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RECV(2) System Calls Manual RECV(2) NAME recv, recvfrom, recvmsg, recvmmsg -- receive message(s) from a socket LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <sys/socket.h> ssize_t recv(int s, void *buf, size_t len, int flags); ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr * restrict from, socklen_t * restrict fromlen); ssize_t recvmsg(int s, struct msghdr *msg, int flags); ssize_t recvmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags, const struct timespec * restrict timeout); DESCRIPTION The recvfrom(), recvmsg(), and recvmmsg() system calls are used to re- ceive 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 a null pointer and the socket is not connection-ori- ented, the source address of the message is filled in. The fromlen ar- gument is a value-result argument, initialized to the size of the buffer associated with from, and modified on return to indicate the ac- tual size of the address stored there. The recv() function is normally used only on a connected socket (see connect(2)) and is identical to recvfrom() with a null pointer passed as its from argument. The recvmmsg() function is used to receive multiple messages at a call. Their number is supplied by vlen. The messages are placed in the buffers described by msgvec vector, after reception. The size of each received message is placed in the msg_len field of each element of the vector. If timeout is NULL the call blocks until the data is available for each supplied message buffer. Otherwise it waits for data for the specified amount of time. If the timeout expired and there is no data received, a value 0 is returned. The ppoll(2) system call is used to implement the timeout mechanism, before first receive is performed. The recv(), recvfrom() and recvmsg() return the length of the message on successful completion, whereas recvmmsg() returns the number of re- ceived messages. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see socket(2)). If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is non-blocking (see fcntl(2)) in which case the value -1 is returned and the global variable errno is set to EAGAIN. The receive calls except recvmmsg() normally return any data available, up to the requested amount, rather than waiting for re- ceipt of the full amount requested; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2). The recvmmsg() function implements this behaviour for each message in the vector. The select(2) