1UNIX(7) Linux Programmer's Manual UNIX(7)
2
6 unix - sockets for local interprocess communication
7
9 #include <sys/socket.h>
10 #include <sys/un.h>
11 unix_socket = socket(AF_UNIX, type, 0);
13 error = socketpair(AF_UNIX, type, 0, int *sv);
14
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 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 UNIX domain sockets support passing file descriptors or process creden‐
31 tials to other processes using ancillary data.
32 Address format
34 A UNIX domain socket address is represented in the following structure:
35 struct sockaddr_un {
37 sa_family_t sun_family; /* AF_UNIX */
38 char sun_path[108]; /* Pathname */
39 };
40 The sun_family field always contains AF_UNIX. On Linux, sun_path is
42 108 bytes in size; see also NOTES, below.
43 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 Three types of address are distinguished in the sockaddr_un structure:
50 * 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 offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
57 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 For further details of pathname sockets, see below.
65 * 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 * 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 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 * The pathname in sun_path should be null-terminated.
88 * The length of the pathname, including the terminating null byte,
90 should not exceed the size of sun_path.
91 * The addrlen argument that describes the enclosing sockaddr_un struc‐
93 ture should have a value of at least:
94 offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
96 or, more simply, addrlen can be specified as sizeof(struct sock‐
98 addr_un).
99 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 When coding portable applications, keep in mind that some implementa‐
106 tions have sun_path as short as 92 bytes.
107 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 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 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 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 The owner, group, and permissions of a pathname socket can be changed
135 (using chown(2) and chmod(2)).
136 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 Abstract sockets automatically disappear when all open references to
144 the socket are closed.
145 The abstract socket namespace is a nonportable Linux extension.
147 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 SO_PASSCRED
155 Enabling this socket option causes receipt of the credentials of
156 the sending process in an SCM_CREDENTIALS ancillary message in
157 each subsequently received message. The returned credentials
158 are those specified by the sender using SCM_CREDENTIALS, or a
159 default that includes the sender's PID, real user ID, and real
160 group ID, if the sender did not specify SCM_CREDENTIALS ancil‐
161 lary data.
162 When this option is set and the socket is not yet connected, a
164 unique name in the abstract namespace will be generated automat‐
165 ically.
166 The value given as an argument to setsockopt(2) and returned as
168 the result of getsockopt(2) is an integer boolean flag.
169 SO_PASSSEC
171 Enables receiving of the SELinux security label of the peer
172 socket in an ancillary message of type SCM_SECURITY (see below).
173 The value given as an argument to setsockopt(2) and returned as
175 the result of getsockopt(2) is an integer boolean flag.
176 The SO_PASSSEC option is supported for UNIX domain datagram
178 sockets since Linux 2.6.18; support for UNIX domain stream sock‐
179 ets was added in Linux 4.2.
180 SO_PEEK_OFF
182 See socket(7).
183 SO_PEERCRED
185 This read-only socket option returns the credentials of the peer
186 process connected to this socket. The returned credentials are
187 those that were in effect at the time of the call to connect(2)
188 or socketpair(2).
189 The argument to getsockopt(2) is a pointer to a ucred structure;
191 define the _GNU_SOURCE feature test macro to obtain the defini‐
192 tion of that structure from <sys/socket.h>.
193 The use of this option is possible only for connected AF_UNIX
195 stream sockets and for AF_UNIX stream and datagram socket pairs
196 created using socketpair(2).
197 Autobind feature
199 If a bind(2) call specifies addrlen as sizeof(sa_family_t), or the
200 SO_PASSCRED socket option was specified for a socket that was not
201 explicitly bound to an address, then the socket is autobound to an
202 abstract address. The address consists of a null byte followed by 5
203 bytes in the character set [0-9a-f]. Thus, there is a limit of 2^20
204 autobind addresses. (From Linux 2.1.15, when the autobind feature was
205 added, 8 bytes were used, and the limit was thus 2^32 autobind
206 addresses. The change to 5 bytes came in Linux 2.3.15.)
207 Sockets API
209 The following paragraphs describe domain-specific details and unsup‐
210 ported features of the sockets API for UNIX domain sockets on Linux.
211 UNIX domain sockets do not support the transmission of out-of-band data
213 (the MSG_OOB flag for send(2) and recv(2)).
214 The send(2) MSG_MORE flag is not supported by UNIX domain sockets.
216 Before Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2)
218 was not supported by UNIX domain sockets.
