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12 .TH GETHOSTBYNAME 3NSL "Sep 10, 2013"
13 .SH NAME
14 gethostbyname, gethostbyname_r, gethostbyaddr, gethostbyaddr_r, gethostent,
15 gethostent_r, sethostent, endhostent \- get network host entry
16 .SH SYNOPSIS
17 .LP
18 .nf
19 \fBcc\fR [ \fIflag\fR... ] \fIfile\fR... \fB-lnsl\fR [ \fIlibrary\fR... ]
20 #include <netdb.h>
21
22 \fBstruct hostent *\fR\fBgethostbyname\fR(\fBconst char *\fR\fIname\fR);
23 .fi
24
25 .LP
26 .nf
27 \fBstruct hostent *\fR\fBgethostbyname_r\fR(\fBconst char *\fR\fIname\fR,
28 \fBstruct hostent *\fR\fIresult\fR, \fBchar *\fR\fIbuffer\fR, \fBint\fR \fIbuflen\fR,
29 \fBint *\fR\fIh_errnop\fR);
30 .fi
31
32 .LP
33 .nf
34 \fBstruct hostent *\fR\fBgethostbyaddr\fR(\fBconst char *\fR\fIaddr\fR, \fBint\fR \fIlen\fR,
35 \fBint\fR \fItype\fR);
36 .fi
37
38 .LP
39 .nf
40 \fBstruct hostent *\fR\fBgethostbyaddr_r\fR(\fBconst char *\fR\fIaddr\fR, \fBint\fR \fIlength\fR,
41 \fBint\fR \fItype\fR, \fBstruct hostent *\fR\fIresult\fR, \fBchar *\fR\fIbuffer\fR,
42 \fBint\fR \fIbuflen\fR, \fBint *\fR\fIh_errnop\fR);
43 .fi
44
45 .LP
46 .nf
47 \fBstruct hostent *\fR\fBgethostent\fR(\fBvoid\fR);
48 .fi
49
50 .LP
51 .nf
52 \fBstruct hostent *\fR\fBgethostent_r\fR(\fBstruct hostent *\fR\fIresult\fR,
53 \fBchar *\fR\fIbuffer\fR, \fBint\fR \fIbuflen\fR, \fBint *\fR\fIh_errnop\fR);
54 .fi
55
56 .LP
57 .nf
58 \fBint\fR \fBsethostent\fR(\fBint\fR \fIstayopen\fR);
59 .fi
60
61 .LP
62 .nf
63 \fBint\fR \fBendhostent\fR(\fBvoid\fR);
64 .fi
65
66 .SH DESCRIPTION
67 .sp
68 .LP
69 These functions are used to obtain entries describing hosts. An entry can come
70 from any of the sources for \fBhosts\fR specified in the
71 \fB/etc/nsswitch.conf\fR file. See \fBnsswitch.conf\fR(4). These functions have
72 been superseded by \fBgetipnodebyname\fR(3SOCKET),
73 \fBgetipnodebyaddr\fR(3SOCKET), and \fBgetaddrinfo\fR(3SOCKET), which provide
74 greater portability to applications when multithreading is performed or
75 technologies such as IPv6 are used. For example, the functions described in the
76 following cannot be used with applications targeted to work with IPv6.
77 .sp
78 .LP
79 The \fBgethostbyname()\fR function searches for information for a host with the
80 hostname specified by the character-string parameter \fIname\fR.
81 .sp
82 .LP
83 The \fBgethostbyaddr()\fR function searches for information for a host with a
84 given host address. The parameter \fBtype\fR specifies the family of the
85 address. This should be one of the address families defined in
86 \fB<sys/socket.h>\fR\&. See the \fBNOTES\fR section for more information. Also
87 see the \fBEXAMPLES\fR section for information on how to convert an Internet
88 \fBIP\fR address notation that is separated by periods (.) into an \fIaddr\fR
89 parameter. The parameter \fIlen\fR specifies the length of the buffer indicated
90 by \fIaddr\fR.
91 .sp
92 .LP
93 All addresses are returned in network order. In order to interpret the
94 addresses, \fBbyteorder\fR(3SOCKET) must be used for byte order conversion.
95 .sp
96 .LP
97 The \fBsethostent()\fR, \fBgethostent()\fR, and \fBendhostent()\fR functions
98 are used to enumerate host entries from the database.
99 .sp
100 .LP
101 The \fBsethostent()\fR function sets or resets the enumeration to the beginning
102 of the set of host entries. This function should be called before the first
103 call to \fBgethostent()\fR. Calls to \fBgethostbyname()\fR and
104 \fBgethostbyaddr()\fR leave the enumeration position in an indeterminate state.
105 If the \fIstayopen\fR flag is non-zero, the system can keep allocated resources
106 such as open file descriptors until a subsequent call to \fBendhostent()\fR.
107 .sp
108 .LP
109 Successive calls to the \fBgethostent()\fR function return either successive
110 entries or \fINULL,\fR indicating the end of the enumeration.
111 .sp
112 .LP
113 The \fBendhostent()\fR function can be called to indicate that the caller
114 expects to do no further host entry retrieval operations; the system can then
115 deallocate resources it was using. It is still allowed, but possibly less
116 efficient, for the process to call more host retrieval functions after calling
117 \fBendhostent()\fR.
118 .SS "Reentrant Interfaces"
119 .sp
120 .LP
121 The \fBgethostbyname()\fR, \fBgethostbyaddr()\fR, and \fBgethostent()\fR
122 functions use static storage that is reused in each call, making these
123 functions unsafe for use in multithreaded applications.
124 .sp
125 .LP
126 The \fBgethostbyname_r()\fR, \fBgethostbyaddr_r()\fR, and \fBgethostent_r()\fR
127 functions provide reentrant interfaces for these operations.
128 .sp
129 .LP
130 Each reentrant interface performs the same operation as its non-reentrant
131 counterpart, named by removing the \fB_r\fR suffix. The reentrant interfaces,
132 however, use buffers supplied by the caller to store returned results and the
133 interfaces are safe for use in both single-threaded and multithreaded
134 applications.
135 .sp
136 .LP
137 Each reentrant interface takes the same parameters as its non-reentrant
138 counterpart, as well as the following additional parameters. The parameter
139 \fIresult\fR must be a pointer to a \fBstruct hostent\fR structure allocated by
140 the caller. On successful completion, the function returns the host entry in
141 this structure. The parameter \fIbuffer\fR must be a pointer to a buffer
142 supplied by the caller. This buffer is used as storage space for the host data.
143 All of the pointers within the returned \fBstruct hostent\fR \fIresult\fR point
144 to data stored within this buffer. See the \fBRETURN VALUES\fR section for more
145 information. The buffer must be large enough to hold all of the data associated
146 with the host entry. The parameter \fIbuflen\fR should give the size in bytes
147 of the buffer indicated by \fIbuffer\fR. The parameter \fIh_errnop\fR should be
148 a pointer to an integer. An integer error status value is stored there on
149 certain error conditions. See the \fBERRORS\fR section for more information.
150 .sp
151 .LP
152 For enumeration in multithreaded applications, the position within the
153 enumeration is a process-wide property shared by all threads. The
154 \fBsethostent()\fR function can be used in a multithreaded application but
155 resets the enumeration position for all threads. If multiple threads interleave
156 calls to \fBgethostent_r()\fR, the threads will enumerate disjoint subsets of
157 the host database.
158 .sp
159 .LP
160 Like their non-reentrant counterparts, \fBgethostbyname_r()\fR and
161 \fBgethostbyaddr_r()\fR leave the enumeration position in an indeterminate
162 state.
163 .SH RETURN VALUES
164 .sp
165 .LP
166 Host entries are represented by the \fBstruct hostent\fR structure defined in
167 \fB<netdb.h>\fR:
168 .sp
169 .in +2
170 .nf
171 struct hostent {
172 char *h_name; /* canonical name of host */
173 char **h_aliases; /* alias list */
174 int h_addrtype; /* host address type */
175 int h_length; /* length of address */
176 char **h_addr_list; /* list of addresses */
177 };
178 .fi
179 .in -2
180
181 .sp
182 .LP
183 See the \fBEXAMPLES\fR section for information about how to retrieve a ``.''
184 separated Internet \fBIP\fR address string from the \fIh_addr_list\fR field of
185 \fBstruct hostent\fR.
186 .sp
187 .LP
188 The \fBgethostbyname()\fR, \fBgethostbyname_r()\fR, \fBgethostbyaddr()\fR, and
189 \fBgethostbyaddr_r()\fR functions each return a pointer to a \fBstruct
190 hostent\fR if they successfully locate the requested entry; otherwise they
191 return \fINULL\fR.
192 .sp
193 .LP
194 The \fBgethostent()\fR and \fBgethostent_r()\fR functions each return a pointer
195 to a \fBstruct hostent\fR if they successfully enumerate an entry; otherwise
196 they return \fINULL\fR, indicating the end of the enumeration.
197 .sp
198 .LP
199 The \fBgethostbyname()\fR, \fBgethostbyaddr()\fR, and \fBgethostent()\fR
200 functions use static storage, so returned data must be copied before a
201 subsequent call to any of these functions if the data is to be saved.
