mirror of https://github.com/mkerrisk/man-pages
535 lines
13 KiB
Groff
535 lines
13 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified 1993-05-22, David Metcalfe
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.\" Modified 1993-07-25, Rik Faith (faith@cs.unc.edu)
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.\" Modified 1997-02-16, Andries Brouwer (aeb@cwi.nl)
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.\" Modified 1998-12-21, Andries Brouwer (aeb@cwi.nl)
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.\" Modified 2000-08-12, Andries Brouwer (aeb@cwi.nl)
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.\" Modified 2001-05-19, Andries Brouwer (aeb@cwi.nl)
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.\" Modified 2002-08-05, Michael Kerrisk
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.\" Modified 2004-10-31, Andries Brouwer
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.\"
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.TH GETHOSTBYNAME 3 2021-03-22 "" "Linux Programmer's Manual"
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.SH NAME
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gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
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h_errno,
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herror, hstrerror,
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gethostbyaddr_r,
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gethostbyname2, gethostbyname2_r, gethostbyname_r,
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gethostent_r \- get network host entry
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.SH SYNOPSIS
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.nf
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.B #include <netdb.h>
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.PP
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.B extern int h_errno;
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.PP
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.BI "struct hostent *gethostbyname(const char *" name );
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.BI "struct hostent *gethostbyaddr(const void *" addr ,
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.BI " socklen_t " len ", int " type );
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.PP
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.BI "void sethostent(int " stayopen );
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.B void endhostent(void);
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.PP
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.BI "void herror(const char *" s );
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.BI "const char *hstrerror(int " err );
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.PP
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/* System V/POSIX extension */
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.B struct hostent *gethostent(void);
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.PP
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/* GNU extensions */
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.BI "struct hostent *gethostbyname2(const char *" name ", int " af );
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.PP
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.BI "int gethostent_r(struct hostent *restrict " ret ,
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.BI " char *restrict " buf ", size_t " buflen ,
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.BI " struct hostent **restrict " result ,
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.BI " int *restrict " h_errnop );
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.PP
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.BI "int gethostbyaddr_r(const void *restrict " addr ", socklen_t " len \
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", int " type ,
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.BI " struct hostent *restrict " ret ,
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.BI " char *restrict " buf ", size_t " buflen ,
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.BI " struct hostent **restrict " result ,
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.BI " int *restrict " h_errnop );
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.BI "int gethostbyname_r(const char *restrict " name ,
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.BI " struct hostent *restrict " ret ,
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.BI " char *restrict " buf ", size_t " buflen ,
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.BI " struct hostent **restrict " result ,
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.BI " int *restrict " h_errnop );
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.BI "int gethostbyname2_r(const char *restrict " name ", int " af,
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.BI " struct hostent *restrict " ret ,
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.BI " char *restrict " buf ", size_t " buflen ,
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.BI " struct hostent **restrict " result ,
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.BI " int *restrict " h_errnop );
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.fi
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.PP
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR gethostbyname2 (),
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.BR gethostent_r (),
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.BR gethostbyaddr_r (),
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.BR gethostbyname_r (),
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.BR gethostbyname2_r ():
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.nf
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Since glibc 2.19:
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_DEFAULT_SOURCE
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Glibc up to and including 2.19:
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_BSD_SOURCE || _SVID_SOURCE
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.fi
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.PP
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.BR herror (),
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.BR hstrerror ():
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.nf
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Since glibc 2.19:
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_DEFAULT_SOURCE
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Glibc 2.8 to 2.19:
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_BSD_SOURCE || _SVID_SOURCE
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Before glibc 2.8:
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none
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.fi
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.PP
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.BR h_errno :
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.nf
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Since glibc 2.19
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_DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
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Glibc 2.12 to 2.19:
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_BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
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Before glibc 2.12:
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none
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.fi
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.SH DESCRIPTION
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The
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.BR gethostbyname* (),
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.BR gethostbyaddr* (),
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.BR herror (),
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and
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.BR hstrerror ()
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functions are obsolete.
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Applications should use
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.BR getaddrinfo (3),
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.BR getnameinfo (3),
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and
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.BR gai_strerror (3)
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instead.
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.PP
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The
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.BR gethostbyname ()
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function returns a structure of type
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.I hostent
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for the given host
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.IR name .
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Here
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.I name
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is either a hostname or an IPv4 address in standard dot notation (as for
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.BR inet_addr (3)).
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If
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.I name
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is an IPv4 address, no lookup is performed and
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.BR gethostbyname ()
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simply copies
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.I name
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into the
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.I h_name
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field and its
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.I struct in_addr
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equivalent into the
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.I h_addr_list[0]
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field of the returned
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.I hostent
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structure.
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If
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.I name
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doesn't end in a dot and the environment variable
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.B HOSTALIASES
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is set, the alias file pointed to by
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.B HOSTALIASES
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will first be searched for
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.I name
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(see
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.BR hostname (7)
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for the file format).
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The current domain and its parents are searched unless \fIname\fP
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ends in a dot.
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.PP
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The
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.BR gethostbyaddr ()
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function returns a structure of type \fIhostent\fP
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for the given host address \fIaddr\fP of length \fIlen\fP and address type
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\fItype\fP.
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Valid address types are
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.B AF_INET
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and
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.BR AF_INET6
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(defined in
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.IR <sys/socket.h> ).
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The host address argument is a pointer to a struct of a type depending
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on the address type, for example a \fIstruct in_addr *\fP (probably
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obtained via a call to
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.BR inet_addr (3))
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for address type
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.BR AF_INET .
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.PP
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The
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.BR sethostent ()
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function specifies, if \fIstayopen\fP is true (1),
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that a connected TCP socket should be used for the name server queries and
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that the connection should remain open during successive queries.
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Otherwise, name server queries will use UDP datagrams.
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.PP
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The
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.BR endhostent ()
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function ends the use of a TCP connection for name
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server queries.
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.PP
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The (obsolete)
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.BR herror ()
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function prints the error message associated
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with the current value of \fIh_errno\fP on \fIstderr\fP.
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.PP
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The (obsolete)
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.BR hstrerror ()
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function takes an error number
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(typically \fIh_errno\fP) and returns the corresponding message string.
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.PP
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The domain name queries carried out by
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.BR gethostbyname ()
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and
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.BR gethostbyaddr ()
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rely on the Name Service Switch
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.RB ( nsswitch.conf (5))
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configured sources or a local name server
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.RB ( named (8)).
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The default action is to query the Name Service Switch
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.RB ( nsswitch.conf (5))
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configured sources, failing that, a local name server
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.RB ( named (8)).
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.\"
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.SS Historical
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The
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.BR nsswitch.conf (5)
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file is the modern way of controlling the order of host lookups.
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.PP
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In glibc 2.4 and earlier, the
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.I order
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keyword was used to control the order of host lookups as defined in
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.IR /etc/host.conf
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.RB ( host.conf (5)).
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.PP
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The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
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.PP
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.in +4n
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.EX
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struct hostent {
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char *h_name; /* official name of host */
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char **h_aliases; /* alias list */
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int h_addrtype; /* host address type */
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int h_length; /* length of address */
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char **h_addr_list; /* list of addresses */
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}
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#define h_addr h_addr_list[0] /* for backward compatibility */
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.EE
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.in
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.PP
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The members of the \fIhostent\fP structure are:
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.TP
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.I h_name
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The official name of the host.
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.TP
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.I h_aliases
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An array of alternative names for the host, terminated by a null pointer.
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.TP
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.I h_addrtype
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The type of address; always
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.B AF_INET
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or
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.B AF_INET6
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at present.
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.TP
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.I h_length
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The length of the address in bytes.
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.TP
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.I h_addr_list
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An array of pointers to network addresses for the host (in network byte
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order), terminated by a null pointer.
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.TP
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.I h_addr
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The first address in \fIh_addr_list\fP for backward compatibility.
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.SH RETURN VALUE
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The
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.BR gethostbyname ()
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and
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.BR gethostbyaddr ()
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functions return the
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.I hostent
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structure or a null pointer if an error occurs.
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On error, the
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.I h_errno
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variable holds an error number.
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When non-NULL, the return value may point at static data, see the notes below.
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.SH ERRORS
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The variable \fIh_errno\fP can have the following values:
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.TP
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.B HOST_NOT_FOUND
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The specified host is unknown.
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.TP
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.BR NO_DATA
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The requested name is valid but does not have an IP address.
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Another type of request to the name server for this domain
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may return an answer.
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The constant
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.BR NO_ADDRESS
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is a synonym for
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.BR NO_DATA .
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.TP
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.B NO_RECOVERY
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A nonrecoverable name server error occurred.
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.TP
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.B TRY_AGAIN
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A temporary error occurred on an authoritative name server.
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Try again later.
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.SH FILES
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.TP
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.I /etc/host.conf
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resolver configuration file
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.TP
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.I /etc/hosts
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host database file
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.TP
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.I /etc/nsswitch.conf
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name service switch configuration
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.ad l
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.nh
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.TS
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allbox;
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lb lb lbx
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l l l.
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Interface Attribute Value
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T{
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.BR gethostbyname ()
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T} Thread safety T{
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MT-Unsafe race:hostbyname env
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locale
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T}
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T{
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.BR gethostbyaddr ()
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T} Thread safety T{
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MT-Unsafe race:hostbyaddr env
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locale
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T}
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T{
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.BR sethostent (),
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.BR endhostent (),
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.BR gethostent_r ()
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T} Thread safety T{
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MT-Unsafe race:hostent env
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locale
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T}
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T{
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.BR herror (),
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.BR hstrerror ()
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T} Thread safety MT-Safe
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T{
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.BR gethostent ()
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T} Thread safety T{
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MT-Unsafe race:hostent
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race:hostentbuf env locale
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T}
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T{
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.BR gethostbyname2 ()
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T} Thread safety T{
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MT-Unsafe race:hostbyname2
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env locale
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T}
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T{
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.BR gethostbyaddr_r (),
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.BR gethostbyname_r (),
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.BR gethostbyname2_r ()
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T} Thread safety MT-Safe env locale
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.TE
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.hy
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.ad
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.sp 1
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In the above table,
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.I hostent
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in
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.I race:hostent
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signifies that if any of the functions
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.BR sethostent (),
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.BR gethostent (),
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.BR gethostent_r (),
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or
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.BR endhostent ()
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are used in parallel in different threads of a program,
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then data races could occur.
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.SH CONFORMING TO
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POSIX.1-2001 specifies
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.BR gethostbyname (),
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.BR gethostbyaddr (),
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.BR sethostent (),
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.BR endhostent (),
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.BR gethostent (),
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and
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.IR h_errno ;
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.BR gethostbyname (),
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.BR gethostbyaddr (),
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and
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.IR h_errno
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are marked obsolescent in that standard.
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|
POSIX.1-2008 removes the specifications of
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.BR gethostbyname (),
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.BR gethostbyaddr (),
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and
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.IR h_errno ,
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recommending the use of
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.BR getaddrinfo (3)
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and
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.BR getnameinfo (3)
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instead.
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.SH NOTES
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|
The functions
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.BR gethostbyname ()
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and
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.BR gethostbyaddr ()
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may return pointers to static data, which may be overwritten by
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later calls.
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Copying the
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.I struct hostent
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does not suffice, since it contains pointers; a deep copy is required.
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.PP
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In the original BSD implementation the
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.I len
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argument
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of
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.BR gethostbyname ()
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|
was an
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.IR int .
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|
The SUSv2 standard is buggy and declares the
|
|
.I len
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|
argument of
|
|
.BR gethostbyaddr ()
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|
to be of type
|
|
.IR size_t .
|
|
(That is wrong, because it has to be
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|
.IR int ,
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|
and
|
|
.I size_t
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|
is not.
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|
POSIX.1-2001 makes it
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|
.IR socklen_t ,
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which is OK.)
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|
See also
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|
.BR accept (2).
|
|
.PP
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|
The BSD prototype for
|
|
.BR gethostbyaddr ()
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|
uses
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|
.I "const char\ *"
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|
for the first argument.
|
|
.SS System V/POSIX extension
|
|
POSIX requires the
|
|
.BR gethostent ()
|
|
call, which should return the next entry in the host data base.
|
|
When using DNS/BIND this does not make much sense, but it may
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|
be reasonable if the host data base is a file that can be read
|
|
line by line.
|
|
On many systems, a routine of this name reads
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|
from the file
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.IR /etc/hosts .
|
|
.\" e.g., Linux, FreeBSD, UnixWare, HP-UX
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|
It may be available only when the library was built without DNS support.
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|
.\" e.g., FreeBSD, AIX
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|
The glibc version will ignore ipv6 entries.
|
|
This function is not reentrant,
|
|
and glibc adds a reentrant version
|
|
.BR gethostent_r ().
|
|
.SS GNU extensions
|
|
Glibc2 also has a
|
|
.BR gethostbyname2 ()
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|
that works like
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|
.BR gethostbyname (),
|
|
but permits to specify the address family to which the address must belong.
|
|
.PP
|
|
Glibc2 also has reentrant versions
|
|
.BR gethostent_r (),
|
|
.BR gethostbyaddr_r (),
|
|
.BR gethostbyname_r (),
|
|
and
|
|
.BR gethostbyname2_r ().
|
|
The caller supplies a
|
|
.I hostent
|
|
structure
|
|
.I ret
|
|
which will be filled in on success, and a temporary work buffer
|
|
.I buf
|
|
of size
|
|
.IR buflen .
|
|
After the call,
|
|
.I result
|
|
will point to the result on success.
|
|
In case of an error
|
|
or if no entry is found
|
|
.I result
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|
will be NULL.
|
|
The functions return 0 on success and a nonzero error number on failure.
|
|
In addition to the errors returned by the nonreentrant
|
|
versions of these functions, if
|
|
.I buf
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|
is too small, the functions will return
|
|
.BR ERANGE ,
|
|
and the call should be retried with a larger buffer.
|
|
The global variable
|
|
.I h_errno
|
|
is not modified, but the address of a variable in which to store error numbers
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|
is passed in
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|
.IR h_errnop .
|
|
.SH BUGS
|
|
.BR gethostbyname ()
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|
does not recognize components of a dotted IPv4 address string
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|
that are expressed in hexadecimal.
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|
.\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482973
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|
.SH SEE ALSO
|
|
.BR getaddrinfo (3),
|
|
.\" .BR getipnodebyaddr (3),
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|
.\" .BR getipnodebyname (3),
|
|
.BR getnameinfo (3),
|
|
.BR inet (3),
|
|
.BR inet_ntop (3),
|
|
.BR inet_pton (3),
|
|
.BR resolver (3),
|
|
.BR hosts (5),
|
|
.BR nsswitch.conf (5),
|
|
.BR hostname (7),
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.BR named (8)
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.\" .BR resolv+ (8)
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