1RECVMSG(3P) POSIX Programmer's Manual RECVMSG(3P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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13 recvmsg — receive a message from a socket
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16 #include <sys/socket.h>
17 ssize_t recvmsg(int socket, struct msghdr *message, int flags);
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21 The recvmsg() function shall receive a message from a connection-mode
22 or connectionless-mode socket. It is normally used with connectionless-
23 mode sockets because it permits the application to retrieve the source
24 address of received data.
25 The recvmsg() function takes the following arguments:
27 socket Specifies the socket file descriptor.
29 message Points to a msghdr structure, containing both the buffer to
31 store the source address and the buffers for the incoming
32 message. The length and format of the address depend on the
33 address family of the socket. The msg_flags member is
34 ignored on input, but may contain meaningful values on out‐
35 put.
36 flags Specifies the type of message reception. Values of this
38 argument are formed by logically OR'ing zero or more of the
39 following values:
40 MSG_OOB Requests out-of-band data. The significance and
42 semantics of out-of-band data are protocol-spe‐
43 cific.
44 MSG_PEEK Peeks at the incoming message.
46 MSG_WAITALL On SOCK_STREAM sockets this requests that the
48 function block until the full amount of data
49 can be returned. The function may return the
50 smaller amount of data if the socket is a mes‐
51 sage-based socket, if a signal is caught, if
52 the connection is terminated, if MSG_PEEK was
53 specified, or if an error is pending for the
54 socket.
55 The recvmsg() function shall receive messages from unconnected or con‐
57 nected sockets and shall return the length of the message.
58 The recvmsg() function shall return the total length of the message.
60 For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the
61 entire message shall be read in a single operation. If a message is too
62 long to fit in the supplied buffers, and MSG_PEEK is not set in the
63 flags argument, the excess bytes shall be discarded, and MSG_TRUNC
64 shall be set in the msg_flags member of the msghdr structure. For
65 stream-based sockets, such as SOCK_STREAM, message boundaries shall be
66 ignored. In this case, data shall be returned to the user as soon as it
67 becomes available, and no data shall be discarded.
68 If the MSG_WAITALL flag is not set, data shall be returned only up to
70 the end of the first message.
71 If no messages are available at the socket and O_NONBLOCK is not set on
73 the socket's file descriptor, recvmsg() shall block until a message
74 arrives. If no messages are available at the socket and O_NONBLOCK is
75 set on the socket's file descriptor, the recvmsg() function shall fail
76 and set errno to [EAGAIN] or [EWOULDBLOCK].
77 In the msghdr structure, the msg_name member may be a null pointer if
79 the source address is not required. Otherwise, if the socket is uncon‐
80 nected, the msg_name member points to a sockaddr structure in which the
81 source address is to be stored, and the msg_namelen member on input
82 specifies the length of the supplied sockaddr structure and on output
83 specifies the length of the stored address. If the actual length of
84 the address is greater than the length of the supplied sockaddr struc‐
85 ture, the stored address shall be truncated. If the socket is con‐
86 nected, the msg_name and msg_namelen members shall be ignored. The
87 msg_iov and msg_iovlen fields are used to specify where the received
88 data shall be stored. The msg_iov member points to an array of iovec
89 structures; the msg_iovlen member shall be set to the dimension of this
90 array. In each iovec structure, the iov_base field specifies a storage
91 area and the iov_len field gives its size in bytes. Each storage area
92 indicated by msg_iov is filled with received data in turn until all of
93 the received data is stored or all of the areas have been filled.
94 Upon successful completion, the msg_flags member of the message header
96 shall be the bitwise-inclusive OR of all of the following flags that
97 indicate conditions detected for the received message:
98 MSG_EOR End-of-record was received (if supported by the protocol).
100 MSG_OOB Out-of-band data was received.
102 MSG_TRUNC Normal data was truncated.
104 MSG_CTRUNC Control data was truncated.
106
108 Upon successful completion, recvmsg() shall return the length of the
109 message in bytes. If no messages are available to be received and the
110 peer has performed an orderly shutdown, recvmsg() shall return 0. Oth‐
111 erwise, −1 shall be returned and errno set to indicate the error.
112
114 The recvmsg() function shall fail if:
115 EAGAIN or EWOULDBLOCK
117 The socket's file descriptor is marked O_NONBLOCK and no data is
118 waiting to be received; or MSG_OOB is set and no out-of-band
119 data is available and either the socket's file descriptor is
120 marked O_NONBLOCK or the socket does not support blocking to
121 await out-of-band data.
122 EBADF The socket argument is not a valid open file descriptor.
124 ECONNRESET
126 A connection was forcibly closed by a peer.
127 EINTR This function was interrupted by a signal before any data was
129 available.
130 EINVAL The sum of the iov_len values overflows a ssize_t, or the
132 MSG_OOB flag is set and no out-of-band data is available.
133 EMSGSIZE
135 The msg_iovlen member of the msghdr structure pointed to by mes‐
136 sage is less than or equal to 0, or is greater than {IOV_MAX}.
137 ENOTCONN
139 A receive is attempted on a connection-mode socket that is not
140 connected.
141 ENOTSOCK
143 The socket argument does not refer to a socket.
144 EOPNOTSUPP
146 The specified flags are not supported for this socket type.
147 ETIMEDOUT
149 The connection timed out during connection establishment, or due
150 to a transmission timeout on active connection.
151 The recvmsg() function may fail if:
153 EIO An I/O error occurred while reading from or writing to the file
155 system.
156 ENOBUFS
158 Insufficient resources were available in the system to perform
159 the operation.
160 ENOMEM Insufficient memory was available to fulfill the request.
162 The following sections are informative.
164
166 None.
167
169 The select() and poll() functions can be used to determine when data is
170 available to be received.
171
173 None.
174
176 None.
177
179 poll(), pselect(), recv(), recvfrom(), send(), sendmsg(), sendto(),
180 shutdown(), socket()
181 The Base Definitions volume of POSIX.1‐2008, <sys_socket.h>
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185 Portions of this text are reprinted and reproduced in electronic form
186 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
187 -- Portable Operating System Interface (POSIX), The Open Group Base
188 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
189 cal and Electronics Engineers, Inc and The Open Group. (This is
190 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
191 event of any discrepancy between this version and the original IEEE and
192 The Open Group Standard, the original IEEE and The Open Group Standard
193 is the referee document. The original Standard can be obtained online
194 at http://www.unix.org/online.html .
195 Any typographical or formatting errors that appear in this page are
197 most likely to have been introduced during the conversion of the source
198 files to man page format. To report such errors, see https://www.ker‐
199 nel.org/doc/man-pages/reporting_bugs.html .
200IEEE/The Open Group 2013 RECVMSG(3P)