1RECV(2)                    Linux Programmer's Manual                   RECV(2)
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NAME

6       recv, recvfrom, recvmsg - receive a message from a socket
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SYNOPSIS

9       #include <sys/types.h>
10       #include <sys/socket.h>
11
12       ssize_t recv(int sockfd, void *buf, size_t len, int flags);
13
14       ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
15                        struct sockaddr *src_addr, socklen_t *addrlen);
16
17       ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
18

DESCRIPTION

20       The  recvfrom() and recvmsg() calls are used to receive messages from a
21       socket, and may be used to receive data on a socket whether or  not  it
22       is connection-oriented.
23
24       If  src_addr  is  not  NULL,  and  the underlying protocol provides the
25       source address, this source address is filled  in.   When  src_addr  is
26       NULL,  nothing  is  filled  in;  in this case, addrlen is not used, and
27       should also be NULL.  The argument addrlen is a value-result  argument,
28       which  the  caller should initialize before the call to the size of the
29       buffer associated with src_addr, and modified on return to indicate the
30       actual  size  of the source address.  The returned address is truncated
31       if the buffer provided is too small; in this case, addrlen will  return
32       a value greater than was supplied to the call.
33
34       The  recv()  call is normally used only on a connected socket (see con‐
35       nect(2)) and is identical to recvfrom() with a NULL src_addr argument.
36
37       All three routines return the length of the message on successful  com‐
38       pletion.   If  a  message  is  too  long to fit in the supplied buffer,
39       excess bytes may be discarded depending on the type of socket the  mes‐
40       sage is received from.
41
42       If  no messages are available at the socket, the receive calls wait for
43       a message to arrive, unless the socket is non-blocking (see  fcntl(2)),
44       in  which case the value -1 is returned and the external variable errno
45       is set to EAGAIN or EWOULDBLOCK.  The receive calls normally return any
46       data  available,  up  to  the requested amount, rather than waiting for
47       receipt of the full amount requested.
48
49       The select(2) or poll(2) call may be used to determine when  more  data
50       arrives.
51
52       The  flags argument to a recv() call is formed by OR'ing one or more of
53       the following values:
54
55       MSG_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)
56              Set the close-on-exec flag for the file descriptor received  via
57              a  Unix  domain  file  descriptor using the SCM_RIGHTS operation
58              (described in unix(7)).  This flag is useful for the  same  rea‐
59              sons as the O_CLOEXEC flag of open(2).
60
61       MSG_DONTWAIT (since Linux 2.2)
62              Enables  non-blocking  operation;  if the operation would block,
63              the call fails with the error EAGAIN or  EWOULDBLOCK  (this  can
64              also  be  enabled  using  the  O_NONBLOCK  flag with the F_SETFL
65              fcntl(2)).
66
67       MSG_ERRQUEUE (since Linux 2.2)
68              This flag specifies that queued errors should be  received  from
69              the  socket  error  queue.   The error is passed in an ancillary
70              message  with  a  type  dependent  on  the  protocol  (for  IPv4
71              IP_RECVERR).   The  user  should  supply  a buffer of sufficient
72              size.  See cmsg(3) and ip(7) for more information.  The  payload
73              of the original packet that caused the error is passed as normal
74              data via msg_iovec.  The original  destination  address  of  the
75              datagram that caused the error is supplied via msg_name.
76
77              For local errors, no address is passed (this can be checked with
78              the cmsg_len member of the cmsghdr).  For  error  receives,  the
79              MSG_ERRQUEUE  is  set  in  the  msghdr.  After an error has been
80              passed, the pending socket error is  regenerated  based  on  the
81              next  queued  error and will be passed on the next socket opera‐
82              tion.
83
84              The error is supplied in a sock_extended_err structure:
85
86                  #define SO_EE_ORIGIN_NONE    0
87                  #define SO_EE_ORIGIN_LOCAL   1
88                  #define SO_EE_ORIGIN_ICMP    2
89                  #define SO_EE_ORIGIN_ICMP6   3
90
91                  struct sock_extended_err
92                  {
93                      uint32_t ee_errno;   /* error number */
94                      uint8_t  ee_origin;  /* where the error originated */
95                      uint8_t  ee_type;    /* type */
96                      uint8_t  ee_code;    /* code */
97                      uint8_t  ee_pad;     /* padding */
98                      uint32_t ee_info;    /* additional information */
99                      uint32_t ee_data;    /* other data */
100                      /* More data may follow */
101                  };
102
103                  struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
104
105              ee_errno contains the errno number of the queued error.  ee_ori‐
106              gin is the origin code of where the error originated.  The other
107              fields  are  protocol-specific.   The   macro   SOCK_EE_OFFENDER
108              returns a pointer to the address of the network object where the
109              error originated from given a pointer to the ancillary  message.
110              If  this address is not known, the sa_family member of the sock‐
111              addr contains AF_UNSPEC and the other fields of the sockaddr are
112              undefined.   The  payload of the packet that caused the error is
113              passed as normal data.
114
115              For local errors, no address is passed (this can be checked with
116              the  cmsg_len  member  of the cmsghdr).  For error receives, the
117              MSG_ERRQUEUE is set in the msghdr.   After  an  error  has  been
118              passed,  the  pending  socket  error is regenerated based on the
119              next queued error and will be passed on the next  socket  opera‐
120              tion.
121
122       MSG_OOB
123              This flag requests receipt of out-of-band data that would not be
124              received in the normal data stream.  Some protocols place  expe‐
125              dited  data  at the head of the normal data queue, and thus this
126              flag cannot be used with such protocols.
127
128       MSG_PEEK
129              This flag causes the receive operation to return data  from  the
130              beginning  of  the receive queue without removing that data from
131              the queue.  Thus, a subsequent receive call will return the same
132              data.
133
134       MSG_TRUNC (since Linux 2.2)
135              For    raw   (AF_PACKET),   Internet   datagram   (since   Linux
136              2.4.27/2.6.8), and netlink (since Linux 2.6.22) sockets:  return
137              the  real  length  of  the  packet or datagram, even when it was
138              longer than the passed buffer.  Not implemented for Unix  domain
139              sockets.
140
141              For use with Internet stream sockets, see tcp(7).
142
143       MSG_WAITALL (since Linux 2.2)
144              This  flag  requests  that  the  operation  block until the full
145              request is satisfied.  However, the call may still  return  less
146              data  than  requested if a signal is caught, an error or discon‐
147              nect occurs, or the next data to be received is of  a  different
148              type than that returned.
149
150       The  recvmsg()  call  uses a msghdr structure to minimize the number of
151       directly supplied arguments.  This structure is defined as  follows  in
152       <sys/socket.h>:
153
154           struct iovec {                    /* Scatter/gather array items */
155               void  *iov_base;              /* Starting address */
156               size_t iov_len;               /* Number of bytes to transfer */
157           };
158
159           struct msghdr {
160               void         *msg_name;       /* optional address */
161               socklen_t     msg_namelen;    /* size of address */
162               struct iovec *msg_iov;        /* scatter/gather array */
163               size_t        msg_iovlen;     /* # elements in msg_iov */
164               void         *msg_control;    /* ancillary data, see below */
165               socklen_t     msg_controllen; /* ancillary data buffer len */
166               int           msg_flags;      /* flags on received message */
167           };
168
169       Here  msg_name and msg_namelen specify the source address if the socket
170       is unconnected; msg_name may be given as a null pointer if no names are
171       desired  or required.  The fields msg_iov and msg_iovlen describe scat‐
172       ter-gather locations, as discussed in readv(2).  The field msg_control,
173       which  has length msg_controllen, points to a buffer for other protocol
174       control-related  messages  or  miscellaneous  ancillary   data.    When
175       recvmsg()  is  called,  msg_controllen should contain the length of the
176       available buffer in msg_control; upon return from a successful call  it
177       will contain the length of the control message sequence.
178
179       The messages are of the form:
180
181           struct cmsghdr {
182               socklen_t     cmsg_len;     /* data byte count, including hdr */
183               int           cmsg_level;   /* originating protocol */
184               int           cmsg_type;    /* protocol-specific type */
185           /* followed by
186               unsigned char cmsg_data[]; */
187           };
188
189       Ancillary  data  should  only  be  accessed  by  the  macros defined in
190       cmsg(3).
191
192       As an example,  Linux  uses  this  auxiliary  data  mechanism  to  pass
193       extended errors, IP options or file descriptors over Unix sockets.
194
195       The  msg_flags  field  in the msghdr is set on return of recvmsg().  It
196       can contain several flags:
197
198       MSG_EOR
199              indicates end-of-record; the data returned  completed  a  record
200              (generally used with sockets of type SOCK_SEQPACKET).
201
202       MSG_TRUNC
203              indicates  that the trailing portion of a datagram was discarded
204              because the datagram was larger than the buffer supplied.
205
206       MSG_CTRUNC
207              indicates that some control data were discarded due to  lack  of
208              space in the buffer for ancillary data.
209
210       MSG_OOB
211              is  returned to indicate that expedited or out-of-band data were
212              received.
213
214       MSG_ERRQUEUE
215              indicates that no data was received but an extended  error  from
216              the socket error queue.
217

RETURN VALUE

219       These  calls  return  the  number  of bytes received, or -1 if an error
220       occurred.  The return value will be 0 when the peer  has  performed  an
221       orderly shutdown.
222

ERRORS

224       These  are  some  standard errors generated by the socket layer.  Addi‐
225       tional errors may be generated and returned from the underlying  proto‐
226       col modules; see their manual pages.
227
228       EAGAIN or EWOULDBLOCK
229              The  socket  is  marked  non-blocking  and the receive operation
230              would block, or a receive timeout had been set and  the  timeout
231              expired  before  data  was received.  POSIX.1-2001 allows either
232              error to be returned for this case, and does not  require  these
233              constants  to  have  the  same  value, so a portable application
234              should check for both possibilities.
235
236       EBADF  The argument sockfd is an invalid descriptor.
237
238       ECONNREFUSED
239              A remote host refused to allow the network connection (typically
240              because it is not running the requested service).
241
242       EFAULT The  receive  buffer  pointer(s)  point  outside  the  process's
243              address space.
244
245       EINTR  The receive was interrupted by delivery of a signal  before  any
246              data were available; see signal(7).
247
248       EINVAL Invalid argument passed.
249
250       ENOMEM Could not allocate memory for recvmsg().
251
252       ENOTCONN
253              The socket is associated with a connection-oriented protocol and
254              has not been connected (see connect(2) and accept(2)).
255
256       ENOTSOCK
257              The argument sockfd does not refer to a socket.
258

CONFORMING TO

260       4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.
261
262       POSIX.1-2001 only describes  the  MSG_OOB,  MSG_PEEK,  and  MSG_WAITALL
263       flags.
264

NOTES

266       The  prototypes  given above follow glibc2.  The Single Unix Specifica‐
267       tion agrees, except that it has return values of  type  ssize_t  (while
268       4.x  BSD  and libc4 and libc5 all have int).  The flags argument is int
269       in 4.x BSD, but unsigned int in libc4 and libc5.  The len  argument  is
270       int in 4.x BSD, but size_t in libc4 and libc5.  The addrlen argument is
271       int * in 4.x BSD,  libc4  and  libc5.   The  present   socklen_t *  was
272       invented by POSIX.  See also accept(2).
273
274       According  to  POSIX.1-2001,  the  msg_controllen  field  of the msghdr
275       structure should be typed as socklen_t, but glibc currently (2.4) types
276       it as size_t.
277

EXAMPLE

279       An example of the use of recvfrom() is shown in getaddrinfo(3).
280

SEE ALSO

282       fcntl(2),  getsockopt(2),  read(2),  select(2), shutdown(2), socket(2),
283       cmsg(3), sockatmark(3), socket(7)
284

COLOPHON

286       This page is part of release 3.22 of the Linux  man-pages  project.   A
287       description  of  the project, and information about reporting bugs, can
288       be found at http://www.kernel.org/doc/man-pages/.
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292Linux                             2009-03-10                           RECV(2)
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