1UNIX(7)                    Linux Programmer's Manual                   UNIX(7)
2
3
4

NAME

6       unix - sockets for local interprocess communication
7

SYNOPSIS

9       #include <sys/socket.h>
10       #include <sys/un.h>
11
12       unix_socket = socket(AF_UNIX, type, 0);
13       error = socketpair(AF_UNIX, type, 0, int *sv);
14

DESCRIPTION

16       The  AF_UNIX (also known as AF_LOCAL) socket family is used to communi‐
17       cate between processes on the same machine efficiently.  Traditionally,
18       UNIX  domain  sockets  can  be either unnamed, or bound to a filesystem
19       pathname (marked as being of type  socket).   Linux  also  supports  an
20       abstract namespace which is independent of the filesystem.
21
22       Valid  socket  types in the UNIX domain are: SOCK_STREAM, for a stream-
23       oriented socket; SOCK_DGRAM, for a datagram-oriented socket  that  pre‐
24       serves message boundaries (as on most UNIX implementations, UNIX domain
25       datagram sockets are always reliable and don't reorder datagrams);  and
26       (since  Linux 2.6.4) SOCK_SEQPACKET, for a sequenced-packet socket that
27       is connection-oriented, preserves message boundaries, and delivers mes‐
28       sages in the order that they were sent.
29
30       UNIX domain sockets support passing file descriptors or process creden‐
31       tials to other processes using ancillary data.
32
33   Address format
34       A UNIX domain socket address is represented in the following structure:
35
36           struct sockaddr_un {
37               sa_family_t sun_family;               /* AF_UNIX */
38               char        sun_path[108];            /* pathname */
39           };
40
41       The sun_family field always contains AF_UNIX.  On Linux sun_path is 108
42       bytes in size; see also NOTES, below.
43
44       Various systems calls (for example, bind(2), connect(2), and sendto(2))
45       take a sockaddr_un argument as input.  Some  other  system  calls  (for
46       example,  getsockname(2),  getpeername(2),  recvfrom(2), and accept(2))
47       return an argument of this type.
48
49       Three types of address are distinguished in the sockaddr_un structure:
50
51       *  pathname: a UNIX domain socket can be  bound  to  a  null-terminated
52          filesystem  pathname  using bind(2).  When the address of a pathname
53          socket is returned (by one of the system  calls  noted  above),  its
54          length is
55
56              offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
57
58          and  sun_path contains the null-terminated pathname.  (On Linux, the
59          above  offsetof()  expression  equates  to   the   same   value   as
60          sizeof(sa_family_t),  but  some  other implementations include other
61          fields before sun_path, so the offsetof() expression  more  portably
62          describes the size of the address structure.)
63
64          For further details of pathname sockets, see below.
65
66       *  unnamed: A stream socket that has not been bound to a pathname using
67          bind(2) has no name.  Likewise, the two sockets created  by  socket‐
68          pair(2)  are  unnamed.   When  the  address  of an unnamed socket is
69          returned, its length is sizeof(sa_family_t), and sun_path should not
70          be inspected.
71
72       *  abstract:  an abstract socket address is distinguished (from a path‐
73          name socket) by the fact that sun_path[0] is  a  null  byte  ('\0').
74          The  socket's  address  in this namespace is given by the additional
75          bytes in sun_path that are covered by the specified  length  of  the
76          address structure.  (Null bytes in the name have no special signifi‐
77          cance.)  The name has no connection with filesystem pathnames.  When
78          the  address of an abstract socket is returned, the returned addrlen
79          is greater than sizeof(sa_family_t) (i.e., greater than 2), and  the
80          name   of   the   socket  is  contained  in  the  first  (addrlen  -
81          sizeof(sa_family_t)) bytes of sun_path.
82
83   Pathname sockets
84       When binding a socket to a pathname, a few rules should be observed for
85       maximum portability and ease of coding:
86
87       *  The pathname in sun_path should be null-terminated.
88
89       *  The  length  of  the  pathname, including the terminating null byte,
90          should not exceed the size of sun_path.
91
92       *  The addrlen argument that describes the enclosing sockaddr_un struc‐
93          ture should have a value of at least:
94
95              offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
96
97          or,  more  simply,  addrlen  can be specified as sizeof(struct sock‐
98          addr_un).
99
100       There is some variation  in  how  implementations  handle  UNIX  domain
101       socket addresses that do not follow the above rules.  For example, some
102       (but not all) implementations append  a  null  terminator  if  none  is
103       present in the supplied sun_path.
104
105       When  coding  portable applications, keep in mind that some implementa‐
106       tions have sun_path as short as 92 bytes.
107
108       Various system calls (accept(2), recvfrom(2), getsockname(2),  getpeer‐
109       name(2)) return socket address structures.  When applied to UNIX domain
110       sockets, the value-result addrlen argument supplied to the call  should
111       be  initialized as above.  Upon return, the argument is set to indicate
112       the actual size of the address structure.  The caller should check  the
113       value  returned in this argument: if the output value exceeds the input
114       value, then there is no guarantee that a null terminator is present  in
115       sun_path.  (See BUGS.)
116
117   Pathname socket ownership and permissions
118       In  the Linux implementation, pathname sockets honor the permissions of
119       the directory they are in.  Creation of  a  new  socket  fails  if  the
120       process  does  not  have  write  and search (execute) permission on the
121       directory in which the socket is created.
122
123       On Linux, connecting to a stream socket object requires  write  permis‐
124       sion  on  that socket; sending a datagram to a datagram socket likewise
125       requires write permission on that socket.   POSIX  does  not  make  any
126       statement  about the effect of the permissions on a socket file, and on
127       some systems (e.g., older BSDs), the socket  permissions  are  ignored.
128       Portable programs should not rely on this feature for security.
129
130       When  creating a new socket, the owner and group of the socket file are
131       set according to the usual rules.  The socket file has all  permissions
132       enabled, other than those that are turned off by the process umask(2).
133
134       The  owner,  group, and permissions of a pathname socket can be changed
135       (using chown(2) and chmod(2)).
136
137   Abstract sockets
138       Socket permissions have no meaning for abstract  sockets:  the  process
139       umask(2)  has  no  effect when binding an abstract socket, and changing
140       the ownership and permissions of the object  (via  fchown(2)  and  fch‐
141       mod(2)) has no effect on the accessibility of the socket.
142
143       Abstract  sockets  automatically  disappear when all open references to
144       the socket are closed.
145
146       The abstract socket namespace is a nonportable Linux extension.
147
148   Socket options
149       For historical reasons, these  socket  options  are  specified  with  a
150       SOL_SOCKET type even though they are AF_UNIX specific.  They can be set
151       with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
152       as the socket family.
153
154       SO_PASSCRED
155              Enables  the receiving of the credentials of the sending process
156              in an ancillary message.  When this option is set and the socket
157              is  not  yet  connected  a unique name in the abstract namespace
158              will be generated automatically.   Expects  an  integer  boolean
159              flag.
160
161   Autobind feature
162       If  a  bind(2)  call  specifies  addrlen as sizeof(sa_family_t), or the
163       SO_PASSCRED socket option was specified  for  a  socket  that  was  not
164       explicitly  bound  to  an  address,  then the socket is autobound to an
165       abstract address.  The address consists of a null byte  followed  by  5
166       bytes  in  the  character set [0-9a-f].  Thus, there is a limit of 2^20
167       autobind addresses.  (From Linux 2.1.15, when the autobind feature  was
168       added,  8  bytes  were  used,  and  the  limit  was  thus 2^32 autobind
169       addresses.  The change to 5 bytes came in Linux 2.3.15.)
170
171   Sockets API
172       The following paragraphs describe domain-specific  details  and  unsup‐
173       ported features of the sockets API for UNIX domain sockets on Linux.
174
175       UNIX domain sockets do not support the transmission of out-of-band data
176       (the MSG_OOB flag for send(2) and recv(2)).
177
178       The send(2) MSG_MORE flag is not supported by UNIX domain sockets.
179
180       Before Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2)
181       was not supported by UNIX domain sockets.
182
183       The  SO_SNDBUF  socket option does have an effect for UNIX domain sock‐
184       ets, but the SO_RCVBUF option does  not.   For  datagram  sockets,  the
185       SO_SNDBUF  value  imposes  an upper limit on the size of outgoing data‐
186       grams.  This limit is calculated as the doubled (see socket(7))  option
187       value less 32 bytes used for overhead.
188
189   Ancillary messages
190       Ancillary  data  is  sent and received using sendmsg(2) and recvmsg(2).
191       For historical reasons the ancillary message  types  listed  below  are
192       specified with a SOL_SOCKET type even though they are AF_UNIX specific.
193       To send them  set  the  cmsg_level  field  of  the  struct  cmsghdr  to
194       SOL_SOCKET  and  the cmsg_type field to the type.  For more information
195       see cmsg(3).
196
197       SCM_RIGHTS
198              Send or receive a set of  open  file  descriptors  from  another
199              process.  The data portion contains an integer array of the file
200              descriptors.  The passed file descriptors behave as though  they
201              have been created with dup(2).
202
203       SCM_CREDENTIALS
204              Send  or receive UNIX credentials.  This can be used for authen‐
205              tication.  The credentials are passed as a struct  ucred  ancil‐
206              lary  message.   Thus  structure is defined in <sys/socket.h> as
207              follows:
208
209                  struct ucred {
210                      pid_t pid;    /* process ID of the sending process */
211                      uid_t uid;    /* user ID of the sending process */
212                      gid_t gid;    /* group ID of the sending process */
213                  };
214
215              Since glibc 2.8, the _GNU_SOURCE  feature  test  macro  must  be
216              defined  (before  including any header files) in order to obtain
217              the definition of this structure.
218
219              The credentials which the sender specifies are  checked  by  the
220              kernel.   A process with effective user ID 0 is allowed to spec‐
221              ify values that do not match its own.  The sender  must  specify
222              its own process ID (unless it has the capability CAP_SYS_ADMIN),
223              its real user  ID,  effective  user  ID,  or  saved  set-user-ID
224              (unless  it  has  CAP_SETUID),  and its real group ID, effective
225              group ID, or saved set-group-ID (unless it has CAP_SETGID).   To
226              receive  a  struct  ucred message the SO_PASSCRED option must be
227              enabled on the socket.
228
229   Ioctls
230       The following ioctl(2) calls return information in value.  The  correct
231       syntax is:
232
233              int value;
234              error = ioctl(unix_socket, ioctl_type, &value);
235
236       ioctl_type can be:
237
238       SIOCINQ
239              For SOCK_STREAM socket the function returns the amount of queued
240              unread data in the receive buffer.  The socket must  not  be  in
241              LISTEN  state, otherwise an error (EINVAL) is returned.  SIOCINQ
242              is defined in <linux/sockios.h>.  Alternatively, you can use the
243              synonymous  FIONREAD,  defined in <sys/ioctl.h>.  For SOCK_DGRAM
244              socket, the returned value is the same as  for  Internet  domain
245              datagram socket; see udp(7).
246

ERRORS

248       EADDRINUSE
249              The  specified local address is already in use or the filesystem
250              socket object already exists.
251
252       EBADF  This error can occur for sendmsg(2) when sending a file descrip‐
253              tor  as  ancillary  data  over  a  UNIX  domain  socket (see the
254              description of SCM_RIGHTS, above), and indicates that  the  file
255              descriptor  number  that is being sent is not valid (e.g., it is
256              not an open file descriptor).
257
258       ECONNREFUSED
259              The remote address specified by connect(2) was not  a  listening
260              socket.  This error can also occur if the target pathname is not
261              a socket.
262
263       ECONNRESET
264              Remote socket was unexpectedly closed.
265
266       EFAULT User memory address was not valid.
267
268       EINVAL Invalid argument passed.  A  common  cause  is  that  the  value
269              AF_UNIX  was  not  specified  in  the  sun_type  field of passed
270              addresses, or the socket was in an invalid state for the applied
271              operation.
272
273       EISCONN
274              connect(2)  called  on  an  already connected socket or a target
275              address was specified on a connected socket.
276
277       ENOENT The pathname in the remote address specified to  connect(2)  did
278              not exist.
279
280       ENOMEM Out of memory.
281
282       ENOTCONN
283              Socket  operation  needs a target address, but the socket is not
284              connected.
285
286       EOPNOTSUPP
287              Stream operation called on non-stream oriented socket  or  tried
288              to use the out-of-band data option.
289
290       EPERM  The sender passed invalid credentials in the struct ucred.
291
292       EPIPE  Remote socket was closed on a stream socket.  If enabled, a SIG‐
293              PIPE is sent as well.   This  can  be  avoided  by  passing  the
294              MSG_NOSIGNAL flag to send(2) or sendmsg(2).
295
296       EPROTONOSUPPORT
297              Passed protocol is not AF_UNIX.
298
299       EPROTOTYPE
300              Remote  socket  does not match the local socket type (SOCK_DGRAM
301              versus SOCK_STREAM).
302
303       ESOCKTNOSUPPORT
304              Unknown socket type.
305
306       ETOOMANYREFS
307              This error can occur for sendmsg(2) when sending a file descrip‐
308              tor  as  ancillary  data  over  a  UNIX  domain  socket (see the
309              description of SCM_RIGHTS, above).  It occurs if the  number  of
310              "in-flight"  file descriptors exceeds the RLIMIT_NOFILE resource
311              limit and the caller does not have the CAP_SYS_RESOURCE capabil‐
312              ity.   An  in-flight  file  descriptor is one that has been sent
313              using sendmsg(2) but has not yet been accepted in the  recipient
314              process using recvmsg(2).
315
316              This  error  is  diagnosed since mainline Linux 4.5 (and in some
317              earlier kernel versions where the fix has been backported).   In
318              earlier  kernel  versions, it was possible to place an unlimited
319              number of file descriptors  in  flight,  by  sending  each  file
320              descriptor  with sendmsg(2) and then closing the file descriptor
321              so that it was not accounted against the RLIMIT_NOFILE  resource
322              limit.
323
324       Other  errors  can  be  generated by the generic socket layer or by the
325       filesystem while generating a filesystem socket object.  See the appro‐
326       priate manual pages for more information.
327

VERSIONS

329       SCM_CREDENTIALS  and  the abstract namespace were introduced with Linux
330       2.2 and should not be used in  portable  programs.   (Some  BSD-derived
331       systems also support credential passing, but the implementation details
332       differ.)
333

NOTES

335       Binding to a socket with a filename creates a socket in the  filesystem
336       that  must  be deleted by the caller when it is no longer needed (using
337       unlink(2)).  The usual UNIX close-behind semantics  apply;  the  socket
338       can  be  unlinked  at  any  time  and  will be finally removed from the
339       filesystem when the last reference to it is closed.
340
341       To pass file descriptors or credentials over a SOCK_STREAM socket,  you
342       must  to  send or receive at least one byte of nonancillary data in the
343       same sendmsg(2) or recvmsg(2) call.
344
345       UNIX domain stream sockets do not support  the  notion  of  out-of-band
346       data.
347

BUGS

349       When  binding  a  socket to an address, Linux is one of the implementa‐
350       tions that appends a null terminator if none is supplied  in  sun_path.
351       In  most  cases  this  is  unproblematic:  when  the  socket address is
352       retrieved, it will be one byte  longer  than  that  supplied  when  the
353       socket  was bound.  However, there is one case where confusing behavior
354       can result: if 108 non-null bytes are supplied when a socket is  bound,
355       then  the addition of the null terminator takes the length of the path‐
356       name beyond sizeof(sun_path).  Consequently, when retrieving the socket
357       address (for example, via accept(2)), if the input addrlen argument for
358       the retrieving call is specified as  sizeof(struct  sockaddr_un),  then
359       the  returned  address  structure  won't  have  a  null  terminator  in
360       sun_path.
361
362       In addition, some implementations don't require a null terminator  when
363       binding  a socket (the addrlen argument is used to determine the length
364       of sun_path) and when the socket address is retrieved on  these  imple‐
365       mentations, there is no null terminator in sun_path.
366
367       Applications that retrieve socket addresses can (portably) code to han‐
368       dle the possibility that there is no null  terminator  in  sun_path  by
369       respecting the fact that the number of valid bytes in the pathname is:
370
371           strnlen(addr.sun_path, addrlen - offsetof(sockaddr_un, sun_path))
372
373       Alternatively,  an application can retrieve the socket address by allo‐
374       cating a buffer of size sizeof(struct sockaddr_un)+1 that is zeroed out
375       before  the  retrieval.   The  retrieving  call  can specify addrlen as
376       sizeof(struct sockaddr_un), and the extra zero byte ensures that  there
377       will be a null terminator for the string returned in sun_path:
378
379           void *addrp;
380
381           addrlen = sizeof(struct sockaddr_un);
382           addrp = malloc(addrlen + 1);
383           if (addrp == NULL)
384               /* Handle error */ ;
385           memset(addrp, 0, addrlen + 1);
386
387           if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
388               /* handle error */ ;
389
390           printf("sun_path = %s\n", ((struct sockaddr_un *) addrp)->sun_path);
391
392       This  sort  of  messiness  can  be avoided if it is guaranteed that the
393       applications that create pathname sockets  follow  the  rules  outlined
394       above under Pathname sockets.
395

EXAMPLE

397       The following code demonstrates the use of sequenced-packet sockets for
398       local interprocess communication.  It consists of  two  programs.   The
399       server  program  waits  for  a connection from the client program.  The
400       client sends each of its command-line arguments in  separate  messages.
401       The  server  treats the incoming messages as integers and adds them up.
402       The client sends the command string "END".  The  server  sends  back  a
403       message containing the sum of the client's integers.  The client prints
404       the sum and exits.  The server waits for the next  client  to  connect.
405       To stop the server, the client is called with the command-line argument
406       "DOWN".
407
408       The following output was recorded while running the server in the back‐
409       ground  and  repeatedly  executing the client.  Execution of the server
410       program ends when it receives the "DOWN" command.
411
412   Example output
413           $ ./server &
414           [1] 25887
415           $ ./client 3 4
416           Result = 7
417           $ ./client 11 -5
418           Result = 6
419           $ ./client DOWN
420           Result = 0
421           [1]+  Done                    ./server
422           $
423
424   Program source
425
426       /*
427        * File connection.h
428        */
429
430       #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
431       #define BUFFER_SIZE 12
432
433       /*
434        * File server.c
435        */
436
437       #include <stdio.h>
438       #include <stdlib.h>
439       #include <string.h>
440       #include <sys/socket.h>
441       #include <sys/un.h>
442       #include <unistd.h>
443       #include "connection.h"
444
445       int
446       main(int argc, char *argv[])
447       {
448           struct sockaddr_un name;
449           int down_flag = 0;
450           int ret;
451           int connection_socket;
452           int data_socket;
453           int result;
454           char buffer[BUFFER_SIZE];
455
456           /*
457            * In case the program exited inadvertently on the last run,
458            * remove the socket.
459            */
460
461           unlink(SOCKET_NAME);
462
463           /* Create local socket. */
464
465           connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
466           if (connection_socket == -1) {
467               perror("socket");
468               exit(EXIT_FAILURE);
469           }
470
471           /*
472            * For portability clear the whole structure, since some
473            * implementations have additional (nonstandard) fields in
474            * the structure.
475            */
476
477           memset(&name, 0, sizeof(struct sockaddr_un));
478
479           /* Bind socket to socket name. */
480
481           name.sun_family = AF_UNIX;
482           strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) - 1);
483
484           ret = bind(connection_socket, (const struct sockaddr *) &name,
485                      sizeof(struct sockaddr_un));
486           if (ret == -1) {
487               perror("bind");
488               exit(EXIT_FAILURE);
489           }
490
491           /*
492            * Prepare for accepting connections. The backlog size is set
493            * to 20. So while one request is being processed other requests
494            * can be waiting.
495            */
496
497           ret = listen(connection_socket, 20);
498           if (ret == -1) {
499               perror("listen");
500               exit(EXIT_FAILURE);
501           }
502
503           /* This is the main loop for handling connections. */
504
505           for (;;) {
506
507               /* Wait for incoming connection. */
508
509               data_socket = accept(connection_socket, NULL, NULL);
510               if (data_socket == -1) {
511                   perror("accept");
512                   exit(EXIT_FAILURE);
513               }
514
515               result = 0;
516               for(;;) {
517
518                   /* Wait for next data packet. */
519
520                   ret = read(data_socket, buffer, BUFFER_SIZE);
521                   if (ret == -1) {
522                       perror("read");
523                       exit(EXIT_FAILURE);
524                   }
525
526                   /* Ensure buffer is 0-terminated. */
527
528                   buffer[BUFFER_SIZE - 1] = 0;
