PROLOG This manual page is part of the POSIX Programmer s Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME gethostbyaddr, gethostbyname - network host database functions

SYNOPSIS #include <netdb.h>

struct hostent *gethostbyaddr(const void *addr, socklen_t len, int type); struct hostent *gethostbyname(const char *name);

DESCRIPTION These functions shall retrieve information about hosts. This informa- tion is considered to be stored in a database that can be accessed sequentially or randomly. Implementation of this database is unspeci- fied.

Note: In many cases it is implemented by the Domain Name System, as documented in RFC 1034, RFC 1035, and RFC 1886.

Entries shall be returned in hostent structures.

The gethostbyaddr() function shall return an entry containing addresses of address family type for the host with address addr. The len argument contains the length of the address pointed to by addr. The gethost- byaddr() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.

The gethostbyname() function shall return an entry containing addresses of address family AF_INET for the host with name name. The gethostby- name() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.

The addr argument of gethostbyaddr() shall be an in_addr structure when type is AF_INET. It contains a binary format (that is, not null-termi- nated) address in network byte order. The gethostbyaddr() function is not guaranteed to return addresses of address families other than AF_INET, even when such addresses exist in the database.

If gethostbyaddr() returns successfully, then the h_addrtype field in the result shall be the same as the type argument that was passed to the function, and the h_addr_list field shall list a single address that is a copy of the addr argument that was passed to the function.

The name argument of gethostbyname() shall be a node name; the behavior of gethostbyname() when passed a numeric address string is unspecified. For IPv4, a numeric address string shall be in the dotted-decimal nota- tion described in inet_addr() .

If name is not a numeric address string and is an alias for a valid host name, then gethostbyname() shall return information about the host name to which the alias refers, and name shall be included in the list of aliases returned.

RETURN VALUE Upon successful completion, these functions shall return a pointer to a hostent structure if the requested entry was found, and a null pointer if the end of the database was reached or the requested entry was not found.

Upon unsuccessful completion, gethostbyaddr() and gethostbyname() shall set h_errno to indicate the error.

ERRORS These functions shall fail in the following cases. The gethostbyaddr() and gethostbyname() functions shall set h_errno to the value shown in the list below. Any changes to errno are unspecified.


No such host is known.

NO_DATA The server recognized the request and the name, but no address is available. Another type of request to the name server for the domain might return an answer.


An unexpected server failure occurred which cannot be recovered.

TRY_AGAIN A temporary and possibly transient error occurred, such as a failure of a server to respond.

The following sections are informative.


APPLICATION USAGE The gethostbyaddr() and gethostbyname() functions may return pointers to static data, which may be overwritten by subsequent calls to any of these functions.

The getaddrinfo() and getnameinfo() functions are preferred over the gethostbyaddr() and gethostbyname() functions.


FUTURE DIRECTIONS The gethostbyaddr() and gethostbyname() functions may be withdrawn in a future version.

SEE ALSO endhostent() , endservent() , gai_strerror() , getaddrinfo() , h_errno() , inet_addr() , the Base Definitions volume of IEEE Std 1003.1-2001, <netdb.h>

COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .