]>
Commit | Line | Data |
---|---|---|
15545eb6 HS |
1 | .\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>, |
2 | .\" Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages@gmail.com>, | |
3 | .\" and Copyright (C) 2016, Heinrich Schuchardt <xypron.glpk@gmx.de> | |
2297bf0e | 4 | .\" |
00acdba1 | 5 | .\" %%%LICENSE_START(VERBATIM_ONE_PARA) |
77117f4f MK |
6 | .\" Permission is granted to distribute possibly modified copies |
7 | .\" of this page provided the header is included verbatim, | |
8 | .\" and in case of nontrivial modification author and date | |
9 | .\" of the modification is added to the header. | |
8ff7380d | 10 | .\" %%%LICENSE_END |
77117f4f MK |
11 | .\" |
12 | .\" Modified, 2003-12-02, Michael Kerrisk, <mtk.manpages@gmail.com> | |
13 | .\" Modified, 2003-09-23, Adam Langley | |
14 | .\" Modified, 2004-05-27, Michael Kerrisk, <mtk.manpages@gmail.com> | |
15 | .\" Added SOCK_SEQPACKET | |
16 | .\" 2008-05-27, mtk, Provide a clear description of the three types of | |
17 | .\" address that can appear in the sockaddr_un structure: pathname, | |
18 | .\" unnamed, and abstract. | |
19 | .\" | |
3df541c0 | 20 | .TH UNIX 7 2016-07-17 "Linux" "Linux Programmer's Manual" |
77117f4f | 21 | .SH NAME |
f68512e9 | 22 | unix \- sockets for local interprocess communication |
77117f4f MK |
23 | .SH SYNOPSIS |
24 | .B #include <sys/socket.h> | |
25 | .br | |
26 | .B #include <sys/un.h> | |
27 | ||
d4c8c97c | 28 | .IB unix_socket " = socket(AF_UNIX, type, 0);" |
77117f4f | 29 | .br |
d4c8c97c | 30 | .IB error " = socketpair(AF_UNIX, type, 0, int *" sv ");" |
77117f4f MK |
31 | .SH DESCRIPTION |
32 | The | |
d4c8c97c | 33 | .B AF_UNIX |
77117f4f | 34 | (also known as |
d4c8c97c | 35 | .BR AF_LOCAL ) |
77117f4f MK |
36 | socket family is used to communicate between processes on the same machine |
37 | efficiently. | |
4891f52a | 38 | Traditionally, UNIX domain sockets can be either unnamed, |
9ee4a2b6 | 39 | or bound to a filesystem pathname (marked as being of type socket). |
77117f4f | 40 | Linux also supports an abstract namespace which is independent of the |
9ee4a2b6 | 41 | filesystem. |
77117f4f | 42 | |
d02879f7 | 43 | Valid socket types in the UNIX domain are: |
77117f4f | 44 | .BR SOCK_STREAM , |
d02879f7 | 45 | for a stream-oriented socket; |
77117f4f MK |
46 | .BR SOCK_DGRAM , |
47 | for a datagram-oriented socket that preserves message boundaries | |
008f1ecc | 48 | (as on most UNIX implementations, UNIX domain datagram |
77117f4f MK |
49 | sockets are always reliable and don't reorder datagrams); |
50 | and (since Linux 2.6.4) | |
51 | .BR SOCK_SEQPACKET , | |
0d7e8d59 MK |
52 | for a sequenced-packet socket that is connection-oriented, |
53 | preserves message boundaries, | |
77117f4f MK |
54 | and delivers messages in the order that they were sent. |
55 | ||
4891f52a | 56 | UNIX domain sockets support passing file descriptors or process credentials |
77117f4f | 57 | to other processes using ancillary data. |
c634028a | 58 | .SS Address format |
008f1ecc | 59 | A UNIX domain socket address is represented in the following structure: |
77117f4f MK |
60 | .in +4n |
61 | .nf | |
62 | ||
63bc262c | 63 | .\" #define UNIX_PATH_MAX 108 |
5ffdc2fd | 64 | .\" |
77117f4f MK |
65 | struct sockaddr_un { |
66 | sa_family_t sun_family; /* AF_UNIX */ | |
63bc262c | 67 | char sun_path[108]; /* pathname */ |
77117f4f MK |
68 | }; |
69 | .fi | |
70 | .in | |
71 | .PP | |
d02879f7 | 72 | The |
77117f4f | 73 | .I sun_family |
d02879f7 | 74 | field always contains |
77117f4f | 75 | .BR AF_UNIX . |
840aa3c7 MK |
76 | On Linux |
77 | .I sun_path | |
78 | is 108 bytes in size; see also NOTES, below. | |
77117f4f | 79 | |
d02879f7 MK |
80 | Various systems calls (for example, |
81 | .BR bind (2), | |
82 | .BR connect (2), | |
83 | and | |
84 | .BR sendto (2)) | |
85 | take a | |
b8017cf5 | 86 | .I sockaddr_un |
d02879f7 MK |
87 | argument as input. |
88 | Some other system calls (for example, | |
89 | .BR getsockname (2), | |
90 | .BR getpeername (2), | |
91 | .BR recvfrom (2), | |
92 | and | |
93 | .BR accept (2)) | |
94 | return an argument of this type. | |
95 | ||
96 | Three types of address are distinguished in the | |
97 | .I sockaddr_un | |
98 | structure: | |
77117f4f MK |
99 | .IP * 3 |
100 | .IR pathname : | |
1c7b2458 MK |
101 | a UNIX domain socket can be bound to a null-terminated |
