# NAME

libmaxminddb - a library for working with MaxMind DB files

# SYNOPSIS

```c
#include <maxminddb.h>

int MMDB_open(
    const char *const filename,
    uint32_t flags,
    MMDB_s *const mmdb);
void MMDB_close(MMDB_s *const mmdb);

MMDB_lookup_result_s MMDB_lookup_string(
    const MMDB_s *const mmdb,
    const char *const ipstr,
    int *const gai_error,
    int *const mmdb_error);
MMDB_lookup_result_s MMDB_lookup_sockaddr(
    const MMDB_s *const mmdb,
    const struct sockaddr *const
    sockaddr,
    int *const mmdb_error);

int MMDB_get_value(
    MMDB_entry_s *const start,
    MMDB_entry_data_s *const entry_data,
    ...);
int MMDB_vget_value(
    MMDB_entry_s *const start,
    MMDB_entry_data_s *const entry_data,
    va_list va_path);
int MMDB_aget_value(
    MMDB_entry_s *const start,
    MMDB_entry_data_s *const entry_data,
    const char *const *const path);

int MMDB_get_entry_data_list(
    MMDB_entry_s *start,
    MMDB_entry_data_list_s **const entry_data_list);
void MMDB_free_entry_data_list(
    MMDB_entry_data_list_s *const entry_data_list);
int MMDB_get_metadata_as_entry_data_list(
    const MMDB_s *const mmdb,
    MMDB_entry_data_list_s **const entry_data_list);
int MMDB_dump_entry_data_list(
    FILE *const stream,
    MMDB_entry_data_list_s *const entry_data_list,
    int indent);

int MMDB_read_node(
    const MMDB_s *const mmdb,
    uint32_t node_number,
    MMDB_search_node_s *const node);

const char *MMDB_lib_version(void);
const char *MMDB_strerror(int error_code);

typedef struct MMDB_lookup_result_s {
    bool found_entry;
    MMDB_entry_s entry;
    uint16_t netmask;
} MMDB_lookup_result_s;

typedef struct MMDB_entry_data_s {
    bool has_data;
    union {
        uint32_t pointer;
        const char *utf8_string;
        double double_value;
        const uint8_t *bytes;
        uint16_t uint16;
        uint32_t uint32;
        int32_t int32;
        uint64_t uint64;
        {mmdb_uint128_t or uint8_t[16]} uint128;
        bool boolean;
        float float_value;
    };
    ...
    uint32_t data_size;
    uint32_t type;
} MMDB_entry_data_s;

typedef struct MMDB_entry_data_list_s {
    MMDB_entry_data_s entry_data;
    struct MMDB_entry_data_list_s *next;
} MMDB_entry_data_list_s;
```

# DESCRIPTION

The libmaxminddb library provides functions for working with MaxMind DB files.
See https://maxmind.github.io/MaxMind-DB/ for the MaxMind DB format
specification. The database and results are all represented by different data
structures. Databases are opened by calling `MMDB_open()`. You can look up IP
addresses as a string with `MMDB_lookup_string()` or as a pointer to a
`sockaddr` structure with `MMDB_lookup_sockaddr()`.

If the lookup finds the IP address in the database, it returns a
`MMDB_lookup_result_s` structure. If that structure indicates that the database
has data for the IP, there are a number of functions that can be used to fetch
that data. These include `MMDB_get_value()` and `MMDB_get_entry_data_list()`.
See the function documentation below for more details.

When you are done with the database handle you should call `MMDB_close()`.

All publicly visible functions, structures, and macros begin with "MMDB\_".

# DATA STRUCTURES

All data structures exported by this library's `maxminddb.h` header are
typedef'd in the form `typedef struct foo_s { ... } foo_s` so you can refer to
them without the `struct` prefix.

This library provides the following data structures:

## `MMDB_s`

This is the handle for a MaxMind DB file. We only document some of this
structure's fields intended for public use. All other fields are subject to
change and are intended only for internal use.

```c
typedef struct MMDB_s {
    uint32_t flags;
    const char *filename;
    ...
    MMDB_metadata_s metadata;
} MMDB_s;
```

- `uint32_t flags` - the flags this database was opened with. See the
  `MMDB_open()` documentation for more details.
- `const char *filename` - the name of the file which was opened, as passed to
  `MMDB_open()`.
- `MMDB_metadata_s metadata` - the metadata for the database.

