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
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.
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.
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)
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)
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.
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
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.
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
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:
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
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.
142              The argument for SO_ATTACH_FILTER  is  a  sock_fprog  structure,
143              defined 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
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.
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
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).
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
190              SO_ATTACH_FILTER and  SO_ATTACH_REUSEPORT_EBPF  takes  the  same
191              argument 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
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.
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
244              option 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, sizeof(cpu));
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.
271       SO_KEEPALIVE
272              Enable  sending  of  keep-alive  messages on connection-oriented
273              sockets.  Expects an integer boolean flag.
275       SO_LINGER
276              Sets or gets the SO_LINGER option.  The  argument  is  a  linger
277              structure.
279                  struct linger {
280                      int l_onoff;    /* linger active */
281                      int l_linger;   /* how many seconds to linger for */
282                  };
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.
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‐
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.
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.
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.
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.
319       SO_PASSCRED
320              Enable  or  disable the receiving of the SCM_CREDENTIALS control
321              message.  For more information see unix(7).
323       SO_PASSSEC
324              Enable or disable the receiving of the SCM_SECURITY control mes‐
325              sage.  For more information see unix(7).
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.
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.
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.
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.
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.
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:
360                  aabbccddeeff
362              The  following  sequence  of recv(2) calls would have the effect
363              noted in the comments:
365                  int ov = 4;                  // Set peek offset to 4
366                  setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
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
373       SO_PEERCRED
374              Return the credentials of the peer  process  connected  to  this
375              socket.  For further details, see unix(7).
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.
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.
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.
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.
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.
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.
419              Specify  the  receiving  or  sending timeouts until reporting an
420              error.  The argument is a struct timeval.  If an input or output
421              function  blocks for this period of time, and data has been sent
422              or received, the return value  of  that  function  will  be  the
423              amount  of data transferred; if no data has been transferred and
424              the timeout has been reached, then -1 is returned with errno set
425              to  EAGAIN  or EWOULDBLOCK, or EINPROGRESS (for connect(2)) just
426              as if the socket was specified to be nonblocking.  If the  time‐
427              out  is set to zero (the default), then the operation will never
428              timeout.  Timeouts only have effect for system calls  that  per‐
429              form   socket   I/O   (e.g.,   read(2),   recvmsg(2),   send(2),
430              sendmsg(2)); timeouts have no  effect  for  select(2),  poll(2),
431              epoll_wait(2), and so on.
433       SO_REUSEADDR
434              Indicates  that  the rules used in validating addresses supplied
435              in a bind(2) call should allow reuse of  local  addresses.   For
436              AF_INET  sockets  this means that a socket may bind, except when
437              there is an active listening socket bound to the address.   When
438              the listening socket is bound to INADDR_ANY with a specific port
439              then it is not possible to bind  to  this  port  for  any  local
440              address.  Argument is an integer boolean flag.
442       SO_REUSEPORT (since Linux 3.9)
443              Permits  multiple  AF_INET or AF_INET6 sockets to be bound to an
444              identical socket address.  This  option  must  be  set  on  each
445              socket  (including the first socket) prior to calling bind(2) on
446              the socket.  To prevent port hijacking,  all  of  the  processes
447              binding  to  the  same address must have the same effective UID.
448              This option can be employed with both TCP and UDP sockets.
450              For TCP sockets, this option allows accept(2) load  distribution
451              in  a  multi-threaded  server to be improved by using a distinct
452              listener socket for each thread.  This  provides  improved  load
453              distribution  as compared to traditional techniques such using a
454              single accept(2)ing thread that distributes connections, or hav‐
455              ing  multiple  threads  that  compete to accept(2) from the same
456              socket.
458              For UDP sockets, the use of this option can provide better  dis‐
459              tribution  of  incoming  datagrams  to  multiple  processes  (or
460              threads) as compared to the traditional technique of having mul‐
461              tiple processes compete to receive datagrams on the same socket.
463       SO_RXQ_OVFL (since Linux 2.6.33)
464              Indicates that an unsigned 32-bit value ancillary message (cmsg)
465              should be attached to received skbs  indicating  the  number  of
466              packets dropped by the socket since its creation.
468       SO_SNDBUF
469              Sets  or gets the maximum socket send buffer in bytes.  The ker‐
470              nel doubles this value (to allow space for bookkeeping overhead)
471              when  it  is  set using setsockopt(2), and this doubled value is
472              returned by getsockopt(2).  The default  value  is  set  by  the
473              /proc/sys/net/core/wmem_default  file  and  the  maximum allowed
474              value is set by the /proc/sys/net/core/wmem_max file.  The mini‐