219 The SO_SNDBUF socket option does have an effect for UNIX domain sock‐
221 ets, but the SO_RCVBUF option does not. For datagram sockets, the
222 SO_SNDBUF value imposes an upper limit on the size of outgoing data‐
223 grams. This limit is calculated as the doubled (see socket(7)) option
224 value less 32 bytes used for overhead.
225 Ancillary messages
227 Ancillary data is sent and received using sendmsg(2) and recvmsg(2).
228 For historical reasons, the ancillary message types listed below are
229 specified with a SOL_SOCKET type even though they are AF_UNIX specific.
230 To send them, set the cmsg_level field of the struct cmsghdr to
231 SOL_SOCKET and the cmsg_type field to the type. For more information,
232 see cmsg(3).
233 SCM_RIGHTS
235 Send or receive a set of open file descriptors from another
236 process. The data portion contains an integer array of the file
237 descriptors.
238 Commonly, this operation is referred to as "passing a file
240 descriptor" to another process. However, more accurately, what
241 is being passed is a reference to an open file description (see
242 open(2)), and in the receiving process it is likely that a dif‐
243 ferent file descriptor number will be used. Semantically, this
244 operation is equivalent to duplicating (dup(2)) a file descrip‐
245 tor into the file descriptor table of another process.
246 If the buffer used to receive the ancillary data containing file
248 descriptors is too small (or is absent), then the ancillary data
249 is truncated (or discarded) and the excess file descriptors are
250 automatically closed in the receiving process.
251 If the number of file descriptors received in the ancillary data
253 would cause the process to exceed its RLIMIT_NOFILE resource
254 limit (see getrlimit(2)), the excess file descriptors are auto‐
255 matically closed in the receiving process.
256 The kernel constant SCM_MAX_FD defines a limit on the number of
258 file descriptors in the array. Attempting to send an array
259 larger than this limit causes sendmsg(2) to fail with the error
260 EINVAL. SCM_MAX_FD has the value 253 (or 255 in kernels before
261 2.6.38).
262 SCM_CREDENTIALS
264 Send or receive UNIX credentials. This can be used for authen‐
265 tication. The credentials are passed as a struct ucred ancil‐
266 lary message. This structure is defined in <sys/socket.h> as
267 follows:
268 struct ucred {
270 pid_t pid; /* Process ID of the sending process */
271 uid_t uid; /* User ID of the sending process */
272 gid_t gid; /* Group ID of the sending process */
273 };
274 Since glibc 2.8, the _GNU_SOURCE feature test macro must be
276 defined (before including any header files) in order to obtain
277 the definition of this structure.
278 The credentials which the sender specifies are checked by the
280 kernel. A privileged process is allowed to specify values that
281 do not match its own. The sender must specify its own process
282 ID (unless it has the capability CAP_SYS_ADMIN, in which case
283 the PID of any existing process may be specified), its real user
284 ID, effective user ID, or saved set-user-ID (unless it has
285 CAP_SETUID), and its real group ID, effective group ID, or saved
286 set-group-ID (unless it has CAP_SETGID).
287 To receive a struct ucred message, the SO_PASSCRED option must
289 be enabled on the socket.
290 SCM_SECURITY
292 Receive the SELinux security context (the security label) of the
293 peer socket. The received ancillary data is a null-terminated
294 string containing the security context. The receiver should
295 allocate at least NAME_MAX bytes in the data portion of the
296 ancillary message for this data.
297 To receive the security context, the SO_PASSSEC option must be
299 enabled on the socket (see above).
300 When sending ancillary data with sendmsg(2), only one item of each of
302 the above types may be included in the sent message.
303 At least one byte of real data should be sent when sending ancillary
305 data. On Linux, this is required to successfully send ancillary data
306 over a UNIX domain stream socket. When sending ancillary data over a
307 UNIX domain datagram socket, it is not necessary on Linux to send any
308 accompanying real data. However, portable applications should also
309 include at least one byte of real data when sending ancillary data over
310 a datagram socket.
311 When receiving from a stream socket, ancillary data forms a kind of
313 barrier for the received data. For example, suppose that the sender
314 transmits as follows:
315 1. sendmsg(2) of four bytes, with no ancillary data.
317 2. sendmsg(2) of one byte, with ancillary data.
318 3. sendmsg(2) of four bytes, with no ancillary data.
319 Suppose that the receiver now performs recvmsg(2) calls each with a
321 buffer size of 20 bytes. The first call will receive five bytes of
322 data, along with the ancillary data sent by the second sendmsg(2) call.