102 | filesystem pathname using | |
77117f4f | 103 | .BR bind (2). |
d02879f7 MK |
104 | When the address of a pathname socket is returned |
105 | (by one of the system calls noted above), | |
77117f4f | 106 | its length is |
6cd06646 MK |
107 | |
108 | offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1 | |
109 | ||
77117f4f MK |
110 | and |
111 | .I sun_path | |
112 | contains the null-terminated pathname. | |
d02879f7 MK |
113 | (On Linux, the above |
114 | .BR offsetof () | |
115 | expression equates to the same value as | |
116 | .IR sizeof(sa_family_t) , | |
117 | but some other implementations include other fields before | |
118 | .IR sun_path , | |
119 | so the | |
120 | .BR offsetof () | |
121 | expression more portably describes the size of the address structure.) | |
122 | .IP | |
123 | For further details of pathname sockets, see below. | |
77117f4f MK |
124 | .IP * |
125 | .IR unnamed : | |
126 | A stream socket that has not been bound to a pathname using | |
127 | .BR bind (2) | |
128 | has no name. | |
129 | Likewise, the two sockets created by | |
130 | .BR socketpair (2) | |
131 | are unnamed. | |
d02879f7 | 132 | When the address of an unnamed socket is returned, |
77117f4f MK |
133 | its length is |
134 | .IR "sizeof(sa_family_t)" , | |
135 | and | |
136 | .I sun_path | |
137 | should not be inspected. | |
138 | .\" There is quite some variation across implementations: FreeBSD | |
139 | .\" says the length is 16 bytes, HP-UX 11 says it's zero bytes. | |
140 | .IP * | |
141 | .IR abstract : | |
d02879f7 MK |
142 | an abstract socket address is distinguished (from a pathname socket) |
143 | by the fact that | |
77117f4f | 144 | .IR sun_path[0] |
836830b4 | 145 | is a null byte (\(aq\\0\(aq). |
156a0e0d MK |
146 | The socket's address in this namespace is given by the additional |
147 | bytes in | |
148 | .IR sun_path | |
149 | that are covered by the specified length of the address structure. | |
77117f4f | 150 | (Null bytes in the name have no special significance.) |
9ee4a2b6 | 151 | The name has no connection with filesystem pathnames. |
d02879f7 | 152 | When the address of an abstract socket is returned, |
156a0e0d MK |
153 | the returned |
154 | .I addrlen | |
155 | is greater than | |
156 | .IR "sizeof(sa_family_t)" | |
157 | (i.e., greater than 2), and the name of the socket is contained in | |
158 | the first | |
159 | .IR "(addrlen \- sizeof(sa_family_t))" | |
160 | bytes of | |
161 | .IR sun_path . | |
d02879f7 MK |
162 | .SS Pathname sockets |
163 | When binding a socket to a pathname, a few rules should be observed | |
164 | for maximum portability and ease of coding: | |
165 | .IP * 3 | |
166 | The pathname in | |
167 | .I sun_path | |
168 | should be null-terminated. | |
169 | .IP * | |
170 | The length of the pathname, including the terminating null byte, | |
171 | should not exceed the size of | |
172 | .IR sun_path . | |
173 | .IP * | |
174 | The | |
175 | .I addrlen | |
176 | argument that describes the enclosing | |
177 | .I sockaddr_un | |
178 | structure should have a value of at least: | |
179 | ||
180 | .nf | |
181 | offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1 | |
182 | .fi | |
183 | .IP | |
b8017cf5 | 184 | or, more simply, |
d02879f7 MK |
185 | .I addrlen |
186 | can be specified as | |
187 | .IR "sizeof(struct sockaddr_un)" . | |
188 | .PP | |
189 | There is some variation in how implementations handle UNIX domain | |
190 | socket addresses that do not follow the above rules. | |
191 | For example, some (but not all) implementations | |
192 | .\" Linux does this, including for the case where the supplied path | |
193 | .\" is 108 bytes | |
194 | append a null terminator if none is present in the supplied | |
195 | .IR sun_path . | |
196 | ||
197 | When coding portable applications, | |
198 | keep in mind that some implementations | |
199 | .\" HP-UX | |
200 | have | |
201 | .I sun_path | |
202 | as short as 92 bytes. | |
203 | .\" Modern BSDs generally have 104, Tru64 and AIX have 104, | |
204 | .\" Solaris and Irix have 108 | |
205 | ||
206 | Various system calls | |
207 | .RB ( accept (2), | |
208 | .BR recvfrom (2), | |
209 | .BR getsockname (2), | |
210 | .BR getpeername (2)) | |
211 | return socket address structures. | |
212 | When applied to UNIX domain sockets, the value-result | |
213 | .I addrlen | |
