1RECV(3P)                   POSIX Programmer's Manual                  RECV(3P)
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PROLOG

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

NAME

12       recv — receive a message from a connected socket
13

SYNOPSIS

15       #include <sys/socket.h>
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17       ssize_t recv(int socket, void *buffer, size_t length, int flags);
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DESCRIPTION

20       The recv() function shall receive a message from a  connection-mode  or
21       connectionless-mode  socket. It is normally used with connected sockets
22       because it does not permit  the  application  to  retrieve  the  source
23       address of received data.
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25       The recv() function takes the following arguments:
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27       socket    Specifies the socket file descriptor.
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29       buffer    Points to a buffer where the message should be stored.
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31       length    Specifies the length in bytes of the buffer pointed to by the
32                 buffer argument.
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34       flags     Specifies the type of message reception. Values of this argu‐
35                 ment  are formed by logically OR'ing zero or more of the fol‐
36                 lowing values:
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38                 MSG_PEEK    Peeks at an incoming message. The data is treated
39                             as unread and the next recv() or similar function
40                             shall still return this data.
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42                 MSG_OOB     Requests out-of-band data. The  significance  and
43                             semantics  of  out-of-band data are protocol-spe‐
44                             cific.
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46                 MSG_WAITALL On SOCK_STREAM sockets  this  requests  that  the
47                             function  block until the full amount of data can
48                             be returned. The function may return the  smaller
49                             amount  of  data if the socket is a message-based
50                             socket, if a signal is caught, if the  connection
51                             is  terminated,  if MSG_PEEK was specified, or if
52                             an error is pending for the socket.
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54       The recv() function shall return the length of the message  written  to
55       the  buffer  pointed to by the buffer argument. For message-based sock‐
56       ets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message shall be
57       read  in  a  single  operation.  If a message is too long to fit in the
58       supplied buffer, and MSG_PEEK is not set in  the  flags  argument,  the
59       excess  bytes  shall  be  discarded.  For stream-based sockets, such as
60       SOCK_STREAM, message boundaries shall be ignored. In  this  case,  data
61       shall  be  returned to the user as soon as it becomes available, and no
62       data shall be discarded.
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64       If the MSG_WAITALL flag is not set, data shall be returned only  up  to
65       the end of the first message.
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67       If no messages are available at the socket and O_NONBLOCK is not set on
68       the socket's file  descriptor,  recv()  shall  block  until  a  message
69       arrives.  If  no messages are available at the socket and O_NONBLOCK is
70       set on the socket's file descriptor, recv() shall fail and set errno to
71       [EAGAIN] or [EWOULDBLOCK].
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RETURN VALUE

74       Upon  successful completion, recv() shall return the length of the mes‐
75       sage in bytes. If no messages are available to be received and the peer
76       has performed an orderly shutdown, recv() shall return 0. Otherwise, -1
77       shall be returned and errno set to indicate the error.
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ERRORS

80       The recv() function shall fail if:
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82       EAGAIN or EWOULDBLOCK
83              The socket's file descriptor is marked O_NONBLOCK and no data is
84              waiting  to  be  received;  or MSG_OOB is set and no out-of-band
85              data is available and either the  socket's  file  descriptor  is
86              marked  O_NONBLOCK  or  the  socket does not support blocking to
87              await out-of-band data.
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89       EBADF  The socket argument is not a valid file descriptor.
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91       ECONNRESET
92              A connection was forcibly closed by a peer.
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94       EINTR  The recv() function was interrupted by a signal that was caught,
95              before any data was available.
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97       EINVAL The MSG_OOB flag is set and no out-of-band data is available.
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99       ENOTCONN
100              A  receive  is attempted on a connection-mode socket that is not
101              connected.
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103       ENOTSOCK
104              The socket argument does not refer to a socket.
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106       EOPNOTSUPP
107              The specified flags are not supported for this  socket  type  or
108              protocol.
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110       ETIMEDOUT
111              The connection timed out during connection establishment, or due
112              to a transmission timeout on active connection.
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114       The recv() function may fail if:
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116       EIO    An I/O error occurred while reading from or writing to the  file
117              system.
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119       ENOBUFS
120              Insufficient  resources  were available in the system to perform
121              the operation.
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123       ENOMEM Insufficient memory was available to fulfill the request.
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125       The following sections are informative.
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EXAMPLES

128       None.
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APPLICATION USAGE

131       The recv() function is  equivalent  to  recvfrom()  with  null  pointer
132       address and address_len arguments, and to read() if the socket argument
133       refers to a socket and the flags argument is 0.
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135       The select() and poll() functions can be used to determine when data is
136       available to be received.
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RATIONALE

139       None.
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FUTURE DIRECTIONS

142       None.
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SEE ALSO

145       poll(),  pselect(),  read(),  recvmsg(), recvfrom(), send(), sendmsg(),
146       sendto(), shutdown(), socket(), write()
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148       The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>
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151       Portions of this text are reprinted and reproduced in  electronic  form
152       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
153       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
154       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
155       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
156       event of any discrepancy between this version and the original IEEE and
157       The Open Group Standard, the original IEEE and The Open Group  Standard
158       is  the  referee document. The original Standard can be obtained online
159       at http://www.opengroup.org/unix/online.html .
160
161       Any typographical or formatting errors that appear  in  this  page  are
162       most likely to have been introduced during the conversion of the source
163       files to man page format. To report such errors,  see  https://www.ker
164       nel.org/doc/man-pages/reporting_bugs.html .
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168IEEE/The Open Group                  2017                             RECV(3P)
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