323 The next call will receive the remaining four bytes of data.
324 If the space allocated for receiving incoming ancillary data is too
326 small then the ancillary data is truncated to the number of headers
327 that will fit in the supplied buffer (or, in the case of an SCM_RIGHTS
328 file descriptor list, the list of file descriptors may be truncated).
329 If no buffer is provided for incoming ancillary data (i.e., the
330 msg_control field of the msghdr structure supplied to recvmsg(2) is
331 NULL), then the incoming ancillary data is discarded. In both of these
332 cases, the MSG_CTRUNC flag will be set in the msg.msg_flags value
333 returned by recvmsg(2).
334 Ioctls
336 The following ioctl(2) calls return information in value. The correct
337 syntax is:
338 int value;
340 error = ioctl(unix_socket, ioctl_type, &value);
341 ioctl_type can be:
343 SIOCINQ
345 For SOCK_STREAM sockets, this call returns the number of unread
346 bytes in the receive buffer. The socket must not be in LISTEN
347 state, otherwise an error (EINVAL) is returned. SIOCINQ is
348 defined in <linux/sockios.h>. Alternatively, you can use the
349 synonymous FIONREAD, defined in <sys/ioctl.h>. For SOCK_DGRAM
350 sockets, the returned value is the same as for Internet domain
351 datagram sockets; see udp(7).
352
354 EADDRINUSE
355 The specified local address is already in use or the filesystem
356 socket object already exists.
357 EBADF This error can occur for sendmsg(2) when sending a file descrip‐
359 tor as ancillary data over a UNIX domain socket (see the
360 description of SCM_RIGHTS, above), and indicates that the file
361 descriptor number that is being sent is not valid (e.g., it is
362 not an open file descriptor).
363 ECONNREFUSED
365 The remote address specified by connect(2) was not a listening
366 socket. This error can also occur if the target pathname is not
367 a socket.
368 ECONNRESET
370 Remote socket was unexpectedly closed.
371 EFAULT User memory address was not valid.
373 EINVAL Invalid argument passed. A common cause is that the value
375 AF_UNIX was not specified in the sun_type field of passed
376 addresses, or the socket was in an invalid state for the applied
377 operation.
378 EISCONN
380 connect(2) called on an already connected socket or a target
381 address was specified on a connected socket.
382 ENOENT The pathname in the remote address specified to connect(2) did
384 not exist.
385 ENOMEM Out of memory.
387 ENOTCONN
389 Socket operation needs a target address, but the socket is not
390 connected.
391 EOPNOTSUPP
393 Stream operation called on non-stream oriented socket or tried
394 to use the out-of-band data option.
395 EPERM The sender passed invalid credentials in the struct ucred.
397 EPIPE Remote socket was closed on a stream socket. If enabled, a SIG‐
399 PIPE is sent as well. This can be avoided by passing the
400 MSG_NOSIGNAL flag to send(2) or sendmsg(2).
401 EPROTONOSUPPORT
403 Passed protocol is not AF_UNIX.
404 EPROTOTYPE
406 Remote socket does not match the local socket type (SOCK_DGRAM
407 versus SOCK_STREAM).
408 ESOCKTNOSUPPORT
410 Unknown socket type.
411 ESRCH While sending an ancillary message containing credentials
413 (SCM_CREDENTIALS), the caller specified a PID that does not
414 match any existing process.
415 ETOOMANYREFS
417 This error can occur for sendmsg(2) when sending a file descrip‐
418 tor as ancillary data over a UNIX domain socket (see the
419 description of SCM_RIGHTS, above). It occurs if the number of
420 "in-flight" file descriptors exceeds the RLIMIT_NOFILE resource
421 limit and the caller does not have the CAP_SYS_RESOURCE capabil‐
422 ity. An in-flight file descriptor is one that has been sent
423 using sendmsg(2) but has not yet been accepted in the recipient
424 process using recvmsg(2).
425 This error is diagnosed since mainline Linux 4.5 (and in some
427 earlier kernel versions where the fix has been backported). In
428 earlier kernel versions, it was possible to place an unlimited
429 number of file descriptors in flight, by sending each file
430 descriptor with sendmsg(2) and then closing the file descriptor
431 so that it was not accounted against the RLIMIT_NOFILE resource
432 limit.
433 Other errors can be generated by the generic socket layer or by the
435 filesystem while generating a filesystem socket object. See the appro‐
436 priate manual pages for more information.
437
439 SCM_CREDENTIALS and the abstract namespace were introduced with Linux
440 2.2 and should not be used in portable programs. (Some BSD-derived
441 systems also support credential passing, but the implementation details
442 differ.)
