1RECV(2) Linux Programmer's Manual RECV(2)
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6 recv, recvfrom, recvmsg - receive a message from a socket
7
9 #include <sys/types.h>
10 #include <sys/socket.h>
11 ssize_t recv(int sockfd, void *buf, size_t len, int flags);
13 ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
15 struct sockaddr *src_addr, socklen_t *addrlen);
16 ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
18
20 The recv(), recvfrom(), and recvmsg() calls are used to receive mes‐
21 sages from a socket. They may be used to receive data on both connec‐
22 tionless and connection-oriented sockets. This page first describes
23 common features of all three system calls, and then describes the dif‐
24 ferences between the calls.
25 The only difference between recv() and read(2) is the presence of
27 flags. With a zero flags argument, recv() is generally equivalent to
28 read(2) (but see NOTES). Also, the following call
29 recv(sockfd, buf, len, flags);
31 is equivalent to
33 recvfrom(sockfd, buf, len, flags, NULL, NULL);
35 All three calls return the length of the message on successful comple‐
37 tion. If a message is too long to fit in the supplied buffer, excess
38 bytes may be discarded depending on the type of socket the message is
39 received from.
40 If no messages are available at the socket, the receive calls wait for
42 a message to arrive, unless the socket is nonblocking (see fcntl(2)),
43 in which case the value -1 is returned and the external variable errno
44 is set to EAGAIN or EWOULDBLOCK. The receive calls normally return any
45 data available, up to the requested amount, rather than waiting for re‐
46 ceipt of the full amount requested.
47 An application can use select(2), poll(2), or epoll(7) to determine
49 when more data arrives on a socket.
50 The flags argument
52 The flags argument is formed by ORing one or more of the following val‐
53 ues:
54 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 MSG_DONTWAIT (since Linux 2.2)
62 Enables nonblocking operation; if the operation would block, the
63 call fails with the error EAGAIN or EWOULDBLOCK. This provides
64 similar behavior to setting the O_NONBLOCK flag (via the fc‐
65 ntl(2) F_SETFL operation), but differs in that MSG_DONTWAIT is a
66 per-call option, whereas O_NONBLOCK is a setting on the open
67 file description (see open(2)), which will affect all threads in
68 the calling process and as well as other processes that hold
69 file descriptors referring to the same open file description.
70 MSG_ERRQUEUE (since Linux 2.2)
72 This flag specifies that queued errors should be received from
73 the socket error queue. The error is passed in an ancillary
74 message with a type dependent on the protocol (for IPv4
75 IP_RECVERR). The user should supply a buffer of sufficient
76 size. See cmsg(3) and ip(7) for more information. The payload
77 of the original packet that caused the error is passed as normal
78 data via msg_iovec. The original destination address of the
79 datagram that caused the error is supplied via msg_name.
80 The error is supplied in a sock_extended_err structure:
82 #define SO_EE_ORIGIN_NONE 0
84 #define SO_EE_ORIGIN_LOCAL 1
85 #define SO_EE_ORIGIN_ICMP 2
86 #define SO_EE_ORIGIN_ICMP6 3
87 struct sock_extended_err
89 {
90 uint32_t ee_errno; /* Error number */
91 uint8_t ee_origin; /* Where the error originated */
92 uint8_t ee_type; /* Type */
93 uint8_t ee_code; /* Code */
94 uint8_t ee_pad; /* Padding */
95 uint32_t ee_info; /* Additional information */
96 uint32_t ee_data; /* Other data */
97 /* More data may follow */
98 };
99 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
101 ee_errno contains the errno number of the queued error. ee_ori‐
103 gin is the origin code of where the error originated. The other
104 fields are protocol-specific. The macro SOCK_EE_OFFENDER re‐
105 turns a pointer to the address of the network object where the
106 error originated from given a pointer to the ancillary message.
107 If this address is not known, the sa_family member of the sock‐
108 addr contains AF_UNSPEC and the other fields of the sockaddr are
109 undefined. The payload of the packet that caused the error is
110 passed as normal data.
