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



PPRROOLLOOGG
       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.

NNAAMMEE
       recvfrom -- receive a message from a socket

SSYYNNOOPPSSIISS
       #include <sys/socket.h>

       ssize_t recvfrom(int _s_o_c_k_e_t, void *restrict _b_u_f_f_e_r, size_t _l_e_n_g_t_h,
           int _f_l_a_g_s, struct sockaddr *restrict _a_d_d_r_e_s_s,
           socklen_t *restrict _a_d_d_r_e_s_s___l_e_n);

DDEESSCCRRIIPPTTIIOONN
       The _r_e_c_v_f_r_o_m() 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 _r_e_c_v_f_r_o_m() function takes the following arguments:

       _s_o_c_k_e_t      Specifies the socket file descriptor.

       _b_u_f_f_e_r      Points to the buffer where the message should be stored.

       _l_e_n_g_t_h      Specifies the length in bytes of the buffer pointed to by
                   the _b_u_f_f_e_r argument.

       _f_l_a_g_s       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 _r_e_c_v_f_r_o_m() 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.

       _a_d_d_r_e_s_s     A null pointer, or points to a ssoocckkaaddddrr 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.

       _a_d_d_r_e_s_s___l_e_n Either a null pointer, if _a_d_d_r_e_s_s is a null pointer, or a
                   pointer to a ssoocckklleenn__tt object which on input specifies the
                   length of the supplied ssoocckkaaddddrr structure, and on output
                   specifies the length of the stored address.

       The _r_e_c_v_f_r_o_m() function shall return the length of the message written
       to the buffer pointed to by the _b_u_f_f_e_r 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 _f_l_a_g_s
       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
       _a_d_d_r_e_s_s 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 ssoocckkaaddddrr structure pointed to by the _a_d_d_r_e_s_s
       argument, and the length of this address shall be stored in the object
       pointed to by the _a_d_d_r_e_s_s___l_e_n argument.

       If the actual length of the address is greater than the length of the
       supplied ssoocckkaaddddrr structure, the stored address shall be truncated.

       If the _a_d_d_r_e_s_s 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 _a_d_d_r_e_s_s is unspecified.

       If no messages are available at the socket and O_NONBLOCK is not set on
       the socket's file descriptor, _r_e_c_v_f_r_o_m() 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, _r_e_c_v_f_r_o_m() shall fail and set
       _e_r_r_n_o to [[EEAAGGAAIINN]] or [[EEWWOOUULLDDBBLLOOCCKK]].

RREETTUURRNN VVAALLUUEE
       Upon successful completion, _r_e_c_v_f_r_o_m() 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, _r_e_c_v_f_r_o_m() shall return 0.
       Otherwise, the function shall return -1 and set _e_r_r_n_o to indicate the
       error.

EERRRROORRSS
       The _r_e_c_v_f_r_o_m() function shall fail if:

       EEAAGGAAIINN or EEWWOOUULLDDBBLLOOCCKK
              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.

       EEBBAADDFF  The _s_o_c_k_e_t argument is not a valid file descriptor.

       EECCOONNNNRREESSEETT
              A connection was forcibly closed by a peer.

       EEIINNTTRR  A signal interrupted _r_e_c_v_f_r_o_m() before any data was available.

       EEIINNVVAALL The MSG_OOB flag is set and no out-of-band data is available.

       EENNOOTTCCOONNNN
              A receive is attempted on a connection-mode socket that is not
              connected.

       EENNOOTTSSOOCCKK
              The _s_o_c_k_e_t argument does not refer to a socket.

       EEOOPPNNOOTTSSUUPPPP
              The specified flags are not supported for this socket type.

       EETTIIMMEEDDOOUUTT
              The connection timed out during connection establishment, or due
              to a transmission timeout on active connection.

       The _r_e_c_v_f_r_o_m() function may fail if:

       EEIIOO    An I/O error occurred while reading from or writing to the file
              system.

       EENNOOBBUUFFSS
              Insufficient resources were available in the system to perform
              the operation.

       EENNOOMMEEMM Insufficient memory was available to fulfill the request.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       The _s_e_l_e_c_t() and _p_o_l_l() functions can be used to determine when data is
       available to be received.

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _p_o_l_l(), _p_s_e_l_e_c_t(), _r_e_a_d(), _r_e_c_v(), _r_e_c_v_m_s_g(), _s_e_n_d(), _s_e_n_d_m_s_g(),
       _s_e_n_d_t_o(), _s_h_u_t_d_o_w_n(), _s_o_c_k_e_t(), _w_r_i_t_e()

       The Base Definitions volume of POSIX.1-2017, <<ssyyss__ssoocckkeett..hh>>

CCOOPPYYRRIIGGHHTT
       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)
