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recvfrom.3p (7403B)

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.TH RECVFROM "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
recvfrom
\(em receive a message from a socket
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.P
ssize_t recvfrom(int \fIsocket\fP, void *restrict \fIbuffer\fP, size_t \fIlength\fP,
    int \fIflags\fP, struct sockaddr *restrict \fIaddress\fP,
    socklen_t *restrict \fIaddress_len\fP);
.fi
.SH DESCRIPTION
The
\fIrecvfrom\fR()
function shall receive a message from a connection-mode or
connectionless-mode socket. It is normally used with
connectionless-mode sockets because it permits the application to
retrieve the source address of received data.
.P
The
\fIrecvfrom\fR()
function takes the following arguments:
.IP "\fIsocket\fR" 12
Specifies the socket file descriptor.
.IP "\fIbuffer\fR" 12
Points to the buffer where the message should be stored.
.IP "\fIlength\fR" 12
Specifies the length in bytes of the buffer pointed to by the
.IR buffer
argument.
.IP "\fIflags\fR" 12
Specifies the type of message reception. Values of this argument are
formed by logically OR'ing zero or more of the following values:
.RS 12 
.IP MSG_PEEK 12
Peeks at an incoming message. The data is treated as unread and the
next
\fIrecvfrom\fR()
or similar function shall still return this data.
.IP MSG_OOB 12
Requests out-of-band data. The significance and semantics of
out-of-band data are protocol-specific.
.IP MSG_WAITALL 12
On SOCK_STREAM sockets this requests that the function block until the
full amount of data can be returned. The function may return the
smaller amount of data if the socket is a message-based socket, if a
signal is caught, if the connection is terminated, if MSG_PEEK was
specified, or if an error is pending for the socket.
.RE
.IP "\fIaddress\fR" 12
A null pointer, or points to a
.BR sockaddr
structure in which the sending address is to be stored. The length and
format of the address depend on the address family of the socket.
.IP "\fIaddress_len\fR" 12
Either a null pointer, if
.IR address
is a null pointer, or a pointer to a
.BR socklen_t
object which on input specifies the length of the supplied
.BR sockaddr
structure, and on output specifies the length of the stored address.
.P
The
\fIrecvfrom\fR()
function shall return the length of the message written to the buffer
pointed to by the
.IR buffer
argument. For message-based sockets, such as
SOCK_RAW,
SOCK_DGRAM, and SOCK_SEQPACKET, the entire message shall be read in a
single operation. If a message is too long to fit in the supplied
buffer, and MSG_PEEK is not set in the
.IR flags
argument, the excess bytes shall be discarded. For stream-based
sockets, such as SOCK_STREAM, message boundaries shall be ignored. In
this case, data shall be returned to the user as soon as it becomes
available, and no data shall be discarded.
.P
If the MSG_WAITALL flag is not set, data shall be returned only up to
the end of the first message.
.P
Not all protocols provide the source address for messages. If the
.IR address
argument is not a null pointer and the protocol provides the source
address of messages, the source address of the received message shall
be stored in the
.BR sockaddr
structure pointed to by the
.IR address
argument, and the length of this address shall be stored in the object
pointed to by the
.IR address_len
argument.
.P
If the actual length of the address is greater than the length of the
supplied
.BR sockaddr
structure, the stored address shall be truncated.
.P
If the
.IR address
argument is not a null pointer and the protocol does not provide the
source address of messages, the value stored in the object pointed to
by
.IR address
is unspecified.
.P
If no messages are available at the socket and O_NONBLOCK is not set on
the socket's file descriptor,
\fIrecvfrom\fR()
shall block until a message arrives. If no messages are available at
the socket and O_NONBLOCK is set on the socket's file descriptor,
\fIrecvfrom\fR()
shall fail and set
.IR errno
to
.BR [EAGAIN] 
or
.BR [EWOULDBLOCK] .
.SH "RETURN VALUE"
Upon successful completion,
\fIrecvfrom\fR()
shall return the length of the message in bytes. If no messages are
available to be received and the peer has performed an orderly
shutdown,
\fIrecvfrom\fR()
shall return 0. Otherwise, the function shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIrecvfrom\fR()
function shall fail if:
.TP
.BR EAGAIN " or " EWOULDBLOCK
.br
The socket's file descriptor is marked O_NONBLOCK and no data is
waiting to be received; or MSG_OOB is set and no out-of-band data is
available and either the socket's file descriptor is marked O_NONBLOCK
or the socket does not support blocking to await out-of-band data.
.TP
.BR EBADF
The
.IR socket
argument is not a valid file descriptor.
.TP
.BR ECONNRESET
A connection was forcibly closed by a peer.
.TP
.BR EINTR
A signal interrupted
\fIrecvfrom\fR()
before any data was available.
.TP
.BR EINVAL
The MSG_OOB flag is set and no out-of-band data is available.
.TP
.BR ENOTCONN
A receive is attempted on a connection-mode socket that is not connected.
.TP
.BR ENOTSOCK
The
.IR socket
argument does not refer to a socket.
.TP
.BR EOPNOTSUPP
The specified flags are not supported for this socket type.
.TP
.BR ETIMEDOUT
The connection timed out during connection establishment, or due to a
transmission timeout on active connection.
.P
The
\fIrecvfrom\fR()
function may fail if:
.TP
.BR EIO
An I/O error occurred while reading from or writing to the file
system.
.TP
.BR ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP
.BR ENOMEM
Insufficient memory was available to fulfill the request.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
None.
.SH "APPLICATION USAGE"
The
\fIselect\fR()
and
\fIpoll\fR()
functions can be used to determine when data is available to be
received.
.SH "RATIONALE"
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIpoll\fR\^(\|)",
.IR "\fIpselect\fR\^(\|)",
.IR "\fIread\fR\^(\|)",
.IR "\fIrecv\fR\^(\|)",
.IR "\fIrecvmsg\fR\^(\|)",
.IR "\fIsend\fR\^(\|)",
.IR "\fIsendmsg\fR\^(\|)",
.IR "\fIsendto\fR\^(\|)",
.IR "\fIshutdown\fR\^(\|)",
.IR "\fIsocket\fR\^(\|)",
.IR "\fIwrite\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_socket.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .