# recvmsg

## 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.
    


## NAME



           recvmsg — receive a message from a socket
    


## SYNOPSIS



           #include <sys/socket.h>
    
           ssize_t recvmsg(int _socket_ , struct msghdr *_message_ , int _flags_);
    


## DESCRIPTION



           The _recvmsg_() 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 _recvmsg_() function takes the following arguments:
    
           _socket_      Specifies the socket file descriptor.
    
           _message_     Points to a **msghdr** structure, containing both the
                       buffer to store the source address and the buffers for
                       the incoming message. The length and format of the
                       address depend on the address family of the socket.
                       The _msg_flags_ member is ignored on input, but may
                       contain meaningful values on output.
    
           _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_OOB     Requests out-of-band data. The
                                   significance and semantics of out-of-band
                                   data are protocol-specific.
    
                       MSG_PEEK    Peeks at the incoming message.
    
                       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.
    
           The _recvmsg_() function shall receive messages from unconnected or
           connected sockets and shall return the length of the message.
    
           The _recvmsg_() function shall return the total length of the
           message. For message-based sockets, such as 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
           buffers, and MSG_PEEK is not set in the _flags_ argument, the excess
           bytes shall be discarded, and MSG_TRUNC shall be set in the
           _msg_flags_ member of the **msghdr** structure. 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.
    
           If no messages are available at the socket and O_NONBLOCK is not
           set on the socket's file descriptor, _recvmsg_() 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, the _recvmsg_()
           function shall fail and set _errno_ to **[EAGAIN]** or **[EWOULDBLOCK]**.
    
           In the **msghdr** structure, the _msg_name_ member may be a null pointer
           if the source address is not required.  Otherwise, if the socket
           is unconnected, the _msg_name_ member points to a **sockaddr** structure
           in which the source address is to be stored, and the _msg_namelen_
           member on input specifies the length of the supplied **sockaddr**
           structure and on output specifies the length of the stored
           address.  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 socket is connected, the _msg_name_ and
           _msg_namelen_ members shall be ignored. The _msg_iov_ and _msg_iovlen_
           fields are used to specify where the received data shall be
           stored.  The _msg_iov_ member points to an array of **iovec**
           structures; the _msg_iovlen_ member shall be set to the dimension of
           this array. In each **iovec** structure, the _iov_base_ field specifies
           a storage area and the _iov_len_ field gives its size in bytes. Each
           storage area indicated by _msg_iov_ is filled with received data in
           turn until all of the received data is stored or all of the areas
           have been filled.
    
           Upon successful completion, the _msg_flags_ member of the message
           header shall be the bitwise-inclusive OR of all of the following
           flags that indicate conditions detected for the received message:
    
           MSG_EOR     End-of-record was received (if supported by the
                       protocol).
    
           MSG_OOB     Out-of-band data was received.
    
           MSG_TRUNC   Normal data was truncated.
    
           MSG_CTRUNC  Control data was truncated.
    


## RETURN VALUE



           Upon successful completion, _recvmsg_() 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, _recvmsg_() shall
           return 0. Otherwise, -1 shall be returned and _errno_ set to
           indicate the error.
    


## ERRORS



           The _recvmsg_() 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 open file descriptor.
    
           **ECONNRESET**
                  A connection was forcibly closed by a peer.
    
           **EINTR** This function was interrupted by a signal before any data
                  was available.
    
           **EINVAL** The sum of the _iov_len_ values overflows a **ssize_t** , or the
                  MSG_OOB flag is set and no out-of-band data is available.
    
           **EMSGSIZE**
                  The _msg_iovlen_ member of the **msghdr** structure pointed to by
                  _message_ is less than or equal to 0, or is greater than
                  {IOV_MAX}.
    
           **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 _recvmsg_() 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.
    


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