443
445 Binding to a socket with a filename creates a socket in the filesystem
446 that must be deleted by the caller when it is no longer needed (using
447 unlink(2)). The usual UNIX close-behind semantics apply; the socket
448 can be unlinked at any time and will be finally removed from the
449 filesystem when the last reference to it is closed.
450 To pass file descriptors or credentials over a SOCK_STREAM socket, you
452 must to send or receive at least one byte of nonancillary data in the
453 same sendmsg(2) or recvmsg(2) call.
454 UNIX domain stream sockets do not support the notion of out-of-band
456 data.
457
459 When binding a socket to an address, Linux is one of the implementa‐
460 tions that appends a null terminator if none is supplied in sun_path.
461 In most cases this is unproblematic: when the socket address is
462 retrieved, it will be one byte longer than that supplied when the
463 socket was bound. However, there is one case where confusing behavior
464 can result: if 108 non-null bytes are supplied when a socket is bound,
465 then the addition of the null terminator takes the length of the path‐
466 name beyond sizeof(sun_path). Consequently, when retrieving the socket
467 address (for example, via accept(2)), if the input addrlen argument for
468 the retrieving call is specified as sizeof(struct sockaddr_un), then
469 the returned address structure won't have a null terminator in
470 sun_path.
471 In addition, some implementations don't require a null terminator when
473 binding a socket (the addrlen argument is used to determine the length
474 of sun_path) and when the socket address is retrieved on these imple‐
475 mentations, there is no null terminator in sun_path.
476 Applications that retrieve socket addresses can (portably) code to han‐
478 dle the possibility that there is no null terminator in sun_path by
479 respecting the fact that the number of valid bytes in the pathname is:
480 strnlen(addr.sun_path, addrlen - offsetof(sockaddr_un, sun_path))
482 Alternatively, an application can retrieve the socket address by allo‐
484 cating a buffer of size sizeof(struct sockaddr_un)+1 that is zeroed out
485 before the retrieval. The retrieving call can specify addrlen as
486 sizeof(struct sockaddr_un), and the extra zero byte ensures that there
487 will be a null terminator for the string returned in sun_path:
488 void *addrp;
490 addrlen = sizeof(struct sockaddr_un);
492 addrp = malloc(addrlen + 1);
493 if (addrp == NULL)
494 /* Handle error */ ;
495 memset(addrp, 0, addrlen + 1);
496 if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
498 /* handle error */ ;
499 printf("sun_path = %s\n", ((struct sockaddr_un *) addrp)->sun_path);
501 This sort of messiness can be avoided if it is guaranteed that the
503 applications that create pathname sockets follow the rules outlined
504 above under Pathname sockets.
505
507 The following code demonstrates the use of sequenced-packet sockets for
508 local interprocess communication. It consists of two programs. The
509 server program waits for a connection from the client program. The
510 client sends each of its command-line arguments in separate messages.
511 The server treats the incoming messages as integers and adds them up.
512 The client sends the command string "END". The server sends back a
513 message containing the sum of the client's integers. The client prints
514 the sum and exits. The server waits for the next client to connect.
515 To stop the server, the client is called with the command-line argument
516 "DOWN".
517 The following output was recorded while running the server in the back‐
519 ground and repeatedly executing the client. Execution of the server
520 program ends when it receives the "DOWN" command.
521 Example output
523 $ ./server &
524 [1] 25887
525 $ ./client 3 4
526 Result = 7
527 $ ./client 11 -5
528 Result = 6
529 $ ./client DOWN
530 Result = 0
531 [1]+ Done ./server
532 $
533 Program source
535 /*
537 * File connection.h
538 */
539 #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
541 #define BUFFER_SIZE 12
542 /*
544 * File server.c
545 */
546 #include <stdio.h>
548 #include <stdlib.h>
549 #include <string.h>
550 #include <sys/socket.h>
551 #include <sys/un.h>
552 #include <unistd.h>
553 #include "connection.h"
554 int
556 main(int argc, char *argv[])
557 {
558 struct sockaddr_un name;
559 int down_flag = 0;
560 int ret;
561 int connection_socket;
562 int data_socket;
563 int result;
564 char buffer[BUFFER_SIZE];
565 /* Create local socket. */
567 connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
569 if (connection_socket == -1) {
570 perror("socket");
571 exit(EXIT_FAILURE);
572 }
573 /*
575 * For portability clear the whole structure, since some
576 * implementations have additional (nonstandard) fields in
577 * the structure.