111 For local errors, no address is passed (this can be checked with
113 the cmsg_len member of the cmsghdr). For error receives, the
114 MSG_ERRQUEUE flag is set in the msghdr. After an error has been
115 passed, the pending socket error is regenerated based on the
116 next queued error and will be passed on the next socket opera‐
117 tion.
118 MSG_OOB
120 This flag requests receipt of out-of-band data that would not be
121 received in the normal data stream. Some protocols place expe‐
122 dited data at the head of the normal data queue, and thus this
123 flag cannot be used with such protocols.
124 MSG_PEEK
126 This flag causes the receive operation to return data from the
127 beginning of the receive queue without removing that data from
128 the queue. Thus, a subsequent receive call will return the same
129 data.
130 MSG_TRUNC (since Linux 2.2)
132 For raw (AF_PACKET), Internet datagram (since Linux
133 2.4.27/2.6.8), netlink (since Linux 2.6.22), and UNIX datagram
134 (since Linux 3.4) sockets: return the real length of the packet
135 or datagram, even when it was longer than the passed buffer.
136 For use with Internet stream sockets, see tcp(7).
138 MSG_WAITALL (since Linux 2.2)
140 This flag requests that the operation block until the full re‐
141 quest is satisfied. However, the call may still return less
142 data than requested if a signal is caught, an error or discon‐
143 nect occurs, or the next data to be received is of a different
144 type than that returned. This flag has no effect for datagram
145 sockets.
146 recvfrom()
148 recvfrom() places the received message into the buffer buf. The caller
149 must specify the size of the buffer in len.
150 If src_addr is not NULL, and the underlying protocol provides the
152 source address of the message, that source address is placed in the
153 buffer pointed to by src_addr. In this case, addrlen is a value-result
154 argument. Before the call, it should be initialized to the size of the
155 buffer associated with src_addr. Upon return, addrlen is updated to
156 contain the actual size of the source address. The returned address is
157 truncated if the buffer provided is too small; in this case, addrlen
158 will return a value greater than was supplied to the call.
159 If the caller is not interested in the source address, src_addr and ad‐
161 drlen should be specified as NULL.
162 recv()
164 The recv() call is normally used only on a connected socket (see con‐
165 nect(2)). It is equivalent to the call:
166 recvfrom(fd, buf, len, flags, NULL, 0);
168 recvmsg()
170 The recvmsg() call uses a msghdr structure to minimize the number of
171 directly supplied arguments. This structure is defined as follows in
172 <sys/socket.h>:
173 struct iovec { /* Scatter/gather array items */
175 void *iov_base; /* Starting address */
176 size_t iov_len; /* Number of bytes to transfer */
177 };
178 struct msghdr {
180 void *msg_name; /* Optional address */
181 socklen_t msg_namelen; /* Size of address */
182 struct iovec *msg_iov; /* Scatter/gather array */
183 size_t msg_iovlen; /* # elements in msg_iov */
184 void *msg_control; /* Ancillary data, see below */
185 size_t msg_controllen; /* Ancillary data buffer len */
186 int msg_flags; /* Flags on received message */
187 };
188 The msg_name field points to a caller-allocated buffer that is used to
190 return the source address if the socket is unconnected. The caller
191 should set msg_namelen to the size of this buffer before this call;
192 upon return from a successful call, msg_namelen will contain the length
193 of the returned address. If the application does not need to know the
194 source address, msg_name can be specified as NULL.
195 The fields msg_iov and msg_iovlen describe scatter-gather locations, as
197 discussed in readv(2).
198 The field msg_control, which has length msg_controllen, points to a
200 buffer for other protocol control-related messages or miscellaneous an‐
201 cillary data. When recvmsg() is called, msg_controllen should contain
202 the length of the available buffer in msg_control; upon return from a
203 successful call it will contain the length of the control message se‐
204 quence.
205 The messages are of the form:
207 struct cmsghdr {
209 size_t cmsg_len; /* Data byte count, including header
210 (type is socklen_t in POSIX) */
211 int cmsg_level; /* Originating protocol */
212 int cmsg_type; /* Protocol-specific type */
213 /* followed by
214 unsigned char cmsg_data[]; */
215 };
216 Ancillary data should be accessed only by the macros defined in
218 cmsg(3).
219 As an example, Linux uses this ancillary data mechanism to pass ex‐
221 tended errors, IP options, or file descriptors over UNIX domain sock‐
222 ets. For further information on the use of ancillary data in various
223 socket domains, see unix(7) and ip(7).
224 The msg_flags field in the msghdr is set on return of recvmsg(). It
226 can contain several flags:
227 MSG_EOR
229 indicates end-of-record; the data returned completed a record
230 (generally used with sockets of type SOCK_SEQPACKET).
231 MSG_TRUNC
233 indicates that the trailing portion of a datagram was discarded
234 because the datagram was larger than the buffer supplied.
235 MSG_CTRUNC
237 indicates that some control data was discarded due to lack of
238 space in the buffer for ancillary data.
239 MSG_OOB
241 is returned to indicate that expedited or out-of-band data was
242 received.
243 MSG_ERRQUEUE
245 indicates that no data was received but an extended error from
246 the socket error queue.
247
249 These calls return the number of bytes received, or -1 if an error oc‐
250 curred. In the event of an error, errno is set to indicate the error.
251 When a stream socket peer has performed an orderly shutdown, the return
253 value will be 0 (the traditional "end-of-file" return).
254 Datagram sockets in various domains (e.g., the UNIX and Internet do‐
256 mains) permit zero-length datagrams. When such a datagram is received,
257 the return value is 0.
258 The value 0 may also be returned if the requested number of bytes to
260 receive from a stream socket was 0.
261
263 These are some standard errors generated by the socket layer. Addi‐
264 tional errors may be generated and returned from the underlying proto‐
265 col modules; see their manual pages.
266 EAGAIN or EWOULDBLOCK
268 The socket is marked nonblocking and the receive operation would
269 block, or a receive timeout had been set and the timeout expired
270 before data was received. POSIX.1 allows either error to be re‐
271 turned for this case, and does not require these constants to
272 have the same value, so a portable application should check for
273 both possibilities.
274 EBADF The argument sockfd is an invalid file descriptor.
276 ECONNREFUSED
278 A remote host refused to allow the network connection (typically
279 because it is not running the requested service).
280 EFAULT The receive buffer pointer(s) point outside the process's ad‐
282 dress space.
283 EINTR The receive was interrupted by delivery of a signal before any
285 data was available; see signal(7).
286 EINVAL Invalid argument passed.
288 ENOMEM Could not allocate memory for recvmsg().
290 ENOTCONN
292 The socket is associated with a connection-oriented protocol and
293 has not been connected (see connect(2) and accept(2)).
294 ENOTSOCK
296 The file descriptor sockfd does not refer to a socket.
297
299 POSIX.1-2001, POSIX.1-2008, 4.4BSD (these interfaces first appeared in
300 4.2BSD).
301 POSIX.1 describes only the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.
303
305 If a zero-length datagram is pending, read(2) and recv() with a flags
306 argument of zero provide different behavior. In this circumstance,
307 read(2) has no effect (the datagram remains pending), while recv() con‐
308 sumes the pending datagram.
309 The socklen_t type was invented by POSIX. See also accept(2).
311 According to POSIX.1, the msg_controllen field of the msghdr structure
313 should be typed as socklen_t, and the msg_iovlen field should be typed
314 as int, but glibc currently types both as size_t.
315 See recvmmsg(2) for information about a Linux-specific system call that
317 can be used to receive multiple datagrams in a single call.
318
320 An example of the use of recvfrom() is shown in getaddrinfo(3).
321
323 fcntl(2), getsockopt(2), read(2), recvmmsg(2), select(2), shutdown(2),
324 socket(2), cmsg(3), sockatmark(3), ip(7), ipv6(7), socket(7), tcp(7),
325 udp(7), unix(7)
326
328 This page is part of release 5.10 of the Linux man-pages project. A
329 description of the project, information about reporting bugs, and the
330 latest version of this page, can be found at
331 https://www.kernel.org/doc/man-pages/.
332Linux 2020-11-01 RECV(2)