202 .sp
203 .LP
204 When the pointer returned by the reentrant functions \fBgethostbyname_r()\fR,
205 \fBgethostbyaddr_r()\fR, and \fBgethostent_r()\fR is not \fINULL\fR, it is
206 always equal to the \fIresult\fR pointer that was supplied by the caller.
207 .sp
208 .LP
209 The \fBsethostent()\fR and \fBendhostent()\fR functions return \fB0\fR on
210 success.
211 .SH ERRORS
212 .sp
213 .LP
214 The reentrant functions \fBgethostbyname_r()\fR, \fBgethostbyaddr_r()\fR, and
215 \fBgethostent_r()\fR will return \fINULL\fR and set \fIerrno\fR to \fBERANGE\fR
216 if the length of the buffer supplied by caller is not large enough to store the
217 result. See \fBIntro\fR(2) for the proper usage and interpretation of
218 \fBerrno\fR in multithreaded applications.
219 .sp
220 .LP
221 The reentrant functions \fBgethostbyname_r()\fR and \fBgethostbyaddr_r()\fR set
222 the integer pointed to by \fIh_errnop\fR to one of these values in case of
223 error.
224 .sp
225 .LP
226 On failures, the non-reentrant functions \fBgethostbyname()\fR and
227 \fBgethostbyaddr()\fR set a global integer \fIh_errno\fR to indicate one of
228 these error codes (defined in \fB<netdb.h>\fR): \fBHOST_NOT_FOUND\fR,
229 \fBTRY_AGAIN\fR, \fBNO_RECOVERY\fR, \fBNO_DATA\fR, and \fBNO_ADDRESS\fR.
230 .sp
231 .LP
232 If a resolver is provided with a malformed address, or if any other error
233 occurs before \fBgethostbyname()\fR is resolved, then \fBgethostbyname()\fR
234 returns an internal error with a value of \(mi1.
235 .sp
236 .LP
237 The \fBgethostbyname()\fR function will set \fIh_errno\fR to
238 \fBNETDB_INTERNAL\fR when it returns a \fINULL\fR value.
239 .SH EXAMPLES
240 .LP
241 \fBExample 1 \fRUsing \fBgethostbyaddr()\fR
242 .sp
243 .LP
244 Here is a sample program that gets the canonical name, aliases, and ``.''
245 separated Internet \fBIP\fR addresses for a given ``.'' separated \fBIP\fR
246 address:
247
248 .sp
249 .in +2
250 .nf
251 #include <stdio.h>
252 #include <stdlib.h
253 #include <string.h>
254 #include <sys/types.h>
255 #include <sys/socket.h>
256 #include <netinet/in.h>
257 #include <arpa/inet.h>
258 #include <netdb.h>
259 int main(int argc, const char **argv)
260 {
261 in_addr_t addr;
262 struct hostent *hp;
263 char **p;
264 if (argc != 2) {
265 (void) printf("usage: %s IP-address\en", argv[0]);
266 exit (1);
267 }
268 if ((int)(addr = inet_addr(argv[1])) == -1) {
269 (void) printf("IP-address must be of the form a.b.c.d\en");
270 exit (2);
271 }
272 hp = gethostbyaddr((char *)&addr, 4, AF_INET);
273 if (hp == NULL) {
274 (void) printf("host information for %s not found\en", argv[1]);
275 exit (3);
276 }
277 for (p = hp->h_addr_list; *p != 0; p++) {
278 struct in_addr in;
279 char **q;
280 (void) memcpy(&in.s_addr, *p, sizeof (in.s_addr));
281 (void) printf("%s\t%s", inet_ntoa(in), hp\(mi>h_name);
282 for (q = hp->h_aliases; *q != 0; q++)
283 (void) printf(" %s", *q);
284 (void) putchar('\en');
285 }
286 exit (0);
287 }
288 .fi
289 .in -2
290
291 .sp
292 .LP
293 Note that the preceding sample program is unsafe for use in multithreaded
294 applications.
295
296 .SH FILES
297 .sp
298 .ne 2
299 .na
300 \fB\fB/etc/hosts\fR\fR
301 .ad
302 .RS 22n
303 hosts file that associates the names of hosts with their Internet Protocol (IP)
304 addresses
305 .RE
306
307 .sp
308 .ne 2
309 .na
310 \fB\fB/etc/netconfig\fR\fR
311 .ad
312 .RS 22n
313 network configuration database
314 .RE
315
316 .sp
317 .ne 2
318 .na
319 \fB\fB/etc/nsswitch.conf\fR\fR
320 .ad
321 .RS 22n
322 configuration file for the name service switch
323 .RE
324
325 .SH ATTRIBUTES
326 .sp
327 .LP
328 See \fBattributes\fR(5) for descriptions of the following attributes:
329 .sp
330
331 .sp
332 .TS
333 box;
334 c | c
335 l | l .
