1SOCKET(7)                  Linux Programmer's Manual                 SOCKET(7)
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NAME

6       socket - Linux socket interface
7

SYNOPSIS

9       #include <sys/socket.h>
10
11       sockfd = socket(int socket_family, int socket_type, int protocol);
12

DESCRIPTION

14       This  manual  page  describes  the  Linux  networking socket layer user
15       interface.  The  BSD  compatible  sockets  are  the  uniform  interface
16       between the user process and the network protocol stacks in the kernel.
17       The protocol  modules  are  grouped  into  protocol  families  such  as
18       AF_INET, AF_IPX, and AF_PACKET, and socket types such as SOCK_STREAM or
19       SOCK_DGRAM.  See socket(2) for more information on families and types.
20
21   Socket-layer functions
22       These functions are used by the user process to send or receive packets
23       and  to  do  other  socket  operations.  For more information see their
24       respective manual pages.
25
26       socket(2) creates a socket, connect(2) connects a socket  to  a  remote
27       socket  address,  the bind(2) function binds a socket to a local socket
28       address, listen(2) tells the  socket  that  new  connections  shall  be
29       accepted, and accept(2) is used to get a new socket with a new incoming
30       connection.  socketpair(2)  returns  two  connected  anonymous  sockets
31       (implemented only for a few local families like AF_UNIX)
32
33       send(2),  sendto(2),  and  sendmsg(2)  send  data  over  a  socket, and
34       recv(2), recvfrom(2), recvmsg(2) receive data from a  socket.   poll(2)
35       and  select(2)  wait for arriving data or a readiness to send data.  In
36       addition, the standard I/O operations like write(2),  writev(2),  send‐
37       file(2), read(2), and readv(2) can be used to read and write data.
38
39       getsockname(2)  returns  the  local  socket  address and getpeername(2)
40       returns the remote socket address.  getsockopt(2) and setsockopt(2) are
41       used  to  set or get socket layer or protocol options.  ioctl(2) can be
42       used to set or read some other options.
43
44       close(2) is used to close a socket.   shutdown(2)  closes  parts  of  a
45       full-duplex socket connection.
46
47       Seeking,  or  calling  pread(2) or pwrite(2) with a nonzero position is
48       not supported on sockets.
49
50       It is possible to do nonblocking I/O on sockets by setting  the  O_NON‐
51       BLOCK flag on a socket file descriptor using fcntl(2).  Then all opera‐
52       tions that would block will (usually)  return  with  EAGAIN  (operation
53       should  be  retried  later);  connect(2) will return EINPROGRESS error.
54       The user can then wait for various events via poll(2) or select(2).
55
56       ┌────────────────────────────────────────────────────────────────────┐
57       │                            I/O events                              │
58       ├───────────┬───────────┬────────────────────────────────────────────┤
59       │Event      │ Poll flag │ Occurrence                                 │
60       ├───────────┼───────────┼────────────────────────────────────────────┤
61       │Read       │ POLLIN    │ New data arrived.                          │
62       ├───────────┼───────────┼────────────────────────────────────────────┤
63       │Read       │ POLLIN    │ A connection setup has been completed (for │
64       │           │           │ connection-oriented sockets)               │
65       ├───────────┼───────────┼────────────────────────────────────────────┤
66       │Read       │ POLLHUP   │ A disconnection request has been initiated │
67       │           │           │ by the other end.                          │
68       ├───────────┼───────────┼────────────────────────────────────────────┤
69       │Read       │ POLLHUP   │ A connection is broken (only  for  connec‐ │
70       │           │           │ tion-oriented protocols).  When the socket │
71       │           │           │ is written SIGPIPE is also sent.           │
72       ├───────────┼───────────┼────────────────────────────────────────────┤
73       │Write      │ POLLOUT   │ Socket has enough send  buffer  space  for │
74       │           │           │ writing new data.                          │
75       ├───────────┼───────────┼────────────────────────────────────────────┤
76       │Read/Write │ POLLIN |  │ An outgoing connect(2) finished.           │
77       │           │ POLLOUT   │                                            │
78       ├───────────┼───────────┼────────────────────────────────────────────┤
79       │Read/Write │ POLLERR   │ An asynchronous error occurred.            │
80       ├───────────┼───────────┼────────────────────────────────────────────┤
81       │Read/Write │ POLLHUP   │ The other end has shut down one direction. │
82       ├───────────┼───────────┼────────────────────────────────────────────┤
83       │Exception  │ POLLPRI   │ Urgent data arrived.  SIGURG is sent then. │
84       └───────────┴───────────┴────────────────────────────────────────────┘
85       An alternative to poll(2) and select(2) is to let the kernel inform the
86       application about events via a SIGIO signal.  For that the O_ASYNC flag
87       must be set on a socket file descriptor via fcntl(2) and a valid signal
88       handler for SIGIO must be installed via sigaction(2).  See the  Signals
89       discussion below.
90
91   Socket address structures
92       Each  socket  domain  has  its  own format for socket addresses, with a
93       domain-specific address structure.  Each  of  these  structures  begins
94       with  an  integer  "family" field (typed as sa_family_t) that indicates
95       the type of the address structure.   This  allows  the  various  system
96       calls  (e.g.,  connect(2), bind(2), accept(2), getsockname(2), getpeer‐
97       name(2)), which are generic to all socket  domains,  to  determine  the
98       domain of a particular socket address.
99
100       To  allow  any type of socket address to be passed to interfaces in the
101       sockets API, the type struct sockaddr is defined.  The purpose of  this
102       type is purely to allow casting of domain-specific socket address types
103       to a "generic" type, so as to avoid compiler warnings about  type  mis‐
104       matches in calls to the sockets API.
105
106       In  addition,  the  sockets  API  provides  the  data type struct sock‐
107       addr_storage.  This type  is  suitable  to  accommodate  all  supported
108       domain-specific  socket  address  structures; it is large enough and is
109       aligned properly.  (In particular, it is  large  enough  to  hold  IPv6
110       socket  addresses.)   The structure includes the following field, which
111       can be used to identify the type of socket address actually  stored  in
112       the structure:
113
114               sa_family_t ss_family;
115
116       The  sockaddr_storage  structure is useful in programs that must handle
117       socket addresses in a generic way (e.g., programs that must  deal  with
118       both IPv4 and IPv6 socket addresses).
119
120   Socket options
121       The  socket  options listed below can be set by using setsockopt(2) and
122       read with getsockopt(2) with the socket level set to SOL_SOCKET for all
123       sockets.  Unless otherwise noted, optval is a pointer to an int.
124
125       SO_ACCEPTCONN
126              Returns  a  value indicating whether or not this socket has been
127              marked to accept connections with listen(2).  The value 0  indi‐
128              cates that this is not a listening socket, the value 1 indicates
129              that this is a listening socket.  This socket  option  is  read-
130              only.
131
132       SO_ATTACH_FILTER (since Linux 2.2), SO_ATTACH_BPF (since Linux 3.19)
133              Attach  a  classic  BPF  (SO_ATTACH_FILTER)  or  an extended BPF
134              (SO_ATTACH_BPF) program to the socket for use  as  a  filter  of
135              incoming  packets.   A packet will be dropped if the filter pro‐
136              gram returns zero.  If the  filter  program  returns  a  nonzero
137              value  which  is  less than the packet's data length, the packet
138              will be truncated to the length returned.  If the value returned
139              by  the  filter  is  greater  than or equal to the packet's data
140              length, the packet is allowed to proceed unmodified.
141
142              The argument for SO_ATTACH_FILTER  is  a  sock_fprog  structure,
143              defined in <linux/filter.h>:
144
145                  struct sock_fprog {
146                      unsigned short      len;
147                      struct sock_filter *filter;
148                  };
149
150              The  argument for SO_ATTACH_BPF is a file descriptor returned by
151              the bpf(2) system call and must  refer  to  a  program  of  type
152              BPF_PROG_TYPE_SOCKET_FILTER.
153
154              These options may be set multiple times for a given socket, each
155              time replacing the previous filter  program.   The  classic  and
156              extended versions may be called on the same socket, but the pre‐
157              vious filter will always be replaced such that  a  socket  never
158              has more than one filter defined.
159
160              Both classic and extended BPF are explained in the kernel source
161              file Documentation/networking/filter.txt
162
163       SO_ATTACH_REUSEPORT_CBPF, SO_ATTACH_REUSEPORT_EBPF
164              For use with the SO_REUSEPORT option, these  options  allow  the
165              user  to  set  a  classic  BPF  (SO_ATTACH_REUSEPORT_CBPF) or an
166              extended BPF (SO_ATTACH_REUSEPORT_EBPF)  program  which  defines
167              how  packets  are assigned to the sockets in the reuseport group
168              (that is, all sockets which have SO_REUSEPORT set and are  using
169              the same local address to receive packets).
170
171              The  BPF  program  must return an index between 0 and N-1 repre‐
172              senting the socket which should receive the packet (where  N  is
173              the number of sockets in the group).  If the BPF program returns
174              an invalid index, socket selection will fall back to  the  plain
175              SO_REUSEPORT mechanism.
176
177              Sockets are numbered in the order in which they are added to the
178              group (that is, the order of bind(2) calls for  UDP  sockets  or
179              the  order  of  listen(2)  calls  for TCP sockets).  New sockets
180              added to a reuseport group will inherit the BPF program.  When a
181              socket  is  removed  from  a reuseport group (via close(2)), the
182              last socket in the group will be moved into the closed  socket's
183              position.
184
185              These options may be set repeatedly at any time on any socket in
186              the group to replace the current BPF program used by all sockets
187              in the group.
188
189              SO_ATTACH_REUSEPORT_CBPF   takes   the  same  argument  type  as
190              SO_ATTACH_FILTER and  SO_ATTACH_REUSEPORT_EBPF  takes  the  same
191              argument type as SO_ATTACH_BPF.
192
193              UDP  support  for this feature is available since Linux 4.5; TCP
194              support is available since Linux 4.6.
195
196       SO_BINDTODEVICE
197              Bind this socket to a particular device like “eth0”,  as  speci‐
198              fied  in  the  passed  interface  name.  If the name is an empty
199              string or the option length is zero, the socket  device  binding
200              is  removed.  The passed option is a variable-length null-termi‐
201              nated interface name string with the maximum size  of  IFNAMSIZ.
202              If a socket is bound to an interface, only packets received from
203              that particular interface are processed  by  the  socket.   Note
204              that this works only for some socket types, particularly AF_INET
205              sockets.  It is not supported for  packet  sockets  (use  normal
206              bind(2) there).
207
208              Before Linux 3.8, this socket option could be set, but could not
209              retrieved with getsockopt(2).  Since Linux 3.8, it is  readable.
210              The  optlen argument should contain the buffer size available to
211              receive the device name and is recommended to be IFNAMSIZ bytes.
212              The real device name length is reported back in the optlen argu‐
213              ment.
214
215       SO_BROADCAST
216              Set or get the broadcast flag.  When enabled,  datagram  sockets
217              are allowed to send packets to a broadcast address.  This option
218              has no effect on stream-oriented sockets.
219
220       SO_BSDCOMPAT
221              Enable BSD bug-to-bug compatibility.  This is used  by  the  UDP
222              protocol  module  in Linux 2.0 and 2.2.  If enabled, ICMP errors
223              received for a UDP socket will not be passed to  the  user  pro‐
224              gram.   In  later  kernel  versions, support for this option has
225              been phased out: Linux 2.4 silently ignores it,  and  Linux  2.6
226              generates  a  kernel  warning  (printk()) if a program uses this
227              option.  Linux 2.0 also  enabled  BSD  bug-to-bug  compatibility
228              options (random header changing, skipping of the broadcast flag)
229              for raw sockets with this option, but that was removed in  Linux
230              2.2.
231
232       SO_DEBUG
233              Enable  socket  debugging.   Allowed only for processes with the
234              CAP_NET_ADMIN capability or an effective user ID of 0.
235
236       SO_DETACH_FILTER (since Linux 2.2), SO_DETACH_BPF (since Linux 3.19)
237              These two options, which are synonyms, may be used to remove the
238              classic or extended BPF program attached to a socket with either
239              SO_ATTACH_FILTER or SO_ATTACH_BPF.  The option value is ignored.
240
241       SO_DOMAIN (since Linux 2.6.32)
242              Retrieves the socket domain as an  integer,  returning  a  value
243              such  as  AF_INET6.   See  socket(2)  for  details.  This socket
244              option is read-only.
245
246       SO_ERROR
247              Get and clear the pending socket error.  This socket  option  is
248              read-only.  Expects an integer.
249
250       SO_DONTROUTE
251              Don't send via a gateway, send only to directly connected hosts.
252              The same effect can be achieved  by  setting  the  MSG_DONTROUTE
253              flag  on a socket send(2) operation.  Expects an integer boolean
254              flag.
255
256       SO_INCOMING_CPU (gettable since Linux 3.19, settable since Linux 4.4)
257              Sets or gets the CPU affinity of a socket.  Expects  an  integer
258              flag.
259
260                  int cpu = 1;
261                  setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, sizeof(cpu));
262
263              Because  all of the packets for a single stream (i.e., all pack‐
264              ets for the same 4-tuple) arrive on the single RX queue that  is
265              associated  with  a  particular  CPU, the typical use case is to
266              employ one listening process per RX  queue,  with  the  incoming
267              flow  being  handled  by a listener on the same CPU that is han‐
268              dling the RX queue.  This provides  optimal  NUMA  behavior  and
269              keeps CPU caches hot.
270
271       SO_KEEPALIVE
272              Enable  sending  of  keep-alive  messages on connection-oriented
273              sockets.  Expects an integer boolean flag.
274
275       SO_LINGER
276              Sets or gets the SO_LINGER option.  The  argument  is  a  linger
277              structure.
278
279                  struct linger {
280                      int l_onoff;    /* linger active */
281                      int l_linger;   /* how many seconds to linger for */
282                  };
283
284              When  enabled,  a  close(2) or shutdown(2) will not return until
285              all queued messages for the socket have been  successfully  sent
286              or  the  linger  timeout  has been reached.  Otherwise, the call
287              returns immediately and the closing is done in  the  background.
288              When  the socket is closed as part of exit(2), it always lingers
289              in the background.
290
291       SO_LOCK_FILTER
292              When set, this option will prevent changing the filters  associ‐
293              ated  with  the socket.  These filters include any set using the
294              socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE‐
295              PORT_CBPF, and SO_ATTACH_REUSEPORT_EBPF.
296
297              The typical use case is for a privileged process to set up a raw
298              socket (an operation that requires the CAP_NET_RAW  capability),
299              apply  a  restrictive filter, set the SO_LOCK_FILTER option, and
300              then either drop its privileges or pass the socket file descrip‐
301              tor to an unprivileged process via a UNIX domain socket.
302
303              Once  the  SO_LOCK_FILTER  option  has been enabled, attempts to
304              change or remove the filter attached to a socket, or to  disable
305              the SO_LOCK_FILTER option will fail with the error EPERM.
306
307       SO_MARK (since Linux 2.6.25)
308              Set  the  mark for each packet sent through this socket (similar
309              to the netfilter MARK target but  socket-based).   Changing  the
310              mark can be used for mark-based routing without netfilter or for
311              packet   filtering.    Setting   this   option   requires    the
312              CAP_NET_ADMIN capability.
313
314       SO_OOBINLINE
315              If  this  option is enabled, out-of-band data is directly placed
316              into the receive data stream.  Otherwise,  out-of-band  data  is
317              passed only when the MSG_OOB flag is set during receiving.
318
319       SO_PASSCRED
320              Enable  or  disable the receiving of the SCM_CREDENTIALS control
321              message.  For more information see unix(7).
322
323       SO_PASSSEC
324              Enable or disable the receiving of the SCM_SECURITY control mes‐
325              sage.  For more information see unix(7).
326
327       SO_PEEK_OFF (since Linux 3.4)
328              This option, which is currently supported only for unix(7) sock‐
329              ets, sets the value of the "peek offset" for the recv(2)  system
330              call when used with MSG_PEEK flag.
331
332              When this option is set to a negative value (it is set to -1 for
333              all new sockets), traditional behavior is provided: recv(2) with
334              the MSG_PEEK flag will peek data from the front of the queue.
335
336              When the option is set to a value greater than or equal to zero,
337              then the next peek at data queued in the socket  will  occur  at
338              the  byte  offset  specified  by  the option value.  At the same
339              time, the "peek offset" will be incremented  by  the  number  of
340              bytes that were peeked from the queue, so that a subsequent peek
341              will return the next data in the queue.
342
343              If data is removed from the front of the queue  via  a  call  to
344              recv(2)  (or  similar) without the MSG_PEEK flag, the "peek off‐
345              set" will be decreased by the number of bytes removed.  In other
346              words,  receiving  data without the MSG_PEEK flag will cause the
347              "peek offset" to be adjusted to maintain  the  correct  relative
348              position  in  the  queued  data,  so that a subsequent peek will
349              retrieve the data that would have been retrieved  had  the  data
350              not been removed.
351
352              For  datagram sockets, if the "peek offset" points to the middle
353              of a packet, the data returned will be marked with the MSG_TRUNC
354              flag.
355
356              The   following   example   serves  to  illustrate  the  use  of
357              SO_PEEK_OFF.  Suppose a stream socket has the  following  queued
358              input data:
359
360                  aabbccddeeff
361
362              The  following  sequence  of recv(2) calls would have the effect
363              noted in the comments:
364
365                  int ov = 4;                  // Set peek offset to 4
366                  setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
367
368                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
369                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
370                  recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
371                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8
372
373       SO_PEERCRED
374              Return the credentials of the peer  process  connected  to  this
375              socket.  For further details, see unix(7).
376
377       SO_PRIORITY
378              Set  the protocol-defined priority for all packets to be sent on
379              this socket.  Linux uses this  value  to  order  the  networking
380              queues:  packets  with  a higher priority may be processed first
381              depending on the selected device queueing discipline.  Setting a
382              priority  outside  the  range  0 to 6 requires the CAP_NET_ADMIN
383              capability.
384
385       SO_PROTOCOL (since Linux 2.6.32)
386              Retrieves the socket protocol as an integer, returning  a  value
387              such  as  IPPROTO_SCTP.  See socket(2) for details.  This socket
388              option is read-only.
389
390       SO_RCVBUF
391              Sets or gets the maximum socket receive buffer  in  bytes.   The
392              kernel  doubles this value (to allow space for bookkeeping over‐
393              head) when it is set using setsockopt(2), and this doubled value
394              is  returned  by getsockopt(2).  The default value is set by the
395              /proc/sys/net/core/rmem_default file, and  the  maximum  allowed
396              value is set by the /proc/sys/net/core/rmem_max file.  The mini‐
397              mum (doubled) value for this option is 256.
398
399       SO_RCVBUFFORCE (since Linux 2.6.14)
400              Using this socket option, a privileged  (CAP_NET_ADMIN)  process
401              can  perform  the same task as SO_RCVBUF, but the rmem_max limit
402              can be overridden.
403
404       SO_RCVLOWAT and SO_SNDLOWAT
405              Specify the minimum number of bytes  in  the  buffer  until  the
406              socket layer will pass the data to the protocol (SO_SNDLOWAT) or
407              the user on receiving (SO_RCVLOWAT).  These two values are  ini‐
408              tialized to 1.  SO_SNDLOWAT is not changeable on Linux (setsock‐
409              opt(2)  fails  with  the  error  ENOPROTOOPT).   SO_RCVLOWAT  is
410              changeable only since Linux 2.4.
