value;
const char* ptr;
size_t sz;
kv->get("key1", [&](string_view v) {
/* Save pointer to the data to use it later outside of a callback scope */
ptr = v.data();
sz = v.size();
});
kv->remove("key");
/* ERROR!
* Using this pointer outside of a callback function may cause access to some random data
* or a segmentation fault. At that point, ptr should be considered as invalid.
*/
value.append(ptr, sz);
Name
pmemkv - Key/Value Datastore for Persistent Memory
See Also
libpmemkv(7), libpmemkv_config(3), libpmemkv_iterator(3) and <https://pmem.io> PMEMKV - pmemkv version 1.5.0 2022-03-09 PMEMKV(3)
Synopsis
#include <libpmemkv.h>
typedef int pmemkv_get_kv_callback(const char *key, size_t keybytes, const char *value,
size_t valuebytes, void *arg);
typedef void pmemkv_get_v_callback(const char *value, size_t valuebytes, void *arg);
int pmemkv_open(const char *engine, pmemkv_config *config, pmemkv_db **db);
void pmemkv_close(pmemkv_db *kv);
int pmemkv_count_all(pmemkv_db *db, size_t *cnt);
int pmemkv_count_above(pmemkv_db *db, const char *k, size_t kb, size_t *cnt);
int pmemkv_count_below(pmemkv_db *db, const char *k, size_t kb, size_t *cnt);
int pmemkv_count_between(pmemkv_db *db, const char *k1, size_t kb1, const char *k2,
size_t kb2, size_t *cnt);
int pmemkv_get_all(pmemkv_db *db, pmemkv_get_kv_callback *c, void *arg);
int pmemkv_get_above(pmemkv_db *db, const char *k, size_t kb, pmemkv_get_kv_callback *c,
void *arg);
int pmemkv_get_below(pmemkv_db *db, const char *k, size_t kb, pmemkv_get_kv_callback *c,
void *arg);
int pmemkv_get_between(pmemkv_db *db, const char *k1, size_t kb1, const char *k2,
size_t kb2, pmemkv_get_kv_callback *c, void *arg);
int pmemkv_exists(pmemkv_db *db, const char *k, size_t kb);
int pmemkv_get(pmemkv_db *db, const char *k, size_t kb, pmemkv_get_v_callback *c,
void *arg);
int pmemkv_get_copy(pmemkv_db *db, const char *k, size_t kb, char *buffer,
size_t buffer_size, size_t *value_size);
int pmemkv_put(pmemkv_db *db, const char *k, size_t kb, const char *v, size_t vb);
int pmemkv_remove(pmemkv_db *db, const char *k, size_t kb);
int pmemkv_defrag(pmemkv_db *db, double start_percent, double amount_percent);
const char *pmemkv_errormsg(void);
For pmemkv configuration API description see libpmemkv_config(3). For pmemkv iterator API description
see libpmemkv_iterator(3). For general pmemkv information, engine descriptions and bindings details see
libpmemkv(7).
PNG Icons
