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<html> <body> <pre> NAME recvfrom - receive a message from a socket SYNOPSIS #include <network.h> int recvfrom(int s, void *buf, int len, unsigned int flags struct sockaddr *from, int *fromlen); DESCRIPTION The recvfrom call is used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented. If from is not NULL, and the socket is not connection-ori ented, the source address of the message is filled in. Fromlen is a value-result parameter, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there. The routine returns the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depend ing on the type of socket the message is received from (see socket(2)). If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1 is returned and the external variable errno set to EAGAIN. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested. The select(2) call may be used to determine when more data arrives. The flags argument to a recvfrom call is formed by OR'ing one or more of the following values: MSG_OOB This flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. MSG_PEEK This flag causes the receive operation to return data from the beginning of the receive queue with out removing that data from the queue. Thus, a subsequent receive call will return the same data. MSG_WAITALL This flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a sig nal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. MSG_ERRQUEUE Receive packet from the error queue MSG_NOSIGNAL This flag turns off raising of SIGPIPE on stream sockets when the other end disappears. MSG_ERRQUEUE This flag specifies that queued errors should be received from the socket error queue. The error is passed in a ancilliary message with a type depen dent on the protocol (for IP IP_RECVERR). The error is supplied in a sock_extended_error struc ture: #define SO_EE_ORIGIN_NONE 0 #define SO_EE_ORIGIN_LOCAL 1 #define SO_EE_ORIGIN_ICMP 2 #define SO_EE_ORIGIN_ICMP6 3 struct sock_extended_err { __u32 ee_errno; /* error number */ __u8 ee_origin; /* where the error originated */ __u8 ee_type; /* type */ __u8 ee_code; /* code */ __u8 ee_pad; __u32 ee_info; /* additional information */ __u32 ee_data; /* other data */ }; struct sockaddr *SOCK_EE_OFFENDER(struct sock_extended_err *); ee_errno contains the errno number of the queued error. ee_origin is the origin code of where the error originated. The other fields are protocol specific. SOCK_EE_OFFENDER returns a pointer to the address of the network object where the error originated from. If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the sockaddr are undefined. The payload of the packet that caused the error is passed as normal data. For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE is set in the msghdr. After a error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the next socket operation. The msg_flags field is set on return according to the mes sage received. MSG_EOR indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET). MSG_TRUNC indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied. MSG_CTRUNC indicates that some control data were discarded due to lack of space in the buffer for ancillary data. MSG_OOB is returned to indicate that expedited or out-of-band data were received. MSG_ERRQUEUE indicates that no data was received but an extended error from the socket error queue. RETURN VALUES These calls return the number of bytes received, or -1 if an error occurred. ERRORS These are some standard errors generated by the socket layer. Additional errors may be generated and returned from the underlying protocol modules; see their manual pages. EBADF The argument s is an invalid descriptor. ENOTSOCK The argument s does not refer to a socket. EAGAIN The socket is marked non-blocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received. EINTR The receive was interrupted by delivery of a sig nal before any data were available. EINVAL Invalid argument passed. </pre> </body> </html>
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