214 | argument supplied to the call should be initialized as above. | |
215 | Upon return, the argument is set to indicate the | |
216 | .I actual | |
217 | size of the address structure. | |
218 | The caller should check the value returned in this argument: | |
219 | if the output value exceeds the input value, | |
220 | then there is no guarantee that a null terminator is present in | |
221 | .IR sun_path . | |
222 | (See BUGS.) | |
9f213833 MK |
223 | .\" |
224 | .SS Pathname socket ownership and permissions | |
225 | In the Linux implementation, | |
226 | pathname sockets honor the permissions of the directory they are in. | |
227 | Creation of a new socket will fail if the process does not have write and | |
228 | search (execute) permission on the directory in which the socket is created. | |
229 | ||
230 | On Linux, | |
231 | connecting to a stream socket object requires write permission on that socket; | |
232 | sending a datagram to a datagram socket likewise | |
233 | requires write permission on that socket. | |
234 | POSIX does not make any statement about the effect of the permissions | |
7f98a239 | 235 | on a socket file, and on some systems (e.g., older BSDs), |
9f213833 MK |
236 | the socket permissions are ignored. |
237 | Portable programs should not rely on | |
238 | this feature for security. | |
239 | ||
240 | When creating a new socket, the owner and group of the socket file | |
241 | are set according to the usual rules. | |
242 | The socket file has all permissions enabled, | |
243 | other than those that are turned off by the process | |
244 | .BR umask (2). | |
245 | ||
246 | The owner, group, and permissions of a pathname socket can be changed (using | |
247 | .BR chown (2) | |
248 | and | |
249 | .BR chmod (2)). | |
250 | .\" However, fchown() and fchmod() do not seem to have an effect | |
251 | .\" | |
d1875c13 | 252 | .SS Abstract sockets |
44cca454 MK |
253 | Socket permissions have no meaning for abstract sockets: |
254 | the process | |
255 | .BR umask (2) | |
256 | has no effect when binding an abstract socket, | |
257 | and changing the ownership and permissions of the object (via | |
258 | .BR fchown (2) | |
259 | and | |
260 | .BR fchmod (2)) | |
261 | has no effect on the accessibility of the socket. | |
262 | ||
d1875c13 MK |
263 | Abstract sockets automatically disappear when all open references |
264 | to the socket are closed. | |
265 | ||
266 | The abstract socket namespace is a nonportable Linux extension. | |
267 | .\" | |
c634028a | 268 | .SS Socket options |
464b254b | 269 | For historical reasons, these socket options are specified with a |
77117f4f MK |
270 | .B SOL_SOCKET |
271 | type even though they are | |
d4c8c97c | 272 | .B AF_UNIX |
77117f4f MK |
273 | specific. |
274 | They can be set with | |
275 | .BR setsockopt (2) | |
276 | and read with | |
277 | .BR getsockopt (2) | |
278 | by specifying | |
279 | .B SOL_SOCKET | |
280 | as the socket family. | |
281 | .TP | |
282 | .B SO_PASSCRED | |
6074c3e6 | 283 | Enables the receiving of the credentials of the sending process in an |
77117f4f MK |
284 | ancillary message. |
285 | When this option is set and the socket is not yet connected | |
286 | a unique name in the abstract namespace will be generated automatically. | |
287 | Expects an integer boolean flag. | |
c634028a | 288 | .SS Autobind feature |
0cf2caa4 | 289 | If a |
0b80cf56 | 290 | .BR bind (2) |
0cf2caa4 MK |
291 | call specifies |
292 | .I addrlen | |
293 | as | |
294 | .IR sizeof(sa_family_t) , | |
449dd4e2 | 295 | .\" i.e., sizeof(short) |
0cf2caa4 MK |
296 | or the |
297 | .BR SO_PASSCRED | |
298 | socket option was specified for a socket that was | |
299 | not explicitly bound to an address, | |
300 | then the socket is autobound to an abstract address. | |
301 | The address consists of a null byte | |
302 | followed by 5 bytes in the character set | |
303 | .IR [0-9a-f] . | |
1e4e3bad MK |
304 | Thus, there is a limit of 2^20 autobind addresses. |
305 | (From Linux 2.1.15, when the autobind feature was added, | |
306 | 8 bytes were used, and the limit was thus 2^32 autobind addresses. | |
307 | The change to 5 bytes came in Linux 2.3.15.) | |
19e19f5f | 308 | .SS Sockets API |
77117f4f | 309 | The following paragraphs describe domain-specific details and |
008f1ecc | 310 | unsupported features of the sockets API for UNIX domain sockets on Linux. |
