The GNU C Library: Host Names

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)

Preliminary: | MT-Unsafe race:hostbyname env locale | AS-Unsafe dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd | See POSIX Safety Concepts.

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)

Preliminary: | MT-Unsafe race:hostbyname2 env locale | AS-Unsafe dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd | See POSIX Safety Concepts.

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)

Preliminary: | MT-Unsafe race:hostbyaddr env locale | AS-Unsafe dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd | See POSIX Safety Concepts.

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)

Preliminary: | MT-Safe env locale | AS-Unsafe dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd | See POSIX Safety Concepts.

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. The buffer passed as the buf parameter can be freed only once the caller has finished with the result hostent struct, or has copied it including all the other memory that it points to. 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;

  hostbuf = malloc (sizeof (struct hostent));
  hstbuflen = 1024;
  tmphstbuf = malloc (hstbuflen);

  while ((res = gethostbyname_r (host, hostbuf, tmphstbuf, hstbuflen,
                                 &hp, &herr)) == ERANGE)
    {
      /* Enlarge the buffer.  */
      hstbuflen *= 2;
      tmphstbuf = realloc (tmphstbuf, hstbuflen);
    }

  free (tmphstbuf);
  /*  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)

Preliminary: | MT-Safe env locale | AS-Unsafe dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd | See POSIX Safety Concepts.

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)

Preliminary: | MT-Safe env locale | AS-Unsafe dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd | See POSIX Safety Concepts.

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)

Preliminary: | MT-Unsafe race:hostent env locale | AS-Unsafe dlopen plugin heap lock | AC-Unsafe corrupt lock fd mem | See POSIX Safety Concepts.

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)

Preliminary: | MT-Unsafe race:hostent race:hostentbuf env locale | AS-Unsafe dlopen plugin heap lock | AC-Unsafe corrupt lock fd mem | See POSIX Safety Concepts.

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)

Preliminary: | MT-Unsafe race:hostent env locale | AS-Unsafe dlopen plugin heap lock | AC-Unsafe corrupt lock fd mem | See POSIX Safety Concepts.

This function closes the hosts database.