1RECVMSG(P) POSIX Programmer's Manual RECVMSG(P)
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6 recvmsg - receive a message from a socket
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9 #include <sys/socket.h>
10 ssize_t recvmsg(int socket, struct msghdr *message, int flags);
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15 The recvmsg() function shall receive a message from a connection-mode
16 or connectionless-mode socket. It is normally used with connectionless-
17 mode sockets because it permits the application to retrieve the source
18 address of received data.
19 The recvmsg() function takes the following arguments:
21 socket Specifies the socket file descriptor.
23 message
25 Points to a msghdr structure, containing both the buffer to
26 store the source address and the buffers for the incoming mes‐
27 sage. The length and format of the address depend on the address
28 family of the socket. The msg_flags member is ignored on input,
29 but may contain meaningful values on output.
30 flags Specifies the type of message reception. Values of this argument
32 are formed by logically OR'ing zero or more of the following
33 values:
34 MSG_OOB
36 Requests out-of-band data. The significance and semantics of
37 out-of-band data are protocol-specific.
38 MSG_PEEK
40 Peeks at the incoming message.
41 MSG_WAITALL
43 On SOCK_STREAM sockets this requests that the function block
44 until the full amount of data can be returned. The function may
45 return the smaller amount of data if the socket is a message-
46 based socket, if a signal is caught, if the connection is termi‐
47 nated, if MSG_PEEK was specified, or if an error is pending for
48 the socket.
49 The recvmsg() function shall receive messages from unconnected or con‐
53 nected sockets and shall return the length of the message.
54 The recvmsg() function shall return the total length of the message.
56 For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the
57 entire message shall be read in a single operation. If a message is
58 too long to fit in the supplied buffers, and MSG_PEEK is not set in the
59 flags argument, the excess bytes shall be discarded, and MSG_TRUNC
60 shall be set in the msg_flags member of the msghdr structure. For
61 stream-based sockets, such as SOCK_STREAM, message boundaries shall be
62 ignored. In this case, data shall be returned to the user as soon as it
63 becomes available, and no data shall be discarded.
64 If the MSG_WAITALL flag is not set, data shall be returned only up to
66 the end of the first message.
67 If no messages are available at the socket and O_NONBLOCK is not set on
69 the socket's file descriptor, recvmsg() shall block until a message
70 arrives. If no messages are available at the socket and O_NONBLOCK is
71 set on the socket's file descriptor, the recvmsg() function shall fail
72 and set errno to [EAGAIN] or [EWOULDBLOCK].
73 In the msghdr structure, the msg_name and msg_namelen members specify
75 the source address if the socket is unconnected. If the socket is con‐
76 nected, the msg_name and msg_namelen members shall be ignored. The
77 msg_name member may be a null pointer if no names are desired or
78 required. The msg_iov and msg_iovlen fields are used to specify where
79 the received data shall be stored. msg_iov points to an array of iovec
80 structures; msg_iovlen shall be set to the dimension of this array. In
81 each iovec structure, the iov_base field specifies a storage area and
82 the iov_len field gives its size in bytes. Each storage area indicated
83 by msg_iov is filled with received data in turn until all of the
84 received data is stored or all of the areas have been filled.
85 Upon successful completion, the msg_flags member of the message header
87 shall be the bitwise-inclusive OR of all of the following flags that
88 indicate conditions detected for the received message:
89 MSG_EOR
91 End-of-record was received (if supported by the protocol).
92 MSG_OOB
94 Out-of-band data was received.
95 MSG_TRUNC
97 Normal data was truncated.
98 MSG_CTRUNC
100 Control data was truncated.
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104 Upon successful completion, recvmsg() shall return the length of the
105 message in bytes. If no messages are available to be received and the
106 peer has performed an orderly shutdown, recvmsg() shall return 0. Oth‐
107 erwise, -1 shall be returned and errno set to indicate the error.
108
110 The recvmsg() function shall fail if:
111 EAGAIN or EWOULDBLOCK
113 The socket's file descriptor is marked O_NONBLOCK and no data is
115 waiting to be received; or MSG_OOB is set and no out-of-band
116 data is available and either the socket's file descriptor is
117 marked O_NONBLOCK or the socket does not support blocking to
118 await out-of-band data.
119 EBADF The socket argument is not a valid open file descriptor.
121 ECONNRESET
123 A connection was forcibly closed by a peer.
124 EINTR This function was interrupted by a signal before any data was
126 available.
127 EINVAL The sum of the iov_len values overflows a ssize_t, or the
129 MSG_OOB flag is set and no out-of-band data is available.
130 EMSGSIZE
132 The msg_iovlen member of the msghdr structure pointed to by mes‐
133 sage is less than or equal to 0, or is greater than {IOV_MAX}.
134 ENOTCONN
136 A receive is attempted on a connection-mode socket that is not
137 connected.
138 ENOTSOCK
140 The socket argument does not refer to a socket.
141 EOPNOTSUPP
143 The specified flags are not supported for this socket type.
144 ETIMEDOUT
146 The connection timed out during connection establishment, or due
147 to a transmission timeout on active connection.
148 The recvmsg() function may fail if:
151 EIO An I/O error occurred while reading from or writing to the file
153 system.
154 ENOBUFS
156 Insufficient resources were available in the system to perform
157 the operation.
158 ENOMEM Insufficient memory was available to fulfill the request.
160 The following sections are informative.
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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.
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178 poll() , recv() , recvfrom() , select() , send() , sendmsg() , sendto()
179 , shutdown() , socket() , the Base Definitions volume of
180 IEEE Std 1003.1-2001, <sys/socket.h>
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183 Portions of this text are reprinted and reproduced in electronic form
184 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
185 -- Portable Operating System Interface (POSIX), The Open Group Base
186 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
187 Electrical and Electronics Engineers, Inc and The Open Group. In the
188 event of any discrepancy between this version and the original IEEE and
189 The Open Group Standard, the original IEEE and The Open Group Standard
190 is the referee document. The original Standard can be obtained online
191 at http://www.opengroup.org/unix/online.html .
192IEEE/The Open Group 2003 RECVMSG(P)