336 ATTRIBUTE TYPE ATTRIBUTE VALUE
337 _
338 MT-Level T{
339 See \fBReentrant Interfaces\fR in the \fBDESCRIPTION\fR section.
340 T}
341 .TE
342
343 .SH SEE ALSO
344 .sp
345 .LP
346 \fBIntro\fR(2), \fBIntro\fR(3), \fBbyteorder\fR(3SOCKET), \fBinet\fR(3SOCKET),
347 \fBnetdb.h\fR(3HEAD), \fBnetdir\fR(3NSL), \fBhosts\fR(4), \fBnetconfig\fR(4),
348 \fBnss\fR(4), \fBnsswitch.conf\fR(4), \fBattributes\fR(5)
349 .SH WARNINGS
350 .sp
351 .LP
352 The reentrant interfaces \fBgethostbyname_r()\fR, \fBgethostbyaddr_r()\fR, and
353 \fBgethostent_r()\fR are included in this release on an uncommitted basis only
354 and are subject to change or removal in future minor releases.
355 .SH NOTES
356 .sp
357 .LP
358 To ensure that they all return consistent results, \fBgethostbyname()\fR,
359 \fBgethostbyname_r()\fR, and \fBnetdir_getbyname()\fR are implemented in terms
360 of the same internal library function. This function obtains the system-wide
361 source lookup policy based on the \fBinet\fR family entries in
362 \fBnetconfig\fR(4) and the \fBhosts:\fR entry in \fBnsswitch.conf\fR(4).
363 Similarly, \fBgethostbyaddr()\fR, \fBgethostbyaddr_r()\fR, and
364 \fBnetdir_getbyaddr()\fR are implemented in terms of the same internal library
365 function. If the \fBinet\fR family entries in \fBnetconfig\fR(4) have a ``-''
366 in the last column for \fBnametoaddr\fR libraries, then the entry for
367 \fBhosts\fR in \fBnsswitch.conf\fR will be used; \fBnametoaddr\fR libraries in
368 that column will be used, and \fBnsswitch.conf\fR will not be consulted.
369 .sp
370 .LP
371 There is no analogue of \fBgethostent()\fR and \fBgethostent_r()\fR in the
372 netdir functions, so these enumeration functions go straight to the \fBhosts\fR
373 entry in \fBnsswitch.conf\fR. Thus enumeration can return results from a
374 different source than that used by \fBgethostbyname()\fR,
375 \fBgethostbyname_r()\fR, \fBgethostbyaddr()\fR, and \fBgethostbyaddr_r()\fR.
376 .sp
377 .LP
378 All the functions that return a \fBstruct hostent\fR must always return the
379 \fIcanonical name\fR in the \fIh_name\fR field. This name, by definition, is
380 the well-known and official hostname shared between all aliases and all
381 addresses. The underlying source that satisfies the request determines the
382 mapping of the input name or address into the set of names and addresses in
383 \fBhostent\fR. Different sources might do that in different ways. If there is
384 more than one alias and more than one address in \fBhostent\fR, no pairing is
385 implied between them.
386 .sp
387 .LP
388 The system attempts to put those addresses that are on the same subnet as the
389 caller before addresses that are on different subnets. However, if address
390 sorting is disabled by setting \fBSORT_ADDRS\fR to FALSE in the
391 \fB/etc/default/nss\fR file, the system does not put the local subnet addresses
392 first. See \fBnss\fR(4) for more information.
393 .sp
394 .LP
395 When compiling multithreaded applications, see \fBIntro\fR(3), \fBMULTITHREADED
396 APPLICATIONS\fR, for information about the use of the \fB_REENTRANT\fR flag.
397 .sp
398 .LP
399 Use of the enumeration interfaces \fBgethostent()\fR and \fBgethostent_r()\fR
400 is discouraged; enumeration might not be supported for all database sources.
401 The semantics of enumeration are discussed further in \fBnsswitch.conf\fR(4).
402 .sp
403 .LP
404 The current implementations of these functions only return or accept addresses
405 for the Internet address family (type \fBAF_INET\fR).
406 .sp
407 .LP
408 The form for an address of type \fBAF_INET\fR is a \fBstruct in_addr\fR defined
409 in <\fBnetinet/in.h\fR>. The functions described in \fBinet\fR(3SOCKET), and
410 illustrated in the \fBEXAMPLES\fR section, are helpful in constructing and
411 manipulating addresses in this form.