## `MMDB_metadata_s` and `MMDB_description_s`

This structure can be retrieved from the `MMDB_s` structure. It contains the
metadata read from the database file. Note that you may find it more convenient
to access this metadata by calling `MMDB_get_metadata_as_entry_data_list()`
instead.

```c
typedef struct MMDB_metadata_s {
    uint32_t node_count;
    uint16_t record_size;
    uint16_t ip_version;
    const char *database_type;
    struct {
        size_t count;
        const char **names;
    } languages;
    uint16_t binary_format_major_version;
    uint16_t binary_format_minor_version;
    uint64_t build_epoch;
    struct {
        size_t count;
        MMDB_description_s **descriptions;
    } description;
} MMDB_metadata_s;

typedef struct MMDB_description_s {
    const char *language;
    const char *description;
} MMDB_description_s;
```

These structures should be mostly self-explanatory.

The `ip_version` member should always be `4` or `6`. The
`binary_format_major_version` should always be `2`.

There is no requirement that the database metadata include languages or
descriptions, so the `count` for these parts of the metadata can be zero. All of
the other `MMDB_metadata_s` fields should be populated.

## `MMDB_lookup_result_s`

This structure is returned as the result of looking up an IP address.

```c
typedef struct MMDB_lookup_result_s {
    bool found_entry;
    MMDB_entry_s entry;
    uint16_t netmask;
} MMDB_lookup_result_s;
```

If the `found_entry` member is false then the other members of this structure do
not contain meaningful values. Always check that `found_entry` is true first.

The `entry` member is used to look up the data associated with the IP address.

The `netmask` member tells you what subnet the IP address belongs to in this
database. For example, if you look up the address `1.1.1.1` in an IPv4 database
and the returned `netmask` is 16, then the address is part of the `1.1.0.0/16`
subnet.

If the database is an IPv6 database, the returned netmask is always an IPv6
prefix length (from 0-128), even if that database _also_ contains IPv4 networks.
If you look up an IPv4 address and would like to turn the netmask into an IPv4
netmask value, you can simply subtract `96` from the value.

## `MMDB_entry_s`

You don't really need to dig around in this structure. You'll get this from a
`MMDB_lookup_result_s` structure and pass it to various functions.

## `MMDB_entry_data_s`

This structure is used to return a single data section entry for an IP. These
entries can in turn point to other entries, as is the case for things like maps
and arrays. Some members of this structure are not documented as they are only
for internal use.

```c
typedef struct MMDB_entry_data_s {
    bool has_data;
    union {
        uint32_t pointer;
        const char *utf8_string;
        double double_value;
        const uint8_t *bytes;
        uint16_t uint16;
        uint32_t uint32;
        int32_t int32;
        uint64_t uint64;
        {mmdb_uint128_t or uint8_t[16]} uint128;
        bool boolean;
        float float_value;
    };
    ...
    uint32_t data_size;
    uint32_t type;
} MMDB_entry_data_s;
```

The `has_data` member is true if data was found for a given lookup. See
`MMDB_get_value()` for more details. If this member is false then none of the
other values in the structure are meaningful.

The union at the beginning of the structure defines the actual data. To
determine which union member is populated you should look at the `type` member.
The `pointer` member of the union should never be populated in any data returned
by the API. Pointers should always be resolved internally.

The `data_size` member is only relevant for `utf8_string` and `bytes` data.
`utf8_string` is not null terminated and `data_size` _must_ be used to determine
its length.

The `type` member can be compared to one of the `MMDB_DATA_TYPE_*` macros. When
`type` is `MMDB_DATA_TYPE_MAP` or `MMDB_DATA_TYPE_ARRAY`, the entry represents a
collection. To access individual items within the collection, use
`MMDB_get_value()` with the appropriate lookup path from the original entry. To
iterate over all items, use `MMDB_get_entry_data_list()`.

### 128-bit Integers

The handling of `uint128` data depends on how your platform supports 128-bit
integers, if it does so at all. With GCC 4.4 and 4.5 we can write
`unsigned int __attribute__ ((__mode__ (TI)))`. With newer versions of GCC
(4.6+) and clang (3.2+) we can simply write "unsigned \_\_int128".

In order to work around these differences, this library defines an
`mmdb_uint128_t` type. This type is defined in the `maxminddb.h` header so you
can use it in your own code.

With older compilers, we can't use an integer so we instead use a 16 byte array
of `uint8_t` values. This is the raw data from the database.

This library provides a public macro `MMDB_UINT128_IS_BYTE_ARRAY` macro. If this
is true (1), then `uint128` values are returned as a byte array, if it is false
then they are returned as a `mmdb_uint128_t` integer.

### Data Type Macros

This library provides a macro for every data type defined by the MaxMind DB
spec.

- `MMDB_DATA_TYPE_UTF8_STRING`
- `MMDB_DATA_TYPE_DOUBLE`
- `MMDB_DATA_TYPE_BYTES`
- `MMDB_DATA_TYPE_UINT16`
- `MMDB_DATA_TYPE_UINT32`
- `MMDB_DATA_TYPE_MAP`
- `MMDB_DATA_TYPE_INT32`
- `MMDB_DATA_TYPE_UINT64`
- `MMDB_DATA_TYPE_UINT128`
- `MMDB_DATA_TYPE_ARRAY`
- `MMDB_DATA_TYPE_BOOLEAN`
- `MMDB_DATA_TYPE_FLOAT`

There are also a few types that are for internal use only:

- `MMDB_DATA_TYPE_EXTENDED`
- `MMDB_DATA_TYPE_POINTER`
- `MMDB_DATA_TYPE_CONTAINER`
- `MMDB_DATA_TYPE_END_MARKER`

If you see one of these in returned data then something has gone very wrong. The
database is damaged or was generated incorrectly or there is a bug in the
libmaxminddb code.

### Pointer Values and `MMDB_close()`

The `utf8_string`, `bytes`, and (maybe) the `uint128` members of this structure
are all pointers directly into the database's data section. This can either be a
`calloc`'d or `mmap`'d block of memory. In either case, these pointers will
become invalid after `MMDB_close()` is called.

If you need to refer to this data after that time you should copy the data with
an appropriate function (`strdup`, `memcpy`, etc.).

## `MMDB_entry_data_list_s`

This structure encapsulates a linked list of `MMDB_entry_data_s` structures.

```c
typedef struct MMDB_entry_data_list_s {
    MMDB_entry_data_s entry_data;
    struct MMDB_entry_data_list_s *next;
} MMDB_entry_data_list_s;
```

This structure lets you look at entire map or array data entry by iterating over
the linked list.

## `MMDB_search_node_s`

This structure encapsulates the two records in a search node. This is really
only useful if you want to write code that iterates over the entire search tree
as opposed to looking up a specific IP address.

```c
typedef struct MMDB_search_node_s {
    uint64_t left_record;
    uint64_t right_record;
    uint8_t left_record_type;
    uint8_t right_record_type;
    MMDB_entry_s left_record_entry;
    MMDB_entry_s right_record_entry;
} MMDB_search_node_s;
```

The two record types will take one of the following values:

- `MMDB_RECORD_TYPE_SEARCH_NODE` - The record points to the next search node.
- `MMDB_RECORD_TYPE_EMPTY` - The record is a placeholder that indicates there is
  no data for the IP address. The search should end here.
- `MMDB_RECORD_TYPE_DATA` - The record is for data in the data section of the
  database. Use the entry for the record when looking up the data for the
  record.
- `MMDB_RECORD_TYPE_INVALID` - The record is invalid. Either an invalid node was
  looked up or the database is corrupt.

The `MMDB_entry_s` for the record is only valid if the type is
`MMDB_RECORD_TYPE_DATA`. Attempts to use an entry for other record types will
result in an error or invalid data.

# STATUS CODES

This library returns (or populates) status codes for many functions. These
status codes are:

- `MMDB_SUCCESS` - everything worked
- `MMDB_FILE_OPEN_ERROR` - there was an error trying to open the MaxMind DB
  file.
- `MMDB_IO_ERROR` - an IO operation failed. Check `errno` for more details.
- `MMDB_CORRUPT_SEARCH_TREE_ERROR` - looking up an IP address in the search tree
  gave us an impossible result. The database is damaged or was generated
  incorrectly or there is a bug in the libmaxminddb code.
- `MMDB_INVALID_METADATA_ERROR` - something in the database is wrong. This
  includes missing metadata keys as well as impossible values (like an
  `ip_version` of 7).
- `MMDB_UNKNOWN_DATABASE_FORMAT_ERROR` - The database metadata indicates that
  its major version is not 2. This library can only handle major version 2.
- `MMDB_OUT_OF_MEMORY_ERROR` - a memory allocation call (`malloc`, etc.) failed.
- `MMDB_INVALID_DATA_ERROR` - an entry in the data section contains invalid
  data. For example, a `uint16` field is claiming to be more than 2 bytes long.
  The database is probably damaged or was generated incorrectly.
- `MMDB_INVALID_LOOKUP_PATH_ERROR` - The lookup path passed to `MMDB_get_value`,
  `MMDB_vget_value`, or `MMDB_aget_value` contains an array offset that is
  larger than LONG_MAX or smaller than LONG_MIN.
- `MMDB_LOOKUP_PATH_DOES_NOT_MATCH_DATA_ERROR` - The lookup path passed to
  `MMDB_get_value`,`MMDB_vget_value`, or `MMDB_aget_value` does not match the
  data structure for the entry. There are number of reasons this can happen. The
  lookup path could include a key not in a map. The lookup path could include an
  array index larger than an array or smaller than the minimum offset from the
  end of an array. It can also happen when the path expects to find a map or
  array where none exist.

All status codes should be treated as `int` values.

## `MMDB_strerror()`

```c
const char *MMDB_strerror(int error_code)
```

This function takes a status code and returns an English string explaining the
status.

# FUNCTIONS

This library provides the following exported functions:

## `MMDB_open()`

```c
int MMDB_open(
    const char *const filename,
    uint32_t flags,
    MMDB_s *const mmdb);
```

This function opens a handle to a MaxMind DB file. Its return value is a status
code as defined above. Always check this call's return value.

```c
MMDB_s mmdb;
int status =
    MMDB_open("/path/to/file.mmdb", MMDB_MODE_MMAP, &mmdb);
if (MMDB_SUCCESS != status) { ... }
...
MMDB_close(&mmdb);
```

`filename` must be encoded as UTF-8 on Windows.

The `MMDB_s` structure you pass in can be on the stack or allocated from the
heap. However, if the open is successful it will contain heap-allocated data, so
you need to close it with `MMDB_close()`. If the status returned is not
`MMDB_SUCCESS` then this library makes sure that all allocated memory is freed
before returning.

The flags currently provided are:

- `MMDB_MODE_MMAP` - open the database with `mmap()`.

Passing in other values for `flags` may yield unpredictable results. In the
future we may add additional flags that you can bitwise-or together with the
mode, as well as additional modes.

You can also pass `0` as the `flags` value in which case the database will be
opened with the default flags. However, these defaults may change in future
releases. The current default is `MMDB_MODE_MMAP`.

## `MMDB_close()`

```c
void MMDB_close(MMDB_s *const mmdb);
```

This frees any allocated or mmap'd memory that is held from the `MMDB_s`
structure. _It does not free the memory allocated for the structure itself!_ If
you allocated the structure from the heap then you are responsible for freeing
it.

## `MMDB_lookup_string()`

```c
MMDB_lookup_result_s MMDB_lookup_string(
    const MMDB_s *const mmdb,
    const char *const ipstr,
    int *const gai_error,
    int *const mmdb_error);
```

This function looks up an IP address that is passed in as a null-terminated
string. Internally it calls `getaddrinfo()` to resolve the address into a binary
form. It then calls `MMDB_lookup_sockaddr()` to look the address up in the
database. If you have already resolved an address you can call
`MMDB_lookup_sockaddr()` directly, rather than resolving the address twice.

```c
int gai_error, mmdb_error;
MMDB_lookup_result_s result =
    MMDB_lookup_string(&mmdb, "1.2.3.4", &gai_error, &mmdb_error);
if (0 != gai_error) { ... }
if (MMDB_SUCCESS != mmdb_error) { ... }

if (result.found_entry) { ... }
```

This function always returns an `MMDB_lookup_result_s` structure, but you should
also check the `gai_error` and `mmdb_error` parameters. If either of these
indicates an error then the returned structure is meaningless.