475              mum (doubled) value for this option is 2048.
477       SO_SNDBUFFORCE (since Linux 2.6.14)
478              Using  this  socket option, a privileged (CAP_NET_ADMIN) process
479              can perform the same task as SO_SNDBUF, but the  wmem_max  limit
480              can be overridden.
482       SO_TIMESTAMP
483              Enable or disable the receiving of the SO_TIMESTAMP control mes‐
484              sage.   The  timestamp  control  message  is  sent  with   level
485              SOL_SOCKET  and the cmsg_data field is a struct timeval indicat‐
486              ing the reception time of the last packet passed to the user  in
487              this call.  See cmsg(3) for details on control messages.
489       SO_TYPE
490              Gets  the  socket  type as an integer (e.g., SOCK_STREAM).  This
491              socket option is read-only.
493       SO_BUSY_POLL (since Linux 3.11)
494              Sets the approximate time in microseconds  to  busy  poll  on  a
495              blocking  receive  when there is no data.  Increasing this value
496              requires CAP_NET_ADMIN.  The default for  this  option  is  con‐
497              trolled by the /proc/sys/net/core/busy_read file.
499              The  value  in  the /proc/sys/net/core/busy_poll file determines
500              how long select(2) and poll(2) will busy poll when they  operate
501              on  sockets  with  SO_BUSY_POLL  set and no events to report are
502              found.
504              In both cases, busy polling will only be done  when  the  socket
505              last  received  data  from  a  network device that supports this
506              option.
508              While busy polling may improve  latency  of  some  applications,
509              care  must  be taken when using it since this will increase both
510              CPU utilization and power usage.
512   Signals
513       When writing onto a connection-oriented socket that has been shut  down
514       (by the local or the remote end) SIGPIPE is sent to the writing process
515       and EPIPE is returned.  The signal is not  sent  when  the  write  call
516       specified the MSG_NOSIGNAL flag.
518       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO
519       is sent when an I/O event occurs.  It is possible  to  use  poll(2)  or
520       select(2)  in  the  signal  handler  to find out which socket the event
521       occurred on.  An alternative (in Linux 2.2) is to set a real-time  sig‐
522       nal  using  the  F_SETSIG fcntl(2); the handler of the real time signal
523       will be called with the file descriptor in the si_fd field of its  sig‐
524       info_t.  See fcntl(2) for more information.
526       Under  some  circumstances (e.g., multiple processes accessing a single
527       socket), the condition that caused the SIGIO may  have  already  disap‐
528       peared  when  the  process  reacts to the signal.  If this happens, the
529       process should wait again because Linux will resend the signal later.
531   /proc interfaces
532       The core socket networking parameters can be accessed via files in  the
533       directory /proc/sys/net/core/.
535       rmem_default
536              contains the default setting in bytes of the socket receive buf‐
537              fer.
539       rmem_max
540              contains the maximum socket receive buffer size in bytes which a
541              user may set by using the SO_RCVBUF socket option.
543       wmem_default
544              contains the default setting in bytes of the socket send buffer.
546       wmem_max
547              contains  the  maximum  socket send buffer size in bytes which a
548              user may set by using the SO_SNDBUF socket option.
550       message_cost and message_burst
551              configure the token bucket filter used  to  load  limit  warning
552              messages caused by external network events.
554       netdev_max_backlog
555              Maximum number of packets in the global input queue.
557       optmem_max
558              Maximum  length of ancillary data and user control data like the
559              iovecs per socket.
561   Ioctls
562       These operations can be accessed using ioctl(2):
564           error = ioctl(ip_socket, ioctl_type, &value_result);
566       SIOCGSTAMP
567              Return a struct timeval with the receive timestamp of  the  last
568              packet  passed  to  the user.  This is useful for accurate round
569              trip time measurements.  See setitimer(2) for a  description  of
570              struct  timeval.   This  ioctl should be used only if the socket
571              option SO_TIMESTAMP is not set on  the  socket.   Otherwise,  it
572              returns the timestamp of the last packet that was received while
573              SO_TIMESTAMP was not set, or it fails if no such packet has been
574              received, (i.e., ioctl(2) returns -1 with errno set to ENOENT).
576       SIOCSPGRP
577              Set  the  process  or  process group that is to receive SIGIO or
578              SIGURG signals when I/O  becomes  possible  or  urgent  data  is
579              available.   The  argument is a pointer to a pid_t.  For further
580              details, see the description of F_SETOWN in fcntl(2).
582       FIOASYNC
583              Change the O_ASYNC flag to enable or  disable  asynchronous  I/O
584              mode  of the socket.  Asynchronous I/O mode means that the SIGIO
585              signal or the signal set with F_SETSIG is raised when a new  I/O
586              event occurs.
588              Argument is an integer boolean flag.  (This operation is synony‐
589              mous with the use of fcntl(2) to set the O_ASYNC flag.)
591       SIOCGPGRP
592              Get the current process or process group that receives SIGIO  or
593              SIGURG signals, or 0 when none is set.
595       Valid fcntl(2) operations:
597       FIOGETOWN
598              The same as the SIOCGPGRP ioctl(2).
600       FIOSETOWN
601              The same as the SIOCSPGRP ioctl(2).


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


611       Linux assumes that half of the send/receive buffer is used for internal
612       kernel structures; thus the values in the corresponding /proc files are
613       twice what can be observed on the wire.
615       Linux will allow port reuse only with the SO_REUSEADDR option when this
616       option was set both in the previous program that performed a bind(2) to
617       the port and in the program that wants to reuse the port.  This differs
618       from some implementations (e.g., FreeBSD) where only the later  program
619       needs  to  set  the  SO_REUSEADDR option.  Typically this difference is
620       invisible, since, for example, a server program is designed  to  always
621       set this option.


624       wireshark(1),   bpf(2),   connect(2),   getsockopt(2),   setsockopt(2),
625       socket(2),  pcap(3),  address_families(7),   capabilities(7),   ddp(7),
626       ip(7), packet(7), tcp(7), udp(7), unix(7), tcpdump(8)


629       This  page  is  part of release 5.04 of the Linux man-pages project.  A
630       description of the project, information about reporting bugs,  and  the
631       latest     version     of     this    page,    can    be    found    at
632       https://www.kernel.org/doc/man-pages/.
636Linux                             2019-08-02                         SOCKET(7)