77117f4f | 311 | |
008f1ecc | 312 | UNIX domain sockets do not support the transmission of |
77117f4f MK |
313 | out-of-band data (the |
314 | .B MSG_OOB | |
315 | flag for | |
316 | .BR send (2) | |
317 | and | |
318 | .BR recv (2)). | |
319 | ||
320 | The | |
321 | .BR send (2) | |
322 | .B MSG_MORE | |
008f1ecc | 323 | flag is not supported by UNIX domain sockets. |
77117f4f | 324 | |
77e75b90 MK |
325 | The use of |
326 | .B MSG_TRUNC | |
327 | in the | |
328 | .I flags | |
329 | argument of | |
330 | .BR recv (2) | |
008f1ecc | 331 | is not supported by UNIX domain sockets. |
77e75b90 | 332 | |
77117f4f MK |
333 | The |
334 | .B SO_SNDBUF | |
008f1ecc | 335 | socket option does have an effect for UNIX domain sockets, but the |
77117f4f MK |
336 | .B SO_RCVBUF |
337 | option does not. | |
338 | For datagram sockets, the | |
339 | .B SO_SNDBUF | |
340 | value imposes an upper limit on the size of outgoing datagrams. | |
341 | This limit is calculated as the doubled (see | |
342 | .BR socket (7)) | |
343 | option value less 32 bytes used for overhead. | |
c634028a | 344 | .SS Ancillary messages |
77117f4f MK |
345 | Ancillary data is sent and received using |
346 | .BR sendmsg (2) | |
347 | and | |
348 | .BR recvmsg (2). | |
349 | For historical reasons the ancillary message types listed below | |
350 | are specified with a | |
351 | .B SOL_SOCKET | |
352 | type even though they are | |
d4c8c97c | 353 | .B AF_UNIX |
77117f4f MK |
354 | specific. |
355 | To send them set the | |
356 | .I cmsg_level | |
357 | field of the struct | |
358 | .I cmsghdr | |
359 | to | |
360 | .B SOL_SOCKET | |
361 | and the | |
362 | .I cmsg_type | |
363 | field to the type. | |
364 | For more information see | |
365 | .BR cmsg (3). | |
366 | .TP | |
367 | .B SCM_RIGHTS | |
368 | Send or receive a set of open file descriptors from another process. | |
369 | The data portion contains an integer array of the file descriptors. | |
370 | The passed file descriptors behave as though they have been created with | |
371 | .BR dup (2). | |
372 | .TP | |
373 | .B SCM_CREDENTIALS | |
008f1ecc | 374 | Send or receive UNIX credentials. |
77117f4f MK |
375 | This can be used for authentication. |
376 | The credentials are passed as a | |
377 | .I struct ucred | |
378 | ancillary message. | |
b1587ca8 MK |
379 | Thus structure is defined in |
380 | .I <sys/socket.h> | |
381 | as follows: | |
77117f4f MK |
382 | |
383 | .in +4n | |
384 | .nf | |
385 | struct ucred { | |
386 | pid_t pid; /* process ID of the sending process */ | |
387 | uid_t uid; /* user ID of the sending process */ | |
388 | gid_t gid; /* group ID of the sending process */ | |
389 | }; | |
390 | .fi | |
391 | .in | |
392 | ||
b1587ca8 | 393 | Since glibc 2.8, the |
1bc510f5 | 394 | .B _GNU_SOURCE |
e417acb0 MK |
395 | feature test macro must be defined (before including |
396 | .I any | |
397 | header files) in order to obtain the definition | |
b1587ca8 MK |
398 | of this structure. |
399 | ||
77117f4f MK |
400 | The credentials which the sender specifies are checked by the kernel. |
401 | A process with effective user ID 0 is allowed to specify values that do | |
402 | not match its own. | |
403 | The sender must specify its own process ID (unless it has the capability | |
404 | .BR CAP_SYS_ADMIN ), | |
405 | its user ID, effective user ID, or saved set-user-ID (unless it has | |
406 | .BR CAP_SETUID ), | |
407 | and its group ID, effective group ID, or saved set-group-ID | |
408 | (unless it has | |
409 | .BR CAP_SETGID ). | |
410 | To receive a | |
411 | .I struct ucred | |
412 | message the | |
413 | .B SO_PASSCRED | |
414 | option must be enabled on the socket. | |
fbea0f81 MK |
415 | .SS Ioctls |
416 | The following | |
417 | .BR ioctl (2) | |
418 | calls return information in | |
419 | .IR value . | |
420 | The correct syntax is: | |
421 | .PP | |
422 | .RS | |
423 | .nf | |
424 | .BI int " value"; | |
f0d77d97 | 425 | .IB error " = ioctl(" unix_socket ", " ioctl_type ", &" value ");" |
fbea0f81 MK |
426 | .fi |
427 | .RE | |
428 | .PP | |
429 | .I ioctl_type | |
430 | can be: | |
431 | .TP | |
432 | .B SIOCINQ | |
170e5f0d JC |
433 | For |
434 | .B SOCK_STREAM | |
435 | socket the function returns the amount of queued unread data in the receive buffer. | |
fbea0f81 MK |
436 | The socket must not be in LISTEN state, otherwise an error |
