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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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RECVFROM(3P) POSIX Programmer's Manual RECVFROM(3P)
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.
recvfrom — receive a message from a socket
#include <sys/socket.h>
ssize_t recvfrom(int socket, void *restrict buffer, size_t length,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
The recvfrom() 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.
The recvfrom() function takes the following arguments:
socket Specifies the socket file descriptor.
buffer Points to the buffer where the message should be
stored.
length Specifies the length in bytes of the buffer pointed to
by the buffer argument.
flags Specifies the type of message reception. Values of
this argument are formed by logically OR'ing zero or
more of the following values:
MSG_PEEK Peeks at an incoming message. The data is
treated as unread and the next recvfrom()
or similar function shall still return
this data.
MSG_OOB Requests out-of-band data. The
significance and semantics of out-of-band
data are protocol-specific.
MSG_WAITALL 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.
address A null pointer, or points to a 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.
address_len Either a null pointer, if address is a null pointer,
or a pointer to a socklen_t object which on input
specifies the length of the supplied sockaddr
structure, and on output specifies the length of the
stored address.
The recvfrom() function shall return the length of the message
written to the buffer pointed to by the 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 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.
If the MSG_WAITALL flag is not set, data shall be returned only up
to the end of the first message.
Not all protocols provide the source address for messages. If the
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 sockaddr structure pointed to by
the address argument, and the length of this address shall be
stored in the object pointed to by the address_len argument.
If the actual length of the address is greater than the length of
the supplied sockaddr structure, the stored address shall be
truncated.
If the 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 address is unspecified.
If no messages are available at the socket and O_NONBLOCK is not
set on the socket's file descriptor, recvfrom() 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, recvfrom()
shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].
Upon successful completion, recvfrom() 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, recvfrom() shall
return 0. Otherwise, the function shall return -1 and set errno to
indicate the error.
The recvfrom() function shall fail if:
EAGAIN or EWOULDBLOCK
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.
EBADF The socket argument is not a valid file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR A signal interrupted recvfrom() before any data was
available.
EINVAL The MSG_OOB flag is set and no out-of-band data is
available.
ENOTCONN
A receive is attempted on a connection-mode socket that is
not connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The specified flags are not supported for this socket type.
ETIMEDOUT
The connection timed out during connection establishment,
or due to a transmission timeout on active connection.
The recvfrom() function may fail if:
EIO An I/O error occurred while reading from or writing to the
file system.
ENOBUFS
Insufficient resources were available in the system to
perform the operation.
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
None.
The select() and poll() functions can be used to determine when
data is available to be received.
None.
None.
poll(3p), pselect(3p), read(3p), recv(3p), recvmsg(3p), send(3p),
sendmsg(3p), sendto(3p), shutdown(3p), socket(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, sys_socket.h(0p)
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 .
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 .
IEEE/The Open Group 2017 RECVFROM(3P)
Pages that refer to this page: sys_socket.h(0p), recv(3p), recvmsg(3p), send(3p), sendmsg(3p), sendto(3p), shutdown(3p), socket(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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