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.
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12 recvmsg - receive a message from a socket
13
15 #include <sys/socket.h>
16 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
31 Points to a msghdr structure, containing both the buffer to
32 store the source address and the buffers for the incoming mes‐
33 sage. The length and format of the address depend on the address
34 family of the socket. The msg_flags member is ignored on input,
35 but may contain meaningful values on output.
36 flags Specifies the type of message reception. Values of this argument
38 are formed by logically OR'ing zero or more of the following
39 values:
40 MSG_OOB
42 Requests out-of-band data. The significance and semantics of
43 out-of-band data are protocol-specific.
44 MSG_PEEK
46 Peeks at the incoming message.
47 MSG_WAITALL
49 On SOCK_STREAM sockets this requests that the function block
50 until the full amount of data can be returned. The function may
51 return the smaller amount of data if the socket is a message-
52 based socket, if a signal is caught, if the connection is termi‐
53 nated, if MSG_PEEK was specified, or if an error is pending for
54 the socket.
55 The recvmsg() function shall receive messages from unconnected or con‐
59 nected sockets and shall return the length of the message.
60 The recvmsg() function shall return the total length of the message.
62 For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the
63 entire message shall be read in a single operation. If a message is
64 too long to fit in the supplied buffers, and MSG_PEEK is not set in the
65 flags argument, the excess bytes shall be discarded, and MSG_TRUNC
66 shall be set in the msg_flags member of the msghdr structure. For
67 stream-based sockets, such as SOCK_STREAM, message boundaries shall be
68 ignored. In this case, data shall be returned to the user as soon as it
69 becomes available, and no data shall be discarded.
70 If the MSG_WAITALL flag is not set, data shall be returned only up to
72 the end of the first message.
73 If no messages are available at the socket and O_NONBLOCK is not set on
75 the socket's file descriptor, recvmsg() shall block until a message
76 arrives. If no messages are available at the socket and O_NONBLOCK is
77 set on the socket's file descriptor, the recvmsg() function shall fail
78 and set errno to [EAGAIN] or [EWOULDBLOCK].
79 In the msghdr structure, the msg_name and msg_namelen members specify
81 the source address if the socket is unconnected. If the socket is con‐
82 nected, the msg_name and msg_namelen members shall be ignored. The
83 msg_name member may be a null pointer if no names are desired or
84 required. The msg_iov and msg_iovlen fields are used to specify where
85 the received data shall be stored. msg_iov points to an array of iovec
86 structures; msg_iovlen shall be set to the dimension of this array. In
87 each iovec structure, the iov_base field specifies a storage area and
88 the iov_len field gives its size in bytes. Each storage area indicated
89 by msg_iov is filled with received data in turn until all of the
90 received data is stored or all of the areas have been filled.
91 Upon successful completion, the msg_flags member of the message header
93 shall be the bitwise-inclusive OR of all of the following flags that
94 indicate conditions detected for the received message:
95 MSG_EOR
97 End-of-record was received (if supported by the protocol).
98 MSG_OOB
100 Out-of-band data was received.
101 MSG_TRUNC
103 Normal data was truncated.
104 MSG_CTRUNC
106 Control data was truncated.
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110 Upon successful completion, recvmsg() shall return the length of the
111 message in bytes. If no messages are available to be received and the
112 peer has performed an orderly shutdown, recvmsg() shall return 0. Oth‐
113 erwise, -1 shall be returned and errno set to indicate the error.
114
116 The recvmsg() function shall fail if:
117 EAGAIN or EWOULDBLOCK
119 The socket's file descriptor is marked O_NONBLOCK and no data is
121 waiting to be received; or MSG_OOB is set and no out-of-band
122 data is available and either the socket's file descriptor is
123 marked O_NONBLOCK or the socket does not support blocking to
124 await out-of-band data.
125 EBADF The socket argument is not a valid open file descriptor.
127 ECONNRESET
129 A connection was forcibly closed by a peer.
130 EINTR This function was interrupted by a signal before any data was
132 available.
133 EINVAL The sum of the iov_len values overflows a ssize_t, or the
135 MSG_OOB flag is set and no out-of-band data is available.
136 EMSGSIZE
138 The msg_iovlen member of the msghdr structure pointed to by mes‐
139 sage is less than or equal to 0, or is greater than {IOV_MAX}.
140 ENOTCONN
142 A receive is attempted on a connection-mode socket that is not
143 connected.
144 ENOTSOCK
146 The socket argument does not refer to a socket.
147 EOPNOTSUPP
149 The specified flags are not supported for this socket type.
150 ETIMEDOUT
152 The connection timed out during connection establishment, or due
153 to a transmission timeout on active connection.
154 The recvmsg() function may fail if:
157 EIO An I/O error occurred while reading from or writing to the file
159 system.
160 ENOBUFS
162 Insufficient resources were available in the system to perform
163 the operation.
164 ENOMEM Insufficient memory was available to fulfill the request.
166 The following sections are informative.
169
171 None.
172
174 The select() and poll() functions can be used to determine when data is
175 available to be received.
176
178 None.
179
181 None.
182
184 poll(), recv(), recvfrom(), select(), send(), sendmsg(), sendto(),
185 shutdown(), socket(), the Base Definitions volume of
186 IEEE Std 1003.1-2001, <sys/socket.h>
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189 Portions of this text are reprinted and reproduced in electronic form
190 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
191 -- Portable Operating System Interface (POSIX), The Open Group Base
192 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
193 Electrical and Electronics Engineers, Inc and The Open Group. In the
194 event of any discrepancy between this version and the original IEEE and
195 The Open Group Standard, the original IEEE and The Open Group Standard
196 is the referee document. The original Standard can be obtained online
197 at http://www.opengroup.org/unix/online.html .
198IEEE/The Open Group 2003 RECVMSG(3P)