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##### 16.6.2.4 Host Names

Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address ‘`158.121.106.19`’ is also known as ‘`alpha.gnu.org`’; and other machines in the ‘`gnu.org`’ domain can refer to it simply as ‘`alpha`’.

Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file `/etc/hosts` or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in `netdb.h`. They are BSD features, defined unconditionally if you include `netdb.h`.

— Data Type: struct hostent

> This data type is used to represent an entry in the hosts database. It has the following members:

> `char *h_name`
>   This is the “official” name of the host.
> `char **h_aliases`
>   These are alternative names for the host, represented as a null-terminated vector of strings.
> `int h_addrtype`
>   This is the host address type; in practice, its value is always either `AF_INET` or `AF_INET6`, with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the database as well as Internet addresses; if this were done, you might find a value in this field other than `AF_INET` or `AF_INET6`. See Socket Addresses.
> `int h_length`
>   This is the length, in bytes, of each address.
> `char **h_addr_list`
>   This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer.
> `char *h_addr`
>   This is a synonym for `h_addr_list[0]`; in other words, it is the first host address.

As far as the host database is concerned, each address is just a block of memory `h_length` bytes long. But in other contexts there is an implicit assumption that you can convert IPv4 addresses to a `struct in_addr` or an `uint32_t`. Host addresses in a `struct hostent` structure are always given in network byte order; see Byte Order.

You can use `gethostbyname`, `gethostbyname2` or `gethostbyaddr` to search the hosts database for information about a particular host. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. You can also use `getaddrinfo` and `getnameinfo` to obtain this information.

— Function: struct hostent * gethostbyname (const char *name)

> The `gethostbyname` function returns information about the host named name. If the lookup fails, it returns a null pointer.

— Function: struct hostent * gethostbyname2 (const char *name, int af)

> The `gethostbyname2` function is like `gethostbyname`, but allows the caller to specify the desired address family (e.g. `AF_INET` or `AF_INET6`) of the result.

— Function: struct hostent * gethostbyaddr (const void *addr, socklen_t length, int format)

> The `gethostbyaddr` function returns information about the host with Internet address addr. The parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The length argument is the size (in bytes) of the address at addr. format specifies the address format; for an IPv4 Internet address, specify a value of `AF_INET`; for an IPv6 Internet address, use `AF_INET6`.

> If the lookup fails, `gethostbyaddr` returns a null pointer.

If the name lookup by `gethostbyname` or `gethostbyaddr` fails, you can find out the reason by looking at the value of the variable `h_errno`. (It would be cleaner design for these functions to set `errno`, but use of `h_errno` is compatible with other systems.)

Here are the error codes that you may find in `h_errno`:

`HOST_NOT_FOUND`
  No such host is known in the database.
`TRY_AGAIN`
  This condition happens when the name server could not be contacted. If you try again later, you may succeed then.
`NO_RECOVERY`
  A non-recoverable error occurred.
`NO_ADDRESS`
  The host database contains an entry for the name, but it doesn't have an associated Internet address.

The lookup functions above all have one in common: they are not reentrant and therefore unusable in multi-threaded applications. Therefore provides the GNU C Library a new set of functions which can be used in this context.

— Function: int gethostbyname_r (const char *restrict name, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

> The `gethostbyname_r` function returns information about the host named name. The caller must pass a pointer to an object of type `struct hostent` in the result_buf parameter. In addition the function may need extra buffer space and the caller must pass an pointer and the size of the buffer in the buf and buflen parameters.

> A pointer to the buffer, in which the result is stored, is available in `*`result after the function call successfully returned. If an error occurs or if no entry is found, the pointer `*`result is a null pointer. Success is signalled by a zero return value. If the function failed the return value is an error number. In addition to the errors defined for `gethostbyname` it can also be `ERANGE`. In this case the call should be repeated with a larger buffer. Additional error information is not stored in the global variable `h_errno` but instead in the object pointed to by h_errnop.

> Here's a small example:

>               struct hostent *
>               gethostname (char *host)
>               {
>                 struct hostent hostbuf, *hp;
>                 size_t hstbuflen;
>                 char *tmphstbuf;
>                 int res;
>                 int herr;

>                 hstbuflen = 1024;
>                 /* Allocate buffer, remember to free it to avoid memory leakage.  */
>                 tmphstbuf = malloc (hstbuflen);

>                 while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
>                                                &hp, &herr)) == ERANGE)
>                   {
>                     /* Enlarge the buffer.  */
>                     hstbuflen *= 2;
>                     tmphstbuf = realloc (tmphstbuf, hstbuflen);
>                   }
>                 /*  Check for errors.  */
>                 if (res || hp == NULL)
>                   return NULL;
>                 return hp;
>               }

— Function: int gethostbyname2_r (const char *name, int af, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

> The `gethostbyname2_r` function is like `gethostbyname_r`, but allows the caller to specify the desired address family (e.g. `AF_INET` or `AF_INET6`) for the result.

— Function: int gethostbyaddr_r (const void *addr, socklen_t length, int format, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

> The `gethostbyaddr_r` function returns information about the host with Internet address addr. The parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The length argument is the size (in bytes) of the address at addr. format specifies the address format; for an IPv4 Internet address, specify a value of `AF_INET`; for an IPv6 Internet address, use `AF_INET6`.

> Similar to the `gethostbyname_r` function, the caller must provide buffers for the result and memory used internally. In case of success the function returns zero. Otherwise the value is an error number where `ERANGE` has the special meaning that the caller-provided buffer is too small.

You can also scan the entire hosts database one entry at a time using `sethostent`, `gethostent` and `endhostent`. Be careful when using these functions because they are not reentrant.

— Function: void sethostent (int stayopen)

> This function opens the hosts database to begin scanning it. You can then call `gethostent` to read the entries.

> If the stayopen argument is nonzero, this sets a flag so that subsequent calls to `gethostbyname` or `gethostbyaddr` will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct hostent * gethostent (void)

> This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries.

— Function: void endhostent (void)

> This function closes the hosts database.