578 */
579 memset(&name, 0, sizeof(struct sockaddr_un));
581 /* Bind socket to socket name. */
583 name.sun_family = AF_UNIX;
585 strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) - 1);
586 ret = bind(connection_socket, (const struct sockaddr *) &name,
588 sizeof(struct sockaddr_un));
589 if (ret == -1) {
590 perror("bind");
591 exit(EXIT_FAILURE);
592 }
593 /*
595 * Prepare for accepting connections. The backlog size is set
596 * to 20. So while one request is being processed other requests
597 * can be waiting.
598 */
599 ret = listen(connection_socket, 20);
601 if (ret == -1) {
602 perror("listen");
603 exit(EXIT_FAILURE);
604 }
605 /* This is the main loop for handling connections. */
607 for (;;) {
609 /* Wait for incoming connection. */
611 data_socket = accept(connection_socket, NULL, NULL);
613 if (data_socket == -1) {
614 perror("accept");
615 exit(EXIT_FAILURE);
616 }
617 result = 0;
619 for (;;) {
620 /* Wait for next data packet. */
622 ret = read(data_socket, buffer, BUFFER_SIZE);
624 if (ret == -1) {
625 perror("read");
626 exit(EXIT_FAILURE);
627 }
628 /* Ensure buffer is 0-terminated. */
630 buffer[BUFFER_SIZE - 1] = 0;
632 /* Handle commands. */
634 if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) {
636 down_flag = 1;
637 break;
638 }
639 if (!strncmp(buffer, "END", BUFFER_SIZE)) {
641 break;
642 }
643 /* Add received summand. */
645 result += atoi(buffer);
647 }
648 /* Send result. */
650 sprintf(buffer, "%d", result);
652 ret = write(data_socket, buffer, BUFFER_SIZE);
653 if (ret == -1) {
654 perror("write");
655 exit(EXIT_FAILURE);
656 }
657 /* Close socket. */
659 close(data_socket);
661 /* Quit on DOWN command. */
663 if (down_flag) {
665 break;
666 }
667 }
668 close(connection_socket);
670 /* Unlink the socket. */
672 unlink(SOCKET_NAME);
674 exit(EXIT_SUCCESS);
676 }
677 /*
679 * File client.c
680 */
681 #include <errno.h>
683 #include <stdio.h>
684 #include <stdlib.h>
685 #include <string.h>
686 #include <sys/socket.h>
687 #include <sys/un.h>
688 #include <unistd.h>
689 #include "connection.h"
690 int
692 main(int argc, char *argv[])
693 {
694 struct sockaddr_un addr;
695 int i;
696 int ret;
697 int data_socket;
698 char buffer[BUFFER_SIZE];
699 /* Create local socket. */
701 data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
703 if (data_socket == -1) {
704 perror("socket");
705 exit(EXIT_FAILURE);
706 }
707 /*
709 * For portability clear the whole structure, since some
710 * implementations have additional (nonstandard) fields in
711 * the structure.
712 */
713 memset(&addr, 0, sizeof(struct sockaddr_un));
715 /* Connect socket to socket address */
717 addr.sun_family = AF_UNIX;
719 strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) - 1);
720 ret = connect (data_socket, (const struct sockaddr *) &addr,
722 sizeof(struct sockaddr_un));
723 if (ret == -1) {
724 fprintf(stderr, "The server is down.\n");
725 exit(EXIT_FAILURE);
726 }
727 /* Send arguments. */
729 for (i = 1; i < argc; ++i) {
731 ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
732 if (ret == -1) {
733 perror("write");
734 break;
735 }
736 }
737 /* Request result. */
739 strcpy (buffer, "END");
741 ret = write(data_socket, buffer, strlen(buffer) + 1);
742 if (ret == -1) {
743 perror("write");
744 exit(EXIT_FAILURE);
745 }
746 /* Receive result. */
748 ret = read(data_socket, buffer, BUFFER_SIZE);
750 if (ret == -1) {
751 perror("read");
752 exit(EXIT_FAILURE);
753 }
754 /* Ensure buffer is 0-terminated. */
756 buffer[BUFFER_SIZE - 1] = 0;
758 printf("Result = %s\n", buffer);
760 /* Close socket. */
762 close(data_socket);
764 exit(EXIT_SUCCESS);
766 }
767 For an example of the use of SCM_RIGHTS see cmsg(3).
769
771 recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili‐
772 ties(7), credentials(7), socket(7), udp(7)
773
775 This page is part of release 5.07 of the Linux man-pages project. A
776 description of the project, information about reporting bugs, and the
777 latest version of this page, can be found at
778 https://www.kernel.org/doc/man-pages/.
779Linux 2020-06-09 UNIX(7)