GETNETBYNAME(3SOCKET) Sockets Library Functions GETNETBYNAME(3SOCKET)


NAME


getnetbyname, getnetbyname_r, getnetbyaddr, getnetbyaddr_r, getnetent,
getnetent_r, setnetent, endnetent - get network entry

SYNOPSIS


cc [ flag ... ] file ... -lsocket -lnsl [ library ... ]
#include <netdb.h>

struct netent *getnetbyname(const char *name);


struct netent *getnetbyname_r(const char *name, struct netent *result,
char *buffer, int buflen);


struct netent *getnetbyaddr(long net, int type);


struct netent *getnetbyaddr_r(long net, int type, struct netent *result,
char *buffer, int buflen);


struct netent *getnetent(void);


struct netent *getnetent_r(struct netent *result, char *buffer,
int buflen);


int setnetent(int stayopen);


int endnetent(void);


DESCRIPTION


These functions are used to obtain entries for networks. An entry may
come from any of the sources for networks specified in the
/etc/nsswitch.conf file. See nsswitch.conf(4).


getnetbyname() searches for a network entry with the network name
specified by the character string parameter name.


getnetbyaddr() searches for a network entry with the network address
specified by net. The parameter type specifies the family of the address.
This should be one of the address families defined in <sys/socket.h>. See
the NOTES section below for more information.


Network numbers and local address parts are returned as machine format
integer values, that is, in host byte order. See also inet(3SOCKET).


The netent.n_net member in the netent structure pointed to by the return
value of the above functions is calculated by inet_network(). The
inet_network() function returns a value in host byte order that is
aligned based upon the input string. For example:


Text Value
"10" 0x0000000a
"10.0" 0x00000a00
"10.0.1" 0a000a0001
"10.0.1.28" 0x0a000180


Commonly, the alignment of the returned value is used as a crude
approximate of pre-CIDR (Classless Inter-Domain Routing) subnet mask. For
example:

in_addr_t addr, mask;

addr = inet_network(net_name);
mask= ~(in_addr_t)0;
if ((addr & IN_CLASSA_NET) == 0)
addr <<= 8, mask <<= 8;
if ((addr & IN_CLASSA_NET) == 0)
addr <<= 8, mask <<= 8;
if ((addr & IN_CLASSA_NET) == 0)
addr <<= 8, mask <<= 8;


This usage is deprecated by the CIDR requirements. See Fuller, V., Li,
T., Yu, J., and Varadhan, K. RFC 1519, Classless Inter-Domain Routing
(CIDR): an Address Assignment and Aggregation Strategy. Network Working
Group. September 1993.


The functions setnetent(), getnetent(), and endnetent() are used to
enumerate network entries from the database.


setnetent() sets (or resets) the enumeration to the beginning of the set
of network entries. This function should be called before the first call
to getnetent(). Calls to getnetbyname() and getnetbyaddr() leave the
enumeration position in an indeterminate state. If the stayopen flag is
non-zero, the system may keep allocated resources such as open file
descriptors until a subsequent call to endnetent().


Successive calls to getnetent() return either successive entries or NULL,
indicating the end of the enumeration.


endnetent() may be called to indicate that the caller expects to do no
further network entry retrieval operations; the system may then
deallocate resources it was using. It is still allowed, but possibly less
efficient, for the process to call more network entry retrieval functions
after calling endnetent().

Reentrant Interfaces


The functions getnetbyname(), getnetbyaddr(), and getnetent() use static
storage that is reused in each call, making these routines unsafe for use
in multi-threaded applications.


The functions getnetbyname_r(), getnetbyaddr_r(), and getnetent_r()
provide reentrant interfaces for these operations.


Each reentrant interface performs the same operation as its non-reentrant
counterpart, named by removing the ``_r'' suffix. The reentrant
interfaces, however, use buffers supplied by the caller to store returned
results, and are safe for use in both single-threaded and multi-threaded
applications.


Each reentrant interface takes the same parameters as its non-reentrant
counterpart, as well as the following additional parameters. The
parameter result must be a pointer to a struct netent structure allocated
by the caller. On successful completion, the function returns the network
entry in this structure. The parameter buffer must be a pointer to a
buffer supplied by the caller. This buffer is used as storage space for
the network entry data. All of the pointers within the returned struct
netent result point to data stored within this buffer. See RETURN VALUES.
The buffer must be large enough to hold all of the data associated with
the network entry. The parameter buflen should give the size in bytes of
the buffer indicated by buffer.


For enumeration in multi-threaded applications, the position within the
enumeration is a process-wide property shared by all threads. setnetent()
may be used in a multi-threaded application but resets the enumeration
position for all threads. If multiple threads interleave calls to
getnetent_r(), the threads will enumerate disjointed subsets of the
network database.


Like their non-reentrant counterparts, getnetbyname_r() and
getnetbyaddr_r() leave the enumeration position in an indeterminate
state.

RETURN VALUES


Network entries are represented by the struct netent structure defined in
<netdb.h>.


The functions getnetbyname(), getnetbyname_r, getnetbyaddr, and
getnetbyaddr_r() each return a pointer to a struct netent if they
successfully locate the requested entry; otherwise they return NULL.


The functions getnetent() and getnetent_r() each return a pointer to a
struct netent if they successfully enumerate an entry; otherwise they
return NULL, indicating the end of the enumeration.


The functions getnetbyname(), getnetbyaddr(), and getnetent() use static
storage, so returned data must be copied before a subsequent call to any
of these functions if the data is to be saved.


When the pointer returned by the reentrant functions getnetbyname_r(),
getnetbyaddr_r(), and getnetent_r() is non-NULL, it is always equal to
the result pointer that was supplied by the caller.


The functions setnetent() and endnetent() return 0 on success.

ERRORS


The reentrant functions getnetbyname_r(), getnetbyaddr_r and
getnetent_r() will return NULL and set errno to ERANGE if the length of
the buffer supplied by caller is not large enough to store the result.
See Intro(2) for the proper usage and interpretation of errno in multi-
threaded applications.

FILES


/etc/networks
network name database


/etc/nsswitch.conf
configuration file for the name service switch


ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|MT-Level | MT-Safe |
+---------------+-----------------+

SEE ALSO


Intro(2), Intro(3), byteorder(3SOCKET), inet(3SOCKET), netdb.h(3HEAD),
networks(4), nsswitch.conf(4), attributes(5)


Fuller, V., Li, T., Yu, J., and Varadhan, K. RFC 1519, Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation Strategy.
Network Working Group. September 1993.

WARNINGS


The reentrant interfaces getnetbyname_r(), getnetbyaddr_r(), and
getnetent_r() are included in this release on an uncommitted basis only,
and are subject to change or removal in future minor releases.

NOTES


The current implementation of these functions only return or accept
network numbers for the Internet address family (type AF_INET). The
functions described in inet(3SOCKET) may be helpful in constructing and
manipulating addresses and network numbers in this form.


When compiling multi-threaded applications, see Intro(3), Notes On
Multithread Applications, for information about the use of the _REENTRANT
flag.


Use of the enumeration interfaces getnetent() and getnetent_r() is
discouraged; enumeration may not be supported for all database sources.
The semantics of enumeration are discussed further in nsswitch.conf(4).


November 4, 2004 GETNETBYNAME(3SOCKET)