437 | .RB ( EINVAL ) | |
438 | is returned. | |
439 | .B SIOCINQ | |
440 | is defined in | |
441 | .IR <linux/sockios.h> . | |
bea08fec | 442 | .\" FIXME . http://sources.redhat.com/bugzilla/show_bug.cgi?id=12002, |
fbea0f81 MK |
443 | .\" filed 2010-09-10, may cause SIOCINQ to be defined in glibc headers |
444 | Alternatively, | |
445 | you can use the synonymous | |
446 | .BR FIONREAD , | |
447 | defined in | |
448 | .IR <sys/ioctl.h> . | |
7aed61d9 | 449 | .\" SIOCOUTQ also has an effect for UNIX domain sockets, but not |
fbea0f81 MK |
450 | .\" quite what userland might expect. It seems to return the number |
451 | .\" of bytes allocated for buffers containing pending output. | |
452 | .\" That number is normally larger than the number of bytes of pending | |
453 | .\" output. Since this info is, from userland's point of view, imprecise, | |
454 | .\" and it may well change, probably best not to document this now. | |
170e5f0d JC |
455 | For |
456 | .B SOCK_DGRAM | |
457 | socket, | |
458 | the returned value is the same as | |
459 | for Internet domain datagram socket; | |
460 | see | |
461 | .BR udp (7). | |
77117f4f MK |
462 | .SH ERRORS |
463 | .TP | |
464 | .B EADDRINUSE | |
9ee4a2b6 | 465 | The specified local address is already in use or the filesystem socket |
77117f4f MK |
466 | object already exists. |
467 | .TP | |
468 | .B ECONNREFUSED | |
1fe284ab | 469 | The remote address specified by |
77117f4f | 470 | .BR connect (2) |
1fe284ab | 471 | was not a listening socket. |
d02879f7 | 472 | This error can also occur if the target pathname is not a socket. |
77117f4f MK |
473 | .TP |
474 | .B ECONNRESET | |
475 | Remote socket was unexpectedly closed. | |
476 | .TP | |
477 | .B EFAULT | |
478 | User memory address was not valid. | |
479 | .TP | |
480 | .B EINVAL | |
481 | Invalid argument passed. | |
1fe284ab | 482 | A common cause is that the value |
40656bc7 | 483 | .B AF_UNIX |
1fe284ab | 484 | was not specified in the |
77117f4f | 485 | .I sun_type |
1fe284ab | 486 | field of passed addresses, or the socket was in an |
77117f4f MK |
487 | invalid state for the applied operation. |
488 | .TP | |
489 | .B EISCONN | |
490 | .BR connect (2) | |
491 | called on an already connected socket or a target address was | |
492 | specified on a connected socket. | |
493 | .TP | |
ec55a2b6 MK |
494 | .B ENOENT |
495 | The pathname in the remote address specified to | |
9470f355 | 496 | .BR connect (2) |
ec55a2b6 MK |
497 | did not exist. |
498 | .TP | |
77117f4f MK |
499 | .B ENOMEM |
500 | Out of memory. | |
501 | .TP | |
502 | .B ENOTCONN | |
503 | Socket operation needs a target address, but the socket is not connected. | |
504 | .TP | |
505 | .B EOPNOTSUPP | |
506 | Stream operation called on non-stream oriented socket or tried to | |
507 | use the out-of-band data option. | |
508 | .TP | |
509 | .B EPERM | |
510 | The sender passed invalid credentials in the | |
511 | .IR "struct ucred" . | |
512 | .TP | |
513 | .B EPIPE | |
514 | Remote socket was closed on a stream socket. | |
515 | If enabled, a | |
516 | .B SIGPIPE | |
517 | is sent as well. | |
518 | This can be avoided by passing the | |
519 | .B MSG_NOSIGNAL | |
520 | flag to | |
110039c1 | 521 | .BR send (2) |
77117f4f | 522 | or |
110039c1 | 523 | .BR sendmsg (2). |
77117f4f MK |
524 | .TP |
525 | .B EPROTONOSUPPORT | |
cd0221ea MK |
526 | Passed protocol is not |
527 | .BR AF_UNIX . | |
77117f4f MK |
528 | .TP |
529 | .B EPROTOTYPE | |
530 | Remote socket does not match the local socket type | |
531 | .RB ( SOCK_DGRAM | |
d1c9ea80 | 532 | versus |
77117f4f MK |
533 | .BR SOCK_STREAM ) |
534 | .TP | |
535 | .B ESOCKTNOSUPPORT | |
536 | Unknown socket type. | |
537 | .PP | |
538 | Other errors can be generated by the generic socket layer or | |
9ee4a2b6 | 539 | by the filesystem while generating a filesystem socket object. |
77117f4f MK |
540 | See the appropriate manual pages for more information. |
541 | .SH VERSIONS | |
542 | .B SCM_CREDENTIALS | |
543 | and the abstract namespace were introduced with Linux 2.2 and should not | |
544 | be used in portable programs. | |
545 | (Some BSD-derived systems also support credential passing, | |
546 | but the implementation details differ.) | |
547 | .SH NOTES | |
00b78c5f MK |
548 | Binding to a socket with a filename creates a socket |
