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


6       socket - Linux socket interface


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


14       This  manual  page describes the Linux networking socket layer user in‐
15       terface.  The BSD compatible sockets are the uniform interface  between
16       the  user  process  and the network protocol stacks in the kernel.  The
17       protocol modules are grouped into protocol families  such  as  AF_INET,
18       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.
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 re‐
24       spective manual pages.
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  ac‐
29       cepted,  and  accept(2) is used to get a new socket with a new incoming
30       connection.  socketpair(2) returns two connected anonymous sockets (im‐
31       plemented only for a few local families like AF_UNIX)
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.
39       getsockname(2)  returns the local socket address and getpeername(2) re‐
40       turns 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.
44       close(2) is used to close a socket.   shutdown(2)  closes  parts  of  a
45       full-duplex socket connection.
47       Seeking,  or  calling  pread(2) or pwrite(2) with a nonzero position is
48       not supported on sockets.
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).
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.
91   Socket address structures
92       Each  socket domain has its own format for socket addresses, with a do‐
93       main-specific address structure.  Each of these structures begins  with
94       an  integer  "family"  field  (typed as sa_family_t) that indicates the
95       type of the address structure.  This allows the  various  system  calls
96       (e.g., connect(2), bind(2), accept(2), getsockname(2), getpeername(2)),
97       which are generic to all socket domains, to determine the domain  of  a
98       particular socket address.
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.
106       In  addition,  the  sockets  API  provides  the  data type struct sock‐
107       addr_storage.  This type is suitable to accommodate all  supported  do‐
108       main-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:
114               sa_family_t ss_family;
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).
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.
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.
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 in‐
135              coming  packets.  A packet will be dropped if the filter program
136              returns zero.  If the filter program  returns  a  nonzero  value
137              which  is less than the packet's data length, the packet will be
138              truncated to the length returned.  If the value returned by  the
139              filter is greater than or equal to the packet's data length, the
140              packet is allowed to proceed unmodified.
142              The argument for SO_ATTACH_FILTER is a sock_fprog structure, de‐
143              fined in <linux/filter.h>:
145                  struct sock_fprog {
146                      unsigned short      len;
147                      struct sock_filter *filter;
148                  };
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
154              These options may be set multiple times for a given socket, each
155              time replacing the previous filter program.  The classic and ex‐
156              tended versions may be called on the same socket, but the previ‐
157              ous filter will always be replaced such that a socket never  has
158              more than one filter defined.
160              Both classic and extended BPF are explained in the kernel source
161              file Documentation/networking/filter.txt
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 ex‐
166              tended BPF (SO_ATTACH_REUSEPORT_EBPF) program which defines  how
167              packets are assigned to the sockets in the reuseport group (that
168              is, all sockets which have SO_REUSEPORT set and  are  using  the
169              same local address to receive packets).
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.
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.
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.
189              SO_ATTACH_REUSEPORT_CBPF  takes the same argument type as SO_AT‐
190              TACH_FILTER and SO_ATTACH_REUSEPORT_EBPF takes the same argument
191              type as SO_ATTACH_BPF.
193              UDP  support  for this feature is available since Linux 4.5; TCP
194              support is available since Linux 4.6.
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).
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.
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.
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 op‐
227              tion.  Linux 2.0 also enabled BSD bug-to-bug  compatibility  op‐
228              tions  (random  header changing, skipping of the broadcast flag)
229              for raw sockets with this option, but that was removed in  Linux
230              2.2.
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.
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.
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 op‐
244              tion is read-only.
246       SO_ERROR
247              Get and clear the pending socket error.  This socket  option  is
248              read-only.  Expects an integer.
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.
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.
260                  int cpu = 1;
261                  setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu,
262                             sizeof(cpu));
264              Because  all of the packets for a single stream (i.e., all pack‐
265              ets for the same 4-tuple) arrive on the single RX queue that  is
266              associated with a particular CPU, the typical use case is to em‐
267              ploy one listening process per RX queue, with the incoming  flow
268              being handled by a listener on the same CPU that is handling the
269              RX queue.  This provides optimal NUMA  behavior  and  keeps  CPU
270              caches hot.
272       SO_INCOMING_NAPI_ID (gettable since Linux 4.12)
273              Returns  a system-level unique ID called NAPI ID that is associ‐
274              ated with a RX queue on which the last  packet  associated  with
275              that socket is received.