When `*gai_error` is non-zero (i.e., `getaddrinfo()` failed), `*mmdb_error` is
set to `MMDB_SUCCESS` because no database error occurred. You should always
check `*gai_error` first.

If no error occurred you still need to make sure that the `found_entry` member
in the returned result is true. If it's not, this means that the IP address does
not have an entry in the database.

This function will work with IPv4 addresses even when the database contains data
for both IPv4 and IPv6 addresses. The IPv4 address will be looked up as
'::xxx.xxx.xxx.xxx' rather than being remapped to the `::ffff:xxx.xxx.xxx.xxx`
block allocated for IPv4-mapped IPv6 addresses.

If you pass an IPv6 address to a database with only IPv4 data then the
`found_entry` member will be false, but the `mmdb_error` status will still be
`MMDB_SUCCESS`.

## `MMDB_lookup_sockaddr()`

```c
MMDB_lookup_result_s MMDB_lookup_sockaddr(
    const MMDB_s *const mmdb,
    const struct sockaddr *const sockaddr,
    int *const mmdb_error);
```

This function looks up an IP address that has already been resolved by
`getaddrinfo()`.

Other than not calling `getaddrinfo()` itself, this function is identical to the
`MMDB_lookup_string()` function.

```c
int mmdb_error;
MMDB_lookup_result_s result =
    MMDB_lookup_sockaddr(&mmdb, address->ai_addr, &mmdb_error);
if (MMDB_SUCCESS != mmdb_error) { ... }

if (result.found_entry) { ... }
```

## Data Lookup Functions

There are three functions for looking up data associated with an IP address.

```c
int MMDB_get_value(
    MMDB_entry_s *const start,
    MMDB_entry_data_s *const entry_data,
    ...);
int MMDB_vget_value(
    MMDB_entry_s *const start,
    MMDB_entry_data_s *const entry_data,
    va_list va_path);
int MMDB_aget_value(
    MMDB_entry_s *const start,
    MMDB_entry_data_s *const entry_data,
    const char *const *const path);
```

The three functions allow three slightly different calling styles, but they all
do the same thing.

The first parameter is a pointer to an `MMDB_entry_s` struct. In most cases this
will be a pointer to the `entry` field of the `MMDB_lookup_result_s` value
returned by `MMDB_lookup_string()` or `MMDB_lookup_sockaddr()`.

The second parameter is a reference to an `MMDB_entry_data_s` structure. This
will be populated with the data that is being looked up, if any is found. If
nothing is found, then the `has_data` member of this structure will be false. If
`has_data` is true then you can look at the `type` member.

The final parameter is a lookup path. The path consists of a set of strings
representing either map keys (e.g, "city") or array indexes (e.g., "0", "1",
"-1") to use in the lookup.

Negative array indexes will be treated as an offset from the end of the array.
For instance, "-1" refers to the last element of the array.

The lookup path allows you to navigate a complex data structure. For example,
given this data:

```js
{
    "names": {
        "en": "Germany",
        "de": "Deutschland"
    },
    "cities": [ "Berlin", "Frankfurt" ]
}
```

We could look up the English name with this code:

```c
MMDB_lookup_result_s result =
    MMDB_lookup_sockaddr(&mmdb, address->ai_addr, &mmdb_error);
MMDB_entry_data_s entry_data;
int status =
    MMDB_get_value(&result.entry, &entry_data,
                   "names", "en", NULL);
if (MMDB_SUCCESS != status) { ... }
if (entry_data.has_data) { ... }
```

If we wanted to find the first city the lookup path would be `"cities", "0"`. If
you don't provide a lookup path at all, you'll get the entry which corresponds
to the top level map. The lookup path must always end with `NULL`, regardless of
which function you call.

The `MMDB_get_value` function takes a variable number of arguments. All of the
arguments after the `MMDB_entry_data_s *` structure pointer are the lookup path.
The last argument must be `NULL`.

The `MMDB_vget_value` function accepts a `va_list` as the lookup path. The last
element retrieved by `va_arg()` must be `NULL`.

Finally, the `MMDB_aget_value` accepts an array of strings as the lookup path.
The last member of this array must be `NULL`.

To look up multiple keys within the same map, use the full lookup path for each
key. For example, to get both the `en` and `de` values from the `names` map,
make separate calls with `"names", "en", NULL` and `"names", "de", NULL`. If you
need all of the data from a complex structure rather than specific values,
`MMDB_get_entry_data_list()` is more efficient than making many individual
calls.