549 | in the filesystem that must be deleted by the caller when it is no | |
550 | longer needed (using | |
551 | .BR unlink (2)). | |
552 | The usual UNIX close-behind semantics apply; the socket can be unlinked | |
553 | at any time and will be finally removed from the filesystem when the last | |
554 | reference to it is closed. | |
555 | ||
556 | To pass file descriptors or credentials over a | |
557 | .BR SOCK_STREAM , | |
558 | you need | |
559 | to send or receive at least one byte of nonancillary data in the same | |
560 | .BR sendmsg (2) | |
561 | or | |
562 | .BR recvmsg (2) | |
563 | call. | |
564 | ||
565 | UNIX domain stream sockets do not support the notion of out-of-band data. | |
566 | .\" | |
d02879f7 MK |
567 | .SH BUGS |
568 | When binding a socket to an address, | |
569 | Linux is one of the implementations that appends a null terminator | |
570 | if none is supplied in | |
571 | .IR sun_path . | |
572 | In most cases this is unproblematic: | |
573 | when the socket address is retrieved, | |
574 | it will be one byte longer than that supplied when the socket was bound. | |
575 | However, there is one case where confusing behavior can result: | |
576 | if 108 non-null bytes are supplied when a socket is bound, | |
577 | then the addition of the null terminator takes the length of | |
578 | the pathname beyond | |
579 | .IR sizeof(sun_path) . | |
580 | Consequently, when retrieving the socket address | |
581 | (for example, via | |
582 | .BR accept (2)), | |
583 | .\" The behavior on Solaris is quite similar. | |
584 | if the input | |
585 | .I addrlen | |
586 | argument for the retrieving call is specified as | |
587 | .IR "sizeof(struct sockaddr_un)" , | |
588 | then the returned address structure | |
589 | .I won't | |
590 | have a null terminator in | |
591 | .IR sun_path . | |
592 | ||
593 | In addition, some implementations | |
594 | .\" i.e., traditional BSD | |
595 | don't require a null terminator when binding a socket (the | |
596 | .I addrlen | |
597 | argument is used to determine the length of | |
598 | .IR sun_path ) | |
599 | and when the socket address is retrieved on these implementations, | |
600 | there is no null terminator in | |
601 | .IR sun_path . | |
602 | ||
b8017cf5 | 603 | Applications that retrieve socket addresses can (portably) code |
d02879f7 MK |
604 | to handle the possibility that there is no null terminator in |
605 | .IR sun_path | |
606 | by respecting the fact that the number of valid bytes in the pathname is: | |
607 | ||
608 | strnlen(addr.sun_path, addrlen \- offsetof(sockaddr_un, sun_path)) | |
609 | .\" The following patch to amend kernel behavior was rejected: | |
610 | .\" http://thread.gmane.org/gmane.linux.kernel.api/2437 | |
611 | .\" Subject: [patch] Fix handling of overlength pathname in AF_UNIX sun_path | |
612 | .\" 2012-04-17 | |
613 | .\" And there was a related discussion in the Austin list: | |
614 | .\" http://thread.gmane.org/gmane.comp.standards.posix.austin.general/5735 | |
615 | .\" Subject: Having a sun_path with no null terminator | |
616 | .\" 2012-04-18 | |
617 | .\" | |
618 | .\" FIXME . Track http://austingroupbugs.net/view.php?id=561 | |
619 | ||
620 | Alternatively, an application can retrieve | |
621 | the socket address by allocating a buffer of size | |
622 | .I "sizeof(struct sockaddr_un)+1" | |
623 | that is zeroed out before the retrieval. | |
624 | The retrieving call can specify | |
625 | .I addrlen | |
626 | as | |
627 | .IR "sizeof(struct sockaddr_un)" , | |
628 | and the extra zero byte ensures that there will be | |
629 | a null terminator for the string returned in | |
630 | .IR sun_path : | |
631 | ||
632 | .nf | |
633 | .in +3 | |
634 | void *addrp; | |
635 | ||
636 | addrlen = sizeof(struct sockaddr_un); | |
637 | addrp = malloc(addrlen + 1); | |
638 | if (addrp == NULL) | |
639 | /* Handle error */ ; | |
640 | memset(addrp, 0, addrlen + 1); | |
641 | ||
3e35b19b | 642 | if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == \-1) |
d02879f7 MK |
643 | /* handle error */ ; |
644 | ||
645 | printf("sun_path = %s\\n", ((struct sockaddr_un *) addrp)\->sun_path); | |
646 | .in | |
647 | .fi | |
648 | ||
649 | This sort of messiness can be avoided if it is guaranteed | |
650 | that the applications that | |
651 | .I create | |
652 | pathname sockets follow the rules outlined above under | |