529
530                   /* Handle commands. */
531
532                   if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) {
533                       down_flag = 1;
534                       break;
535                   }
536
537                   if (!strncmp(buffer, "END", BUFFER_SIZE)) {
538                       break;
539                   }
540
541                   /* Add received summand. */
542
543                   result += atoi(buffer);
544               }
545
546               /* Send result. */
547
548               sprintf(buffer, "%d", result);
549               ret = write(data_socket, buffer, BUFFER_SIZE);
550
551               if (ret == -1) {
552                   perror("write");
553                   exit(EXIT_FAILURE);
554               }
555
556               /* Close socket. */
557
558               close(data_socket);
559
560               /* Quit on DOWN command. */
561
562               if (down_flag) {
563                   break;
564               }
565           }
566
567           close(connection_socket);
568
569           /* Unlink the socket. */
570
571           unlink(SOCKET_NAME);
572
573           exit(EXIT_SUCCESS);
574       }
575
576       /*
577        * File client.c
578        */
579
580       #include <errno.h>
581       #include <stdio.h>
582       #include <stdlib.h>
583       #include <string.h>
584       #include <sys/socket.h>
585       #include <sys/un.h>
586       #include <unistd.h>
587       #include "connection.h"
588
589       int
590       main(int argc, char *argv[])
591       {
592           struct sockaddr_un addr;
593           int i;
594           int ret;
595           int data_socket;
596           char buffer[BUFFER_SIZE];
597
598           /* Create local socket. */
599
600           data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
601           if (data_socket == -1) {
602               perror("socket");
603               exit(EXIT_FAILURE);
604           }
605
606           /*
607            * For portability clear the whole structure, since some
608            * implementations have additional (nonstandard) fields in
609            * the structure.
610            */
611
612           memset(&addr, 0, sizeof(struct sockaddr_un));
613
614           /* Connect socket to socket address */
615
616           addr.sun_family = AF_UNIX;
617           strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) - 1);
618
619           ret = connect (data_socket, (const struct sockaddr *) &addr,
620                          sizeof(struct sockaddr_un));
621           if (ret == -1) {
622               fprintf(stderr, "The server is down.\n");
623               exit(EXIT_FAILURE);
624           }
625
626           /* Send arguments. */
627
628           for (i = 1; i < argc; ++i) {
629               ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
630               if (ret == -1) {
631                   perror("write");
632                   break;
633               }
634           }
635
636           /* Request result. */
637
638           strcpy (buffer, "END");
639           ret = write(data_socket, buffer, strlen(buffer) + 1);
640           if (ret == -1) {
641               perror("write");
642               exit(EXIT_FAILURE);
643           }
644
645           /* Receive result. */
646
647           ret = read(data_socket, buffer, BUFFER_SIZE);
648           if (ret == -1) {
649               perror("read");
650               exit(EXIT_FAILURE);
651           }
652
653           /* Ensure buffer is 0-terminated. */
654
655           buffer[BUFFER_SIZE - 1] = 0;
656
657           printf("Result = %s\n", buffer);
658
659           /* Close socket. */
660
661           close(data_socket);
662
663           exit(EXIT_SUCCESS);
664       }
665
666       For an example of the use of SCM_RIGHTS see cmsg(3).
667

SEE ALSO

669       recvmsg(2), sendmsg(2), socket(2),  socketpair(2),  cmsg(3),  capabili‐
670       ties(7), credentials(7), socket(7), udp(7)
671

COLOPHON

673       This  page  is  part of release 4.16 of the Linux man-pages project.  A
674       description of the project, information about reporting bugs,  and  the
675       latest     version     of     this    page,    can    be    found    at
676       https://www.kernel.org/doc/man-pages/.
677
678
679
680Linux                             2018-04-30                           UNIX(7)
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