(libc.info.gz) Host Names
Info Catalog
(libc.info.gz) Host Address Functions
(libc.info.gz) Host Addresses
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'. 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 |
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 |
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 |
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 | 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 | 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 | 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 | 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 |
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 | POSIX
Safety Concepts.
This function closes the hosts database.
Info Catalog
(libc.info.gz) Host Address Functions
(libc.info.gz) Host Addresses
automatically generated by
info2html