653 | .IR "Pathname sockets" . | |
77117f4f | 654 | .SH EXAMPLE |
84c8cae2 MK |
655 | The following code demonstrates the use of sequenced-packet |
656 | sockets for local interprocess communication. | |
eb73e8ad | 657 | It consists of two programs. |
15545eb6 | 658 | The server program waits for a connection from the client program. |
84c8cae2 MK |
659 | The client sends each of its command-line arguments in separate messages. |
660 | The server treats the incoming messages as integers and adds them up. | |
eb73e8ad | 661 | The client sends the command string "END". |
84c8cae2 MK |
662 | The server sends back a message containing the sum of the client's integers. |
663 | The client prints the sum and exits. | |
15545eb6 | 664 | The server waits for the next client to connect. |
84c8cae2 | 665 | To stop the server, the client is called with the command-line argument "DOWN". |
15545eb6 HS |
666 | .PP |
667 | The following output was recorded while running the server in the background | |
84c8cae2 MK |
668 | and repeatedly executing the client. |
669 | Execution of the server program ends when it receives the "DOWN" command. | |
15545eb6 HS |
670 | .SS Example output |
671 | .in +4n | |
672 | .nf | |
eb73e8ad | 673 | $ \fB./server &\fP |
15545eb6 | 674 | [1] 25887 |
eb73e8ad | 675 | $ \fB./client 3 4\fP |
15545eb6 | 676 | Result = 7 |
eb73e8ad | 677 | $ \fB./client 11 \-5\fP |
15545eb6 | 678 | Result = 6 |
eb73e8ad | 679 | $ \fB./client DOWN\fP |
15545eb6 HS |
680 | Result = 0 |
681 | [1]+ Done ./server | |
682 | $ | |
683 | .fi | |
684 | .in | |
685 | .SS Program source | |
686 | .nf | |
687 | /* | |
688 | * File connection.h | |
689 | */ | |
690 | ||
691 | #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket" | |
692 | #define BUFFER_SIZE 12 | |
693 | ||
694 | /* | |
695 | * File server.c | |
696 | */ | |
697 | ||
698 | #include <stdio.h> | |
699 | #include <stdlib.h> | |
700 | #include <string.h> | |
701 | #include <sys/socket.h> | |
702 | #include <sys/un.h> | |
703 | #include <unistd.h> | |
704 | #include "connection.h" | |
705 | ||
706 | int | |
707 | main(int argc, char *argv[]) | |
708 | { | |
709 | struct sockaddr_un name; | |
710 | int down_flag = 0; | |
711 | int ret; | |
712 | int connection_socket; | |
713 | int data_socket; | |
714 | int result; | |
715 | char buffer[BUFFER_SIZE]; | |
716 | ||
717 | /* | |
eb73e8ad | 718 | * In case the program exited inadvertently on the last run, |
15545eb6 HS |
719 | * remove the socket. |
720 | */ | |
721 | ||
722 | unlink(SOCKET_NAME); | |
723 | ||
724 | /* Create local socket. */ | |
725 | ||
726 | connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0); | |
727 | if (connection_socket == \-1) { | |
728 | perror("socket"); | |
729 | exit(EXIT_FAILURE); | |
730 | } | |
731 | ||
732 | /* | |
eb73e8ad MK |
733 | * For portability clear the whole structure, since some |
734 | * implementations have additional (nonstandard) fields in | |
735 | * the structure. | |
15545eb6 HS |
736 | */ |
737 | ||
738 | memset(&name, 0, sizeof(struct sockaddr_un)); | |
739 | ||
740 | /* Bind socket to socket name. */ | |
741 | ||
742 | name.sun_family = AF_UNIX; | |
743 | strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) \- 1); | |
744 | ||
745 | ret = bind(connection_socket, (const struct sockaddr *) &name, | |
746 | sizeof(struct sockaddr_un)); | |
747 | if (ret == \-1) { | |
748 | perror("bind"); | |
749 | exit(EXIT_FAILURE); | |
750 | } | |
751 | ||
752 | /* | |
eb73e8ad MK |
753 | * Prepare for accepting connections. The backlog size is set |
754 | * to 20. So while one request is being processed other requests | |
755 | * can be waiting. | |
15545eb6 HS |
756 | */ |
757 | ||
758 | ret = listen(connection_socket, 20); | |
759 | if (ret == \-1) { | |
760 | perror("listen"); | |
761 | exit(EXIT_FAILURE); | |
762 | } | |
763 | ||
764 | /* This is the main loop for handling connections. */ | |
765 | ||
766 | for (;;) { | |
767 | ||
15545eb6 HS |
768 | /* Wait for incoming connection. */ |
769 | ||
770 | data_socket = accept(connection_socket, NULL, NULL); | |
3cb43b95 | 771 | if (data_socket == \-1) { |
15545eb6 HS |
772 | perror("accept"); |
773 | exit(EXIT_FAILURE); | |
774 | } | |
775 | ||
776 | result = 0; | |
777 | for(;;) { | |
778 | ||
779 | /* Wait for next data packet. */ | |
780 | ||
781 | ret = read(data_socket, buffer, BUFFER_SIZE); | |