For each of the three functions, the return value is a status code as defined
above.

## `MMDB_get_entry_data_list()`

```c
int MMDB_get_entry_data_list(
    MMDB_entry_s *start,
    MMDB_entry_data_list_s **const entry_data_list);
```

The `start` parameter is a pointer to an `MMDB_entry_s` struct. In most cases
this will be a pointer to the `entry` field of the `MMDB_lookup_result_s` value
returned by `MMDB_lookup_string()` or `MMDB_lookup_sockaddr()`.

This function allows you to get all of the data for a complex data structure at
once, rather than looking up each piece using repeated calls to
`MMDB_get_value()`.

```c
MMDB_lookup_result_s result =
    MMDB_lookup_sockaddr(&mmdb, address->ai_addr, &mmdb_error);
MMDB_entry_data_list_s *entry_data_list, *first;
int status =
    MMDB_get_entry_data_list(&result.entry, &entry_data_list);
if (MMDB_SUCCESS != status) { ... }
// save this so we can free this data later
first = entry_data_list;

while (1) {
    MMDB_entry_data_list_s *next = entry_data_list = entry_data_list->next;
    if (NULL == next) {
        break;
    }

    switch (next->entry_data.type) {
        case MMDB_DATA_TYPE_MAP: { ... }
        case MMDB_DATA_TYPE_UTF8_STRING: { ... }
        ...
    }

}

MMDB_free_entry_data_list(first);
```

It's up to you to interpret the `entry_data_list` data structure. The list is
linked in a depth-first traversal. Let's use this structure as an example:

```js
{
    "names": {
        "en": "Germany",
        "de": "Deutschland"
    },
    "cities": [ "Berlin", "Frankfurt" ]
}
```

The list will consist of the following items:

1. MAP - top level map
2. UTF8_STRING - "names" key
3. MAP - map for "names" key
4. UTF8_STRING - "en" key
5. UTF8_STRING - value for "en" key
6. UTF8_STRING - "de" key
7. UTF8_STRING - value for "de" key
8. UTF8_STRING - "cities" key
9. ARRAY - value for "cities" key
10. UTF8_STRING - array[0]
11. UTF8_STRING - array[1]

The return value of the function is a status code as defined above.

## `MMDB_free_entry_data_list()`

```c
void MMDB_free_entry_data_list(
    MMDB_entry_data_list_s *const entry_data_list);
```

The `MMDB_get_entry_data_list()` and `MMDB_get_metadata_as_entry_data_list()`
functions will allocate the linked list structure from the heap. Call this
function to free the `MMDB_entry_data_list_s` structure.

## `MMDB_get_metadata_as_entry_data_list()`

```c
int MMDB_get_metadata_as_entry_data_list(
    const MMDB_s *const mmdb,
    MMDB_entry_data_list_s **const entry_data_list);
```

This function allows you to retrieve the database metadata as a linked list of
`MMDB_entry_data_list_s` structures. This can be a more convenient way to deal
with the metadata than using the metadata structure directly.

```c
    MMDB_entry_data_list_s *entry_data_list, *first;
    int status =
        MMDB_get_metadata_as_entry_data_list(&mmdb, &entry_data_list);
    if (MMDB_SUCCESS != status) { ... }
    first = entry_data_list;
    ... // do something with the data
    MMDB_free_entry_data_list(first);
```

The return value of the function is a status code as defined above.

## `MMDB_dump_entry_data_list()`

```c
int MMDB_dump_entry_data_list(
    FILE *const stream,
    MMDB_entry_data_list_s *const entry_data_list,
    int indent);
```

This function takes a linked list of `MMDB_entry_data_list_s` structures and
stringifies it to the given `stream`. The `indent` parameter is the starting
indent level for the generated output. It is incremented for nested data
structures (maps, array, etc.).

The `stream` must be a file handle (`stdout`, etc). If your platform provides
something like the GNU `open_memstream()` you can use that to capture the output
as a string.

The output is formatted in a JSON-ish fashion, but values are marked with their
data type (except for maps and arrays which are shown with "{}" and "[]"
respectively).

The specific output format may change in future releases, so you should not rely
on the specific formatting produced by this function. It is intended to be used
to show data to users in a readable way and for debugging purposes.

