1RECVMSG(3P) POSIX Programmer's Manual RECVMSG(3P)
2
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.
10
12 recvmsg — receive a message from a socket
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15 #include <sys/socket.h>
16 ssize_t recvmsg(int socket, struct msghdr *message, int flags);
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20 The recvmsg() function shall receive a message from a connection-mode
21 or connectionless-mode socket. It is normally used with connectionless-
22 mode sockets because it permits the application to retrieve the source
23 address of received data.
24 The recvmsg() function takes the following arguments:
26 socket Specifies the socket file descriptor.
28 message Points to a msghdr structure, containing both the buffer to
30 store the source address and the buffers for the incoming
31 message. The length and format of the address depend on the
32 address family of the socket. The msg_flags member is
33 ignored on input, but may contain meaningful values on out‐
34 put.
35 flags Specifies the type of message reception. Values of this
37 argument are formed by logically OR'ing zero or more of the
38 following values:
39 MSG_OOB Requests out-of-band data. The significance and
41 semantics of out-of-band data are protocol-spe‐
42 cific.
43 MSG_PEEK Peeks at the incoming message.
45 MSG_WAITALL On SOCK_STREAM sockets this requests that the
47 function block until the full amount of data
48 can be returned. The function may return the
49 smaller amount of data if the socket is a mes‐
50 sage-based socket, if a signal is caught, if
51 the connection is terminated, if MSG_PEEK was
52 specified, or if an error is pending for the
53 socket.
54 The recvmsg() function shall receive messages from unconnected or con‐
56 nected sockets and shall return the length of the message.
57 The recvmsg() function shall return the total length of the message.
59 For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the
60 entire message shall be read in a single operation. If a message is too
61 long to fit in the supplied buffers, and MSG_PEEK is not set in the
62 flags argument, the excess bytes shall be discarded, and MSG_TRUNC
63 shall be set in the msg_flags member of the msghdr structure. For
64 stream-based sockets, such as SOCK_STREAM, message boundaries shall be
65 ignored. In this case, data shall be returned to the user as soon as it
66 becomes available, and no data shall be discarded.
67 If the MSG_WAITALL flag is not set, data shall be returned only up to
69 the end of the first message.
70 If no messages are available at the socket and O_NONBLOCK is not set on
72 the socket's file descriptor, recvmsg() shall block until a message
73 arrives. If no messages are available at the socket and O_NONBLOCK is
74 set on the socket's file descriptor, the recvmsg() function shall fail
75 and set errno to [EAGAIN] or [EWOULDBLOCK].
76 In the msghdr structure, the msg_name member may be a null pointer if
78 the source address is not required. Otherwise, if the socket is uncon‐
79 nected, the msg_name member points to a sockaddr structure in which the
80 source address is to be stored, and the msg_namelen member on input
81 specifies the length of the supplied sockaddr structure and on output
82 specifies the length of the stored address. If the actual length of
83 the address is greater than the length of the supplied sockaddr struc‐
84 ture, the stored address shall be truncated. If the socket is con‐
85 nected, the msg_name and msg_namelen members shall be ignored. The
86 msg_iov and msg_iovlen fields are used to specify where the received
87 data shall be stored. The msg_iov member points to an array of iovec
88 structures; the msg_iovlen member shall be set to the dimension of this
89 array. In each iovec structure, the iov_base field specifies a storage
90 area and the iov_len field gives its size in bytes. Each storage area
91 indicated by msg_iov is filled with received data in turn until all of
92 the received data is stored or all of the areas have been filled.
93 Upon successful completion, the msg_flags member of the message header
95 shall be the bitwise-inclusive OR of all of the following flags that
96 indicate conditions detected for the received message:
97 MSG_EOR End-of-record was received (if supported by the protocol).
99 MSG_OOB Out-of-band data was received.
101 MSG_TRUNC Normal data was truncated.
103 MSG_CTRUNC Control data was truncated.
105
107 Upon successful completion, recvmsg() shall return the length of the
108 message in bytes. If no messages are available to be received and the
109 peer has performed an orderly shutdown, recvmsg() shall return 0. Oth‐
110 erwise, -1 shall be returned and errno set to indicate the error.
111
113 The recvmsg() function shall fail if:
114 EAGAIN or EWOULDBLOCK
116 The socket's file descriptor is marked O_NONBLOCK and no data is
117 waiting to be received; or MSG_OOB is set and no out-of-band
118 data is available and either the socket's file descriptor is
119 marked O_NONBLOCK or the socket does not support blocking to
120 await out-of-band data.
121 EBADF The socket argument is not a valid open file descriptor.
123 ECONNRESET
125 A connection was forcibly closed by a peer.
126 EINTR This function was interrupted by a signal before any data was
128 available.
129 EINVAL The sum of the iov_len values overflows a ssize_t, or the
131 MSG_OOB flag is set and no out-of-band data is available.
132 EMSGSIZE
134 The msg_iovlen member of the msghdr structure pointed to by mes‐
135 sage is less than or equal to 0, or is greater than {IOV_MAX}.
136 ENOTCONN
138 A receive is attempted on a connection-mode socket that is not
139 connected.
140 ENOTSOCK
142 The socket argument does not refer to a socket.
143 EOPNOTSUPP
145 The specified flags are not supported for this socket type.
146 ETIMEDOUT
148 The connection timed out during connection establishment, or due
149 to a transmission timeout on active connection.
150 The recvmsg() function may fail if:
152 EIO An I/O error occurred while reading from or writing to the file
154 system.
155 ENOBUFS
157 Insufficient resources were available in the system to perform
158 the operation.
159 ENOMEM Insufficient memory was available to fulfill the request.
161 The following sections are informative.
163
165 None.
166
168 The select() and poll() functions can be used to determine when data is
169 available to be received.
170
172 None.
173
175 None.
176
178 poll(), pselect(), recv(), recvfrom(), send(), sendmsg(), sendto(),
179 shutdown(), socket()
180 The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>
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184 Portions of this text are reprinted and reproduced in electronic form
185 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
186 table Operating System Interface (POSIX), The Open Group Base Specifi‐
187 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
188 Electrical and Electronics Engineers, Inc and The Open Group. In the
189 event of any discrepancy between this version and the original IEEE and
190 The Open Group Standard, the original IEEE and The Open Group Standard
191 is the referee document. The original Standard can be obtained online
192 at http://www.opengroup.org/unix/online.html .
193 Any typographical or formatting errors that appear in this page are
195 most likely to have been introduced during the conversion of the source
196 files to man page format. To report such errors, see https://www.ker‐
197 nel.org/doc/man-pages/reporting_bugs.html .
198IEEE/The Open Group 2017 RECVMSG(3P)