782 | if (ret == \-1) { | |
eb73e8ad | 783 | perror("read"); |
15545eb6 HS |
784 | exit(EXIT_FAILURE); |
785 | } | |
786 | ||
787 | /* Ensure buffer is 0\-terminated. */ | |
788 | ||
789 | buffer[BUFFER_SIZE \- 1] = 0; | |
790 | ||
791 | /* Handle commands. */ | |
792 | ||
793 | if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) { | |
794 | down_flag = 1; | |
795 | break; | |
796 | } | |
797 | ||
798 | if (!strncmp(buffer, "END", BUFFER_SIZE)) { | |
799 | break; | |
800 | } | |
801 | ||
802 | /* Add received summand. */ | |
803 | ||
804 | result += atoi(buffer); | |
805 | } | |
c751683c | 806 | |
15545eb6 HS |
807 | /* Send result. */ |
808 | ||
809 | sprintf(buffer, "%d", result); | |
810 | ret = write(data_socket, buffer, BUFFER_SIZE); | |
811 | ||
812 | if (ret == \-1) { | |
eb73e8ad | 813 | perror("write"); |
15545eb6 HS |
814 | exit(EXIT_FAILURE); |
815 | } | |
816 | ||
817 | /* Close socket. */ | |
818 | ||
819 | close(data_socket); | |
820 | ||
821 | /* Quit on DOWN command. */ | |
822 | ||
823 | if (down_flag) { | |
824 | break; | |
825 | } | |
826 | } | |
827 | ||
828 | close(connection_socket); | |
829 | ||
830 | /* Unlink the socket. */ | |
831 | ||
832 | unlink(SOCKET_NAME); | |
833 | ||
834 | exit(EXIT_SUCCESS); | |
835 | } | |
836 | ||
837 | /* | |
838 | * File client.c | |
839 | */ | |
840 | ||
841 | #include <errno.h> | |
842 | #include <stdio.h> | |
843 | #include <stdlib.h> | |
844 | #include <string.h> | |
845 | #include <sys/socket.h> | |
846 | #include <sys/un.h> | |
847 | #include <unistd.h> | |
848 | #include "connection.h" | |
849 | ||
850 | int | |
851 | main(int argc, char *argv[]) | |
852 | { | |
24a31d63 | 853 | struct sockaddr_un addr; |
15545eb6 HS |
854 | int i; |
855 | int ret; | |
856 | int data_socket; | |
857 | char buffer[BUFFER_SIZE]; | |
858 | ||
859 | /* Create local socket. */ | |
860 | ||
861 | data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0); | |
862 | if (data_socket == \-1) { | |
863 | perror("socket"); | |
864 | exit(EXIT_FAILURE); | |
865 | } | |
866 | ||
867 | /* | |
eb73e8ad MK |
868 | * For portability clear the whole structure, since some |
869 | * implementations have additional (nonstandard) fields in | |
870 | * the structure. | |
15545eb6 HS |
871 | */ |
872 | ||
24a31d63 | 873 | memset(&addr, 0, sizeof(struct sockaddr_un)); |
15545eb6 | 874 | |
24a31d63 | 875 | /* Connect socket to socket address */ |
15545eb6 | 876 | |
24a31d63 MK |
877 | addr.sun_family = AF_UNIX; |
878 | strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) \- 1); | |
15545eb6 | 879 | |
24a31d63 | 880 | ret = connect (data_socket, (const struct sockaddr *) &addr, |
15545eb6 HS |
881 | sizeof(struct sockaddr_un)); |
882 | if (ret == \-1) { | |
883 | fprintf(stderr, "The server is down.\\n"); | |
884 | exit(EXIT_FAILURE); | |
885 | } | |
886 | ||
887 | /* Send arguments. */ | |
888 | ||
889 | for (i = 1; i < argc; ++i) { | |
890 | ret = write(data_socket, argv[i], strlen(argv[i]) + 1); | |
891 | if (ret == \-1) { | |
eb73e8ad | 892 | perror("write"); |
15545eb6 HS |
893 | break; |
894 | } | |
895 | } | |
896 | ||
897 | /* Request result. */ | |
898 | ||
899 | strcpy (buffer, "END"); | |
900 | ret = write(data_socket, buffer, strlen(buffer) + 1); | |
901 | if (ret == \-1) { | |
eb73e8ad | 902 | perror("write"); |
15545eb6 HS |
903 | exit(EXIT_FAILURE); |
904 | } | |
905 | ||
15545eb6 HS |
906 | /* Receive result. */ |
907 | ||
908 | ret = read(data_socket, buffer, BUFFER_SIZE); | |
909 | if (ret == \-1) { | |
eb73e8ad | 910 | perror("read"); |
15545eb6 HS |
911 | exit(EXIT_FAILURE); |
912 | } | |
913 | ||
914 | /* Ensure buffer is 0\-terminated. */ | |
915 | ||
916 | buffer[BUFFER_SIZE \- 1] = 0; | |
917 | ||
918 | printf("Result = %s\\n", buffer); | |
919 | ||
920 | /* Close socket. */ | |
921 | ||
922 | close(data_socket); | |
923 | ||
924 | exit(EXIT_SUCCESS); | |
925 | } | |
15545eb6 HS |
926 | .fi |
927 | .PP | |
c751683c MK |
928 | For an example of the use of |
929 | .BR SCM_RIGHTS | |
930 | see | |
931 | .BR cmsg (3). | |
47297adb | 932 | .SH SEE ALSO |
77117f4f MK |
933 | .BR recvmsg (2), |
934 | .BR sendmsg (2), | |
935 | .BR socket (2), | |
936 | .BR socketpair (2), | |
937 | .BR cmsg (3), | |
938 | .BR capabilities (7), | |
939 | .BR credentials (7), | |
170e5f0d JC |
940 | .BR socket (7), |
941 | .BR udp (7) |