The return value of the function is a status code as defined above.

## `MMDB_read_node()`

```c
int MMDB_read_node(
    const MMDB_s *const mmdb,
    uint32_t node_number,
    MMDB_search_node_s *const node);
```

This reads a specific node in the search tree. The third argument is a reference
to an `MMDB_search_node_s` structure that will be populated by this function.

The return value is a status code. If you pass a `node_number` that is greater
than or equal to the number of nodes in the database, this function will return
`MMDB_INVALID_NODE_NUMBER_ERROR`, otherwise it will return `MMDB_SUCCESS`.

The first node in the search tree is always node 0. If you wanted to iterate
over the whole search tree, you would start by reading node 0 and then following
the records that make up this node, based on the type of each record. If the
type is `MMDB_RECORD_TYPE_SEARCH_NODE` then the record contains an integer for
the next node to look up.

## `MMDB_lib_version()`

```c
const char *MMDB_lib_version(void)
```

This function returns the library version as a string, something like "2.0.0".

# EXAMPLE

```c
#include <errno.h>
#include <maxminddb.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{
    char *filename = argv[1];
    char *ip_address = argv[2];

    MMDB_s mmdb;
    int status = MMDB_open(filename, MMDB_MODE_MMAP, &mmdb);

    if (MMDB_SUCCESS != status) {
        fprintf(stderr, "\n  Can't open %s - %s\n",
                filename, MMDB_strerror(status));

        if (MMDB_IO_ERROR == status) {
            fprintf(stderr, "    IO error: %s\n", strerror(errno));
        }
        exit(1);
    }

    int gai_error, mmdb_error;
    MMDB_lookup_result_s result =
        MMDB_lookup_string(&mmdb, ip_address, &gai_error, &mmdb_error);

    if (0 != gai_error) {
        fprintf(stderr,
                "\n  Error from getaddrinfo for %s - %s\n\n",
                ip_address, gai_strerror(gai_error));
        exit(2);
    }

    if (MMDB_SUCCESS != mmdb_error) {
        fprintf(stderr,
                "\n  Got an error from libmaxminddb: %s\n\n",
                MMDB_strerror(mmdb_error));
        exit(3);
    }

    MMDB_entry_data_list_s *entry_data_list = NULL;

    int exit_code = 0;
    if (result.found_entry) {
        int status = MMDB_get_entry_data_list(&result.entry,
                                              &entry_data_list);

        if (MMDB_SUCCESS != status) {
            fprintf(
                stderr,
                "Got an error looking up the entry data - %s\n",
                MMDB_strerror(status));
            exit_code = 4;
            goto end;
        }

        if (NULL != entry_data_list) {
            MMDB_dump_entry_data_list(stdout, entry_data_list, 2);
        }
    } else {
        fprintf(
            stderr,
            "\n  No entry for this IP address (%s) was found\n\n",
            ip_address);
        exit_code = 5;
    }

    end:
        MMDB_free_entry_data_list(entry_data_list);
        MMDB_close(&mmdb);
        exit(exit_code);
}
```

# REQUIREMENTS

libmaxminddb requires a minimum of POSIX.1-2001 support. If not specified at
compilation time, it defaults to requesting POSIX.1-2008 support.

# THREAD SAFETY

This library is thread safe when compiled and linked with a thread-safe `malloc`
and `free` implementation.

# INSTALLATION AND SOURCE

You can download the latest release of libmaxminddb
[from GitHub](https://github.com/maxmind/libmaxminddb/releases).

[Our GitHub repo](https://github.com/maxmind/libmaxminddb) is publicly
available. Please fork it!

# BUG REPORTS AND PULL REQUESTS

Please report all issues to
[our GitHub issue tracker](https://github.com/maxmind/libmaxminddb/issues). We
welcome bug reports and pull requests. Please note that pull requests are
greatly preferred over patches.

# AUTHORS

This library was written by Boris Zentner (bzentner@maxmind.com) and Dave Rolsky
(drolsky@maxmind.com).

# COPYRIGHT AND LICENSE

Copyright 2013-2026 MaxMind, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use
this file except in compliance with the License. You may obtain a copy of the
License at

    https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed
under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.

# SEE ALSO

mmdblookup(1)