411
412              Before  Linux  2.6.28  select(2),  poll(2), and epoll(7) did not
413              respect the SO_RCVLOWAT setting on Linux, and indicated a socket
414              as  readable  when  even a single byte of data was available.  A
415              subsequent  read  from  the  socket  would  then   block   until
416              SO_RCVLOWAT bytes are available.  Since Linux 2.6.28, select(2),
417              poll(2), and epoll(7) indicate a socket as readable only  if  at
418              least SO_RCVLOWAT bytes are available.
419
420       SO_RCVTIMEO and SO_SNDTIMEO
421              Specify  the  receiving  or  sending timeouts until reporting an
422              error.  The argument is a struct timeval.  If an input or output
423              function  blocks for this period of time, and data has been sent
424              or received, the return value  of  that  function  will  be  the
425              amount  of data transferred; if no data has been transferred and
426              the timeout has been reached, then -1 is returned with errno set
427              to  EAGAIN  or EWOULDBLOCK, or EINPROGRESS (for connect(2)) just
428              as if the socket was specified to be nonblocking.  If the  time‐
429              out  is set to zero (the default), then the operation will never
430              timeout.  Timeouts only have effect for system calls  that  per‐
431              form   socket   I/O   (e.g.,   read(2),   recvmsg(2),   send(2),
432              sendmsg(2)); timeouts have no  effect  for  select(2),  poll(2),
433              epoll_wait(2), and so on.
434
435       SO_REUSEADDR
436              Indicates  that  the rules used in validating addresses supplied
437              in a bind(2) call should allow reuse of  local  addresses.   For
438              AF_INET  sockets  this means that a socket may bind, except when
439              there is an active listening socket bound to the address.   When
440              the listening socket is bound to INADDR_ANY with a specific port
441              then it is not possible to bind  to  this  port  for  any  local
442              address.  Argument is an integer boolean flag.
443
444       SO_REUSEPORT (since Linux 3.9)
445              Permits  multiple  AF_INET or AF_INET6 sockets to be bound to an
446              identical socket address.  This  option  must  be  set  on  each
447              socket  (including the first socket) prior to calling bind(2) on
448              the socket.  To prevent port hijacking,  all  of  the  processes
449              binding  to  the  same address must have the same effective UID.
450              This option can be employed with both TCP and UDP sockets.
451
452              For TCP sockets, this option allows accept(2) load  distribution
453              in  a  multi-threaded  server to be improved by using a distinct
454              listener socket for each thread.  This  provides  improved  load
455              distribution  as compared to traditional techniques such using a
456              single accept(2)ing thread that distributes connections, or hav‐
457              ing  multiple  threads  that  compete to accept(2) from the same
458              socket.
459
460              For UDP sockets, the use of this option can provide better  dis‐
461              tribution  of  incoming  datagrams  to  multiple  processes  (or
462              threads) as compared to the traditional technique of having mul‐
463              tiple processes compete to receive datagrams on the same socket.
464
465       SO_RXQ_OVFL (since Linux 2.6.33)
466              Indicates that an unsigned 32-bit value ancillary message (cmsg)
467              should be attached to received skbs  indicating  the  number  of
468              packets dropped by the socket since its creation.
469
470       SO_SELECT_ERR_QUEUE (since Linux 3.10)
471              When  this  option  is  set on a socket, an error condition on a
472              socket causes notification not only via  the  exceptfds  set  of
473              select(2).   Similarly,  poll(2) also returns a POLLPRI whenever
474              an POLLERR event is returned.
475
476              Background: this option was added when waking  up  on  an  error
477              condition  occurred  only  via  the readfds and writefds sets of
478              select(2).  The option was added to allow monitoring  for  error
479              conditions  via  the  exceptfds  argument without simultaneously
480              having to receive notifications (via readfds) for  regular  data
481              that  can be read from the socket.  After changes in Linux 4.16,
482              the use of this flag to achieve the desired notifications is  no
483              longer  necessary.   This  option  is  nevertheless retained for
484              backwards compatibility.
485
486       SO_SNDBUF
487              Sets or gets the maximum socket send buffer in bytes.  The  ker‐
488              nel doubles this value (to allow space for bookkeeping overhead)
489              when it is set using setsockopt(2), and this  doubled  value  is
490              returned  by  getsockopt(2).   The  default  value is set by the
491              /proc/sys/net/core/wmem_default file  and  the  maximum  allowed
492              value is set by the /proc/sys/net/core/wmem_max file.  The mini‐
493              mum (doubled) value for this option is 2048.
494
495       SO_SNDBUFFORCE (since Linux 2.6.14)
496              Using this socket option, a privileged  (CAP_NET_ADMIN)  process
497              can  perform  the same task as SO_SNDBUF, but the wmem_max limit
498              can be overridden.
499
500       SO_TIMESTAMP
501              Enable or disable the receiving of the SO_TIMESTAMP control mes‐
502              sage.    The  timestamp  control  message  is  sent  with  level
503              SOL_SOCKET and a  cmsg_type  of  SCM_TIMESTAMP.   The  cmsg_data
504              field  is  a struct timeval indicating the reception time of the
505              last packet passed to the user in this call.   See  cmsg(3)  for
506              details on control messages.
507
508       SO_TIMESTAMPNS (since Linux 2.6.22)
509              Enable  or  disable  the receiving of the SO_TIMESTAMPNS control
510              message.  The timestamp  control  message  is  sent  with  level
511              SOL_SOCKET  and  a  cmsg_type of SCM_TIMESTAMPNS.  The cmsg_data
512              field is a struct timespec indicating the reception time of  the
513              last packet passed to the user in this call.  The clock used for
514              the timestamp is CLOCK_REALTIME.  See  cmsg(3)  for  details  on
515              control messages.
516
517              A  socket  cannot  mix  SO_TIMESTAMP and SO_TIMESTAMPNS: the two
518              modes are mutually exclusive.
519
520       SO_TYPE
521              Gets the socket type as an integer  (e.g.,  SOCK_STREAM).   This
522              socket option is read-only.
523
524       SO_BUSY_POLL (since Linux 3.11)
525              Sets  the  approximate  time  in  microseconds to busy poll on a
526              blocking receive when there is no data.  Increasing  this  value
527              requires  CAP_NET_ADMIN.   The  default  for this option is con‐
528              trolled by the /proc/sys/net/core/busy_read file.
529
530              The value in the  /proc/sys/net/core/busy_poll  file  determines
531              how  long select(2) and poll(2) will busy poll when they operate
532              on sockets with SO_BUSY_POLL set and no  events  to  report  are
533              found.
534
535              In  both  cases,  busy polling will only be done when the socket
536              last received data from a  network  device  that  supports  this
537              option.
538
539              While  busy  polling  may  improve latency of some applications,
540              care must be taken when using it since this will  increase  both
541              CPU utilization and power usage.
542
543   Signals
544       When  writing onto a connection-oriented socket that has been shut down
545       (by the local or the remote end) SIGPIPE is sent to the writing process
546       and  EPIPE  is  returned.   The  signal is not sent when the write call
547       specified the MSG_NOSIGNAL flag.
548
549       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO
550       is  sent  when  an  I/O event occurs.  It is possible to use poll(2) or
551       select(2) in the signal handler to find  out  which  socket  the  event
552       occurred  on.  An alternative (in Linux 2.2) is to set a real-time sig‐
553       nal using the F_SETSIG fcntl(2); the handler of the  real  time  signal
554       will  be called with the file descriptor in the si_fd field of its sig‐
555       info_t.  See fcntl(2) for more information.
556
557       Under some circumstances (e.g., multiple processes accessing  a  single
558       socket),  the  condition  that caused the SIGIO may have already disap‐
559       peared when the process reacts to the signal.   If  this  happens,  the
560       process should wait again because Linux will resend the signal later.
561
562   /proc interfaces
563       The  core socket networking parameters can be accessed via files in the
564       directory /proc/sys/net/core/.
565
566       rmem_default
567              contains the default setting in bytes of the socket receive buf‐
568              fer.
569
570       rmem_max
571              contains the maximum socket receive buffer size in bytes which a
572              user may set by using the SO_RCVBUF socket option.
573
574       wmem_default
575              contains the default setting in bytes of the socket send buffer.
576
577       wmem_max
578              contains the maximum socket send buffer size in  bytes  which  a
579              user may set by using the SO_SNDBUF socket option.
580
581       message_cost and message_burst
582              configure  the  token  bucket  filter used to load limit warning
583              messages caused by external network events.
584
585       netdev_max_backlog
586              Maximum number of packets in the global input queue.
587
588       optmem_max
589              Maximum length of ancillary data and user control data like  the
590              iovecs per socket.
591
592   Ioctls
593       These operations can be accessed using ioctl(2):
594
595           error = ioctl(ip_socket, ioctl_type, &value_result);
596
597       SIOCGSTAMP
598              Return  a  struct timeval with the receive timestamp of the last
599              packet passed to the user.  This is useful  for  accurate  round
600              trip  time  measurements.  See setitimer(2) for a description of
601              struct timeval.  This ioctl should be used only  if  the  socket
602              options  SO_TIMESTAMP  and  SO_TIMESTAMPNS  are  not  set on the
603              socket.  Otherwise, it returns the timestamp of the last  packet
604              that was received while SO_TIMESTAMP and SO_TIMESTAMPNS were not
605              set, or it fails if no such packet  has  been  received,  (i.e.,
606              ioctl(2) returns -1 with errno set to ENOENT).
607
608       SIOCSPGRP
609              Set  the  process  or  process group that is to receive SIGIO or
610              SIGURG signals when I/O  becomes  possible  or  urgent  data  is
611              available.   The  argument is a pointer to a pid_t.  For further
612              details, see the description of F_SETOWN in fcntl(2).
613
614       FIOASYNC
615              Change the O_ASYNC flag to enable or  disable  asynchronous  I/O
616              mode  of the socket.  Asynchronous I/O mode means that the SIGIO
617              signal or the signal set with F_SETSIG is raised when a new  I/O
618              event occurs.
619
620              Argument is an integer boolean flag.  (This operation is synony‐
621              mous with the use of fcntl(2) to set the O_ASYNC flag.)
622
623       SIOCGPGRP
624              Get the current process or process group that receives SIGIO  or
625              SIGURG signals, or 0 when none is set.
626
627       Valid fcntl(2) operations:
628
629       FIOGETOWN
630              The same as the SIOCGPGRP ioctl(2).
631
632       FIOSETOWN
633              The same as the SIOCSPGRP ioctl(2).
634

VERSIONS

636       SO_BINDTODEVICE  was introduced in Linux 2.0.30.  SO_PASSCRED is new in
637       Linux 2.2.  The /proc interfaces were introduced in Linux 2.2.  SO_RCV‐
638       TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41.  Earlier, time‐
639       outs were fixed to a protocol-specific setting, and could not  be  read
640       or written.
641

NOTES

643       Linux assumes that half of the send/receive buffer is used for internal
644       kernel structures; thus the values in the corresponding /proc files are
645       twice what can be observed on the wire.
646
647       Linux will allow port reuse only with the SO_REUSEADDR option when this
648       option was set both in the previous program that performed a bind(2) to
649       the port and in the program that wants to reuse the port.  This differs
650       from some implementations (e.g., FreeBSD) where only the later  program
651       needs  to  set  the  SO_REUSEADDR option.  Typically this difference is
652       invisible, since, for example, a server program is designed  to  always
653       set this option.
654

SEE ALSO

656       wireshark(1),   bpf(2),   connect(2),   getsockopt(2),   setsockopt(2),
657       socket(2),  pcap(3),  address_families(7),   capabilities(7),   ddp(7),
658       ip(7), packet(7), tcp(7), udp(7), unix(7), tcpdump(8)
659

COLOPHON

661       This  page  is  part of release 5.07 of the Linux man-pages project.  A
662       description of the project, information about reporting bugs,  and  the
663       latest     version     of     this    page,    can    be    found    at
664       https://www.kernel.org/doc/man-pages/.
665
666
667
668Linux                             2020-04-11                         SOCKET(7)
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