RECVFROM(3)                POSIX Programmer's Manual               RECVFROM(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       recvfrom -- receive a message from a socket

SYNOPSIS
       #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);

DESCRIPTION
       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].

RETURN VALUE
       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.

ERRORS
       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.

EXAMPLES
       None.

APPLICATION USAGE
       The select() and poll() functions can be used to determine when data is
       available to be received.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       poll(), pselect(), read(), recv(), recvmsg(), send(), sendmsg(),
       sendto(), shutdown(), socket(), write()

       The Base Definitions volume of POSIX.1-2017, <sys_socket.h>

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 .

       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(3)