277              This  can  be used by an application to split the incoming flows
278              among worker threads based on the RX queue on which the  packets
279              associated  with  the flows are received.  It allows each worker
280              thread to be associated with a NIC HW receive queue and  service
281              all  the  connection  requests  received on that RX queue.  This
282              mapping between a app thread and a HW NIC queue streamlines  the
283              flow of data from the NIC to the application.
285       SO_KEEPALIVE
286              Enable  sending  of  keep-alive  messages on connection-oriented
287              sockets.  Expects an integer boolean flag.
289       SO_LINGER
290              Sets or gets the SO_LINGER option.  The  argument  is  a  linger
291              structure.
293                  struct linger {
294                      int l_onoff;    /* linger active */
295                      int l_linger;   /* how many seconds to linger for */
296                  };
298              When  enabled,  a  close(2) or shutdown(2) will not return until
299              all queued messages for the socket have been  successfully  sent
300              or the linger timeout has been reached.  Otherwise, the call re‐
301              turns immediately and the closing is  done  in  the  background.
302              When  the socket is closed as part of exit(2), it always lingers
303              in the background.
305       SO_LOCK_FILTER
306              When set, this option will prevent changing the filters  associ‐
307              ated  with  the socket.  These filters include any set using the
308              socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE‐
311              The typical use case is for a privileged process to set up a raw
312              socket (an operation that requires the CAP_NET_RAW  capability),
313              apply  a  restrictive filter, set the SO_LOCK_FILTER option, and
314              then either drop its privileges or pass the socket file descrip‐
315              tor to an unprivileged process via a UNIX domain socket.
317              Once  the  SO_LOCK_FILTER  option  has been enabled, attempts to
318              change or remove the filter attached to a socket, or to  disable
319              the SO_LOCK_FILTER option will fail with the error EPERM.
321       SO_MARK (since Linux 2.6.25)
322              Set  the  mark for each packet sent through this socket (similar
323              to the netfilter MARK target but  socket-based).   Changing  the
324              mark can be used for mark-based routing without netfilter or for
325              packet filtering.  Setting this option requires the  CAP_NET_AD‐
326              MIN capability.
328       SO_OOBINLINE
329              If  this  option is enabled, out-of-band data is directly placed
330              into the receive data stream.  Otherwise,  out-of-band  data  is
331              passed only when the MSG_OOB flag is set during receiving.
333       SO_PASSCRED
334              Enable  or  disable the receiving of the SCM_CREDENTIALS control
335              message.  For more information see unix(7).
337       SO_PASSSEC
338              Enable or disable the receiving of the SCM_SECURITY control mes‐
339              sage.  For more information see unix(7).
341       SO_PEEK_OFF (since Linux 3.4)
342              This option, which is currently supported only for unix(7) sock‐
343              ets, sets the value of the "peek offset" for the recv(2)  system
344              call when used with MSG_PEEK flag.
346              When this option is set to a negative value (it is set to -1 for
347              all new sockets), traditional behavior is provided: recv(2) with
348              the MSG_PEEK flag will peek data from the front of the queue.
350              When the option is set to a value greater than or equal to zero,
351              then the next peek at data queued in the socket  will  occur  at
352              the  byte  offset  specified  by  the option value.  At the same
353              time, the "peek offset" will be incremented  by  the  number  of
354              bytes that were peeked from the queue, so that a subsequent peek
355              will return the next data in the queue.
357              If data is removed from the front of the queue  via  a  call  to
358              recv(2)  (or  similar) without the MSG_PEEK flag, the "peek off‐
359              set" will be decreased by the number of bytes removed.  In other
360              words,  receiving  data without the MSG_PEEK flag will cause the
361              "peek offset" to be adjusted to maintain  the  correct  relative
362              position  in the queued data, so that a subsequent peek will re‐
363              trieve the data that would have been retrieved had the data  not
364              been removed.
366              For  datagram sockets, if the "peek offset" points to the middle
367              of a packet, the data returned will be marked with the MSG_TRUNC
368              flag.
370              The   following   example   serves  to  illustrate  the  use  of
371              SO_PEEK_OFF.  Suppose a stream socket has the  following  queued
372              input data:
374                  aabbccddeeff
376              The  following  sequence  of recv(2) calls would have the effect
377              noted in the comments:
379                  int ov = 4;                  // Set peek offset to 4
380                  setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
382                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
383                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
384                  recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
385                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8
387       SO_PEERCRED
388              Return the credentials of the peer  process  connected  to  this
389              socket.  For further details, see unix(7).
391       SO_PEERSEC (since Linux 2.6.2)
392              Return the security context of the peer socket connected to this
393              socket.  For further details, see unix(7) and ip(7).
395       SO_PRIORITY
396              Set the protocol-defined priority for all packets to be sent  on
397              this  socket.   Linux  uses  this  value to order the networking
398              queues: packets with a higher priority may  be  processed  first
399              depending on the selected device queueing discipline.  Setting a
400              priority outside the range 0 to 6 requires the CAP_NET_ADMIN ca‐
401              pability.
403       SO_PROTOCOL (since Linux 2.6.32)
404              Retrieves  the  socket protocol as an integer, returning a value
405              such as IPPROTO_SCTP.  See socket(2) for details.   This  socket
406              option is read-only.
408       SO_RCVBUF
409              Sets  or  gets  the maximum socket receive buffer in bytes.  The
410              kernel doubles this value (to allow space for bookkeeping  over‐
411              head) when it is set using setsockopt(2), and this doubled value
412              is returned by getsockopt(2).  The default value is set  by  the
413              /proc/sys/net/core/rmem_default  file,  and  the maximum allowed
414              value is set by the /proc/sys/net/core/rmem_max file.  The mini‐
415              mum (doubled) value for this option is 256.
417       SO_RCVBUFFORCE (since Linux 2.6.14)
418              Using  this  socket option, a privileged (CAP_NET_ADMIN) process
419              can perform the same task as SO_RCVBUF, but the  rmem_max  limit
420              can be overridden.
423              Specify  the  minimum  number  of  bytes in the buffer until the
424              socket layer will pass the data to the protocol (SO_SNDLOWAT) or
425              the  user on receiving (SO_RCVLOWAT).  These two values are ini‐
426              tialized to 1.  SO_SNDLOWAT is not changeable on Linux (setsock‐
427              opt(2)  fails  with  the  error  ENOPROTOOPT).   SO_RCVLOWAT  is
428              changeable only since Linux 2.4.
430              Before Linux 2.6.28 select(2), poll(2), and epoll(7) did not re‐
431              spect  the  SO_RCVLOWAT setting on Linux, and indicated a socket
432              as readable when even a single byte of data  was  available.   A
433              subsequent   read   from  the  socket  would  then  block  until
434              SO_RCVLOWAT bytes are available.  Since Linux 2.6.28, select(2),
435              poll(2),  and  epoll(7) indicate a socket as readable only if at
436              least SO_RCVLOWAT bytes are available.
439              Specify the receiving or sending timeouts until reporting an er‐
440              ror.   The  argument is a struct timeval.  If an input or output
441              function blocks for this period of time, and data has been  sent
442              or  received,  the  return  value  of  that function will be the
443              amount of data transferred; if no data has been transferred  and
444              the timeout has been reached, then -1 is returned with errno set
445              to EAGAIN or EWOULDBLOCK, or EINPROGRESS (for  connect(2))  just
446              as  if the socket was specified to be nonblocking.  If the time‐
447              out is set to zero (the default), then the operation will  never
448              timeout.   Timeouts  only have effect for system calls that per‐
449              form   socket   I/O   (e.g.,   read(2),   recvmsg(2),   send(2),
450              sendmsg(2));  timeouts  have  no  effect for select(2), poll(2),
451              epoll_wait(2), and so on.
453       SO_REUSEADDR
454              Indicates that the rules used in validating  addresses  supplied
455              in  a  bind(2)  call should allow reuse of local addresses.  For
456              AF_INET sockets this means that a socket may bind,  except  when
457              there  is an active listening socket bound to the address.  When
458              the listening socket is bound to INADDR_ANY with a specific port
459              then  it  is not possible to bind to this port for any local ad‐
460              dress.  Argument is an integer boolean flag.
462       SO_REUSEPORT (since Linux 3.9)
463              Permits multiple AF_INET or AF_INET6 sockets to be bound  to  an
464              identical  socket  address.   This  option  must  be set on each
465              socket (including the first socket) prior to calling bind(2)  on
466              the  socket.   To  prevent  port hijacking, all of the processes
467              binding to the same address must have the  same  effective  UID.
468              This option can be employed with both TCP and UDP sockets.
470              For  TCP sockets, this option allows accept(2) load distribution
471              in a multi-threaded server to be improved by  using  a  distinct
472              listener  socket  for  each thread.  This provides improved load
473              distribution as compared to traditional techniques such using  a
474              single accept(2)ing thread that distributes connections, or hav‐
475              ing multiple threads that compete to  accept(2)  from  the  same
476              socket.
478              For  UDP sockets, the use of this option can provide better dis‐
479              tribution  of  incoming  datagrams  to  multiple  processes  (or
480              threads) as compared to the traditional technique of having mul‐
481              tiple processes compete to receive datagrams on the same socket.
483       SO_RXQ_OVFL (since Linux 2.6.33)
484              Indicates that an unsigned 32-bit value ancillary message (cmsg)
485              should  be  attached  to  received skbs indicating the number of
486              packets dropped by the socket since its creation.
488       SO_SELECT_ERR_QUEUE (since Linux 3.10)
489              When this option is set on a socket, an  error  condition  on  a
490              socket causes notification not only via the exceptfds set of se‐
491              lect(2).  Similarly, poll(2) also returns a POLLPRI whenever  an
492              POLLERR event is returned.
494              Background:  this  option  was  added when waking up on an error
495              condition occurred only via the readfds and writefds sets of se‐
496              lect(2).   The  option  was  added to allow monitoring for error
497              conditions via the  exceptfds  argument  without  simultaneously
498              having  to  receive notifications (via readfds) for regular data
499              that can be read from the socket.  After changes in Linux  4.16,
500              the  use of this flag to achieve the desired notifications is no
501              longer necessary.  This  option  is  nevertheless  retained  for
502              backwards compatibility.
504       SO_SNDBUF
505              Sets  or gets the maximum socket send buffer in bytes.  The ker‐
506              nel doubles this value (to allow space for bookkeeping overhead)
507              when  it  is  set using setsockopt(2), and this doubled value is
508              returned by getsockopt(2).  The default  value  is  set  by  the
509              /proc/sys/net/core/wmem_default  file  and  the  maximum allowed
510              value is set by the /proc/sys/net/core/wmem_max file.  The mini‐
511              mum (doubled) value for this option is 2048.
513       SO_SNDBUFFORCE (since Linux 2.6.14)
514              Using  this  socket option, a privileged (CAP_NET_ADMIN) process
515              can perform the same task as SO_SNDBUF, but the  wmem_max  limit
516              can be overridden.
518       SO_TIMESTAMP
519              Enable or disable the receiving of the SO_TIMESTAMP control mes‐
520              sage.   The  timestamp  control  message  is  sent  with   level
521              SOL_SOCKET  and  a  cmsg_type  of  SCM_TIMESTAMP.  The cmsg_data
522              field is a struct timeval indicating the reception time  of  the
523              last  packet  passed  to the user in this call.  See cmsg(3) for
524              details on control messages.
526       SO_TIMESTAMPNS (since Linux 2.6.22)
527              Enable or disable the receiving of  the  SO_TIMESTAMPNS  control
528              message.   The  timestamp  control  message  is  sent with level
529              SOL_SOCKET and a cmsg_type of  SCM_TIMESTAMPNS.   The  cmsg_data
530              field  is a struct timespec indicating the reception time of the
531              last packet passed to the user in this call.  The clock used for
532              the  timestamp  is  CLOCK_REALTIME.   See cmsg(3) for details on
533              control messages.
535              A socket cannot mix SO_TIMESTAMP  and  SO_TIMESTAMPNS:  the  two
536              modes are mutually exclusive.
538       SO_TYPE
539              Gets  the  socket  type as an integer (e.g., SOCK_STREAM).  This
540              socket option is read-only.
542       SO_BUSY_POLL (since Linux 3.11)
543              Sets the approximate time in microseconds  to  busy  poll  on  a
544              blocking  receive  when there is no data.  Increasing this value
545              requires CAP_NET_ADMIN.  The default for  this  option  is  con‐
546              trolled by the /proc/sys/net/core/busy_read file.
548              The  value  in  the /proc/sys/net/core/busy_poll file determines
549              how long select(2) and poll(2) will busy poll when they  operate
550              on  sockets  with  SO_BUSY_POLL  set and no events to report are
551              found.
553              In both cases, busy polling will only be done  when  the  socket
554              last  received data from a network device that supports this op‐
555              tion.
557              While busy polling may improve  latency  of  some  applications,
558              care  must  be taken when using it since this will increase both
559              CPU utilization and power usage.
561   Signals
562       When writing onto a connection-oriented socket that has been shut  down
563       (by the local or the remote end) SIGPIPE is sent to the writing process
564       and EPIPE is returned.  The signal is not  sent  when  the  write  call
565       specified the MSG_NOSIGNAL flag.
567       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO
568       is sent when an I/O event occurs.  It is possible to use poll(2) or se‐
569       lect(2)  in  the  signal handler to find out which socket the event oc‐
570       curred on.  An alternative (in Linux 2.2) is to set a real-time  signal
571       using  the  F_SETSIG fcntl(2); the handler of the real time signal will
572       be called with the file descriptor in the si_fd field of its siginfo_t.
573       See fcntl(2) for more information.
575       Under  some  circumstances (e.g., multiple processes accessing a single
576       socket), the condition that caused the SIGIO may  have  already  disap‐
577       peared  when  the  process  reacts to the signal.  If this happens, the
578       process should wait again because Linux will resend the signal later.
580   /proc interfaces
581       The core socket networking parameters can be accessed via files in  the
582       directory /proc/sys/net/core/.
584       rmem_default
585              contains the default setting in bytes of the socket receive buf‐
586              fer.
588       rmem_max
589              contains the maximum socket receive buffer size in bytes which a
590              user may set by using the SO_RCVBUF socket option.
592       wmem_default
593              contains the default setting in bytes of the socket send buffer.
595       wmem_max
596              contains  the  maximum  socket send buffer size in bytes which a
597              user may set by using the SO_SNDBUF socket option.
599       message_cost and message_burst
600              configure the token bucket filter used  to  load  limit  warning
601              messages caused by external network events.
603       netdev_max_backlog
604              Maximum number of packets in the global input queue.
606       optmem_max
607              Maximum  length of ancillary data and user control data like the
608              iovecs per socket.
610   Ioctls
611       These operations can be accessed using ioctl(2):
613           error = ioctl(ip_socket, ioctl_type, &value_result);
615       SIOCGSTAMP
616              Return a struct timeval with the receive timestamp of  the  last
617              packet  passed  to  the user.  This is useful for accurate round
618              trip time measurements.  See setitimer(2) for a  description  of
619              struct  timeval.   This  ioctl should be used only if the socket
620              options SO_TIMESTAMP and  SO_TIMESTAMPNS  are  not  set  on  the
621              socket.   Otherwise, it returns the timestamp of the last packet
622              that was received while SO_TIMESTAMP and SO_TIMESTAMPNS were not
623              set,  or  it  fails  if no such packet has been received, (i.e.,
624              ioctl(2) returns -1 with errno set to ENOENT).
626       SIOCSPGRP
627              Set the process or process group that is  to  receive  SIGIO  or
628              SIGURG  signals  when  I/O  becomes  possible  or urgent data is
629              available.  The argument is a pointer to a pid_t.   For  further
630              details, see the description of F_SETOWN in fcntl(2).
632       FIOASYNC
633              Change  the  O_ASYNC  flag to enable or disable asynchronous I/O
634              mode of the socket.  Asynchronous I/O mode means that the  SIGIO
635              signal  or the signal set with F_SETSIG is raised when a new I/O
636              event occurs.
638              Argument is an integer boolean flag.  (This operation is synony‐
639              mous with the use of fcntl(2) to set the O_ASYNC flag.)
641       SIOCGPGRP
642              Get  the current process or process group that receives SIGIO or
643              SIGURG signals, or 0 when none is set.
645       Valid fcntl(2) operations:
647       FIOGETOWN
648              The same as the SIOCGPGRP ioctl(2).
650       FIOSETOWN
651              The same as the SIOCSPGRP ioctl(2).


654       SO_BINDTODEVICE was introduced in Linux 2.0.30.  SO_PASSCRED is new  in
655       Linux 2.2.  The /proc interfaces were introduced in Linux 2.2.  SO_RCV‐
656       TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41.  Earlier, time‐
657       outs  were  fixed to a protocol-specific setting, and could not be read
658       or written.


661       Linux assumes that half of the send/receive buffer is used for internal
662       kernel structures; thus the values in the corresponding /proc files are
663       twice what can be observed on the wire.
665       Linux will allow port reuse only with the SO_REUSEADDR option when this
666       option was set both in the previous program that performed a bind(2) to
667       the port and in the program that wants to reuse the port.  This differs
668       from  some implementations (e.g., FreeBSD) where only the later program
669       needs to set the SO_REUSEADDR option.  Typically this difference is in‐
670       visible, since, for example, a server program is designed to always set
671       this option.


674       wireshark(1),   bpf(2),   connect(2),   getsockopt(2),   setsockopt(2),
675       socket(2),   pcap(3),   address_families(7),  capabilities(7),  ddp(7),
676       ip(7), ipv6(7), packet(7), tcp(7), udp(7), unix(7), tcpdump(8)


679       This page is part of release 5.10 of the Linux  man-pages  project.   A
680       description  of  the project, information about reporting bugs, and the
681       latest    version    of    this    page,    can     be     found     at
682       https://www.kernel.org/doc/man-pages/.
686Linux                             2020-08-13                         SOCKET(7)