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32 .\" $Id: recv.2,v 1.3 1999/05/13 11:33:38 freitag Exp $
34 .\" Modified Sat Jul 24 00:22:20 1993 by Rik Faith <faith@cs.unc.edu>
35 .\" Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esr@thyrsus.com>
36 .\" Modified 1998,1999 by Andi Kleen
37 .\" 2001-06-19 corrected SO_EE_OFFENDER, bug report by James Hawtin
39 .TH RECV 2 2011-09-16 "Linux" "Linux Programmer's Manual"
41 recv, recvfrom, recvmsg \- receive a message from a socket
43 .\" .B #include <sys/uio.h>
46 .B #include <sys/types.h>
48 .B #include <sys/socket.h>
50 .BI "ssize_t recv(int " sockfd ", void *" buf ", size_t " len ", int " flags );
52 .BI "ssize_t recvfrom(int " sockfd ", void *" buf ", size_t " len ", int " flags ,
53 .BI " struct sockaddr *" src_addr ", socklen_t *" addrlen );
55 .BI "ssize_t recvmsg(int " sockfd ", struct msghdr *" msg ", int " flags );
62 calls are used to receive messages from a socket, and may be used
63 to receive data on a socket whether or not it is connection-oriented.
67 is not NULL, and the underlying protocol provides the source address,
68 this source address is filled in.
71 is NULL, nothing is filled in; in this case,
73 is not used, and should also be NULL.
74 .\" (Note: for datagram sockets in both the UNIX and Internet domains,
78 .\" is also filled in for stream sockets in the UNIX domain, but is not
79 .\" filled in for stream sockets in the Internet domain.)
80 .\" [The above notes on AF_UNIX and AF_INET sockets apply as at
81 .\" Kernel 2.4.18. (MTK, 22 Jul 02)]
84 is a value-result argument,
85 which the caller should initialize before the call to
86 the size of the buffer associated with
88 and modified on return to indicate the actual size of the source address.
89 The returned address is truncated if the buffer provided is too small;
92 will return a value greater than was supplied to the call.
96 call is normally used only on a
106 All three routines return the length of the message on successful
108 If a message is too long to fit in the supplied buffer, excess
109 bytes may be discarded depending on the type of socket the message is
112 If no messages are available at the socket, the receive calls wait for a
113 message to arrive, unless the socket is nonblocking (see
115 in which case the value \-1 is returned and the external variable
118 .BR EAGAIN " or " EWOULDBLOCK .
119 The receive calls normally return any data available, up to the requested
120 amount, rather than waiting for receipt of the full amount requested.
126 call may be used to determine when more data arrives.
132 call is formed by ORing one or more of the following values:
134 .BR MSG_CMSG_CLOEXEC " (" recvmsg "() only; since Linux 2.6.23)"
135 Set the close-on-exec flag for the file descriptor received
136 via a UNIX domain file descriptor using the
138 operation (described in
140 This flag is useful for the same reasons as the
145 .BR MSG_DONTWAIT " (since Linux 2.2)"
146 Enables nonblocking operation; if the operation would block,
147 the call fails with the error
148 .BR EAGAIN " or " EWOULDBLOCK
149 (this can also be enabled using the
155 .BR MSG_ERRQUEUE " (since Linux 2.2)"
157 specifies that queued errors should be received from the socket error queue.
158 The error is passed in
159 an ancillary message with a type dependent on the protocol (for IPv4
161 The user should supply a buffer of sufficient size.
166 for more information.
167 The payload of the original packet that caused the error
168 is passed as normal data via
170 The original destination address of the datagram that caused the error
174 For local errors, no address is passed (this can be checked with the
178 For error receives, the
182 After an error has been passed, the pending socket error
183 is regenerated based on the next queued error and will be passed
184 on the next socket operation.
186 The error is supplied in a
192 #define SO_EE_ORIGIN_NONE 0
193 #define SO_EE_ORIGIN_LOCAL 1
194 #define SO_EE_ORIGIN_ICMP 2
195 #define SO_EE_ORIGIN_ICMP6 3
197 struct sock_extended_err
199 uint32_t ee_errno; /* error number */
200 uint8_t ee_origin; /* where the error originated */
201 uint8_t ee_type; /* type */
202 uint8_t ee_code; /* code */
203 uint8_t ee_pad; /* padding */
204 uint32_t ee_info; /* additional information */
205 uint32_t ee_data; /* other data */
206 /* More data may follow */
209 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
216 number of the queued error.
218 is the origin code of where the error originated.
219 The other fields are protocol-specific.
222 returns a pointer to the address of the network object
223 where the error originated from given a pointer to the ancillary message.
224 If this address is not known, the
230 and the other fields of the
233 The payload of the packet that caused the error is passed as normal data.
235 For local errors, no address is passed (this
236 can be checked with the
245 After an error has been passed, the pending socket error
246 is regenerated based on the next queued error and will be passed
247 on the next socket operation.
250 This flag requests receipt of out-of-band data that would not be received
251 in the normal data stream.
252 Some protocols place expedited data
253 at the head of the normal data queue, and thus this flag cannot
254 be used with such protocols.
257 This flag causes the receive operation to
258 return data from the beginning of the
259 receive queue without removing that data from the queue.
261 subsequent receive call will return the same data.
263 .BR MSG_TRUNC " (since Linux 2.2)"
266 Internet datagram (since Linux 2.4.27/2.6.8),
267 and netlink (since Linux 2.6.22) sockets:
268 return the real length of the packet or datagram,
269 even when it was longer than the passed buffer.
270 Not implemented for UNIX domain
274 For use with Internet stream sockets, see
277 .BR MSG_WAITALL " (since Linux 2.2)"
278 This flag requests that the operation block until the full request is
280 However, the call may still return less data than requested if
281 a signal is caught, an error or disconnect occurs, or the next data to be
282 received is of a different type than that returned.
288 structure to minimize the number of directly supplied arguments.
289 This structure is defined as follows in
294 struct iovec { /* Scatter/gather array items */
295 void *iov_base; /* Starting address */
296 size_t iov_len; /* Number of bytes to transfer */
300 void *msg_name; /* optional address */
301 socklen_t msg_namelen; /* size of address */
302 struct iovec *msg_iov; /* scatter/gather array */
303 size_t msg_iovlen; /* # elements in msg_iov */
304 void *msg_control; /* ancillary data, see below */
305 size_t msg_controllen; /* ancillary data buffer len */
306 int msg_flags; /* flags on received message */
315 specify the source address if the socket is unconnected;
317 may be given as a NULL pointer if no names are desired or required.
322 describe scatter-gather locations, as discussed in
328 points to a buffer for other protocol control-related messages or
329 miscellaneous ancillary data.
334 should contain the length of the available buffer in
336 upon return from a successful call it will contain the length
337 of the control message sequence.
339 The messages are of the form:
344 socklen_t cmsg_len; /* data byte count, including hdr */
345 int cmsg_level; /* originating protocol */
346 int cmsg_type; /* protocol-specific type */
348 unsigned char cmsg_data[]; */
353 Ancillary data should only be accessed by the macros defined in
356 As an example, Linux uses this ancillary data mechanism to pass extended
357 errors, IP options, or file descriptors over UNIX domain sockets.
365 It can contain several flags:
368 indicates end-of-record; the data returned completed a record (generally
369 used with sockets of type
370 .BR SOCK_SEQPACKET ).
373 indicates that the trailing portion of a datagram was discarded because the
374 datagram was larger than the buffer supplied.
377 indicates that some control data were discarded due to lack of space in the
378 buffer for ancillary data.
381 is returned to indicate that expedited or out-of-band data were received.
384 indicates that no data was received but an extended error from the socket
387 These calls return the number of bytes received, or \-1
388 if an error occurred.
389 The return value will be 0 when the
390 peer has performed an orderly shutdown.
392 These are some standard errors generated by the socket layer.
394 may be generated and returned from the underlying protocol modules;
395 see their manual pages.
397 .BR EAGAIN " or " EWOULDBLOCK
398 .\" Actually EAGAIN on Linux
399 The socket is marked nonblocking and the receive operation
400 would block, or a receive timeout had been set and the timeout expired
401 before data was received.
402 POSIX.1-2001 allows either error to be returned for this case,
403 and does not require these constants to have the same value,
404 so a portable application should check for both possibilities.
409 is an invalid descriptor.
412 A remote host refused to allow the network connection (typically
413 because it is not running the requested service).
416 The receive buffer pointer(s) point outside the process's
420 The receive was interrupted by delivery of a signal before
421 any data were available; see
425 Invalid argument passed.
426 .\" e.g., msg_namelen < 0 for recvmsg() or addrlen < 0 for recvfrom()
429 Could not allocate memory for
433 The socket is associated with a connection-oriented protocol
434 and has not been connected (see
442 does not refer to a socket.
444 4.4BSD (these function calls first appeared in 4.2BSD),
447 POSIX.1-2001 only describes the
454 The prototypes given above follow glibc2.
455 The Single UNIX Specification agrees, except that it has return values
456 of type \fIssize_t\fP (while 4.x BSD and libc4 and libc5 all have \fIint\fP).
459 argument is \fIint\fP in 4.x BSD, but \fIunsigned int\fP in libc4 and libc5.
462 argument is \fIint\fP in 4.x BSD, but \fIsize_t\fP in libc4 and libc5.
465 argument is \fIint\ *\fP in 4.x BSD, libc4 and libc5.
466 The present \fIsocklen_t\ *\fP was invented by POSIX.
470 According to POSIX.1-2001, the
474 structure should be typed as
476 but glibc currently types it as
478 .\" glibc bug raised 12 Mar 2006
479 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
480 .\" The problem is an underlying kernel issue: the size of the
481 .\" __kernel_size_t type used to type this field varies
482 .\" across architectures, but socklen_t is always 32 bits.
486 for information about a Linux-specific system call
487 that can be used to receive multiple datagrams in a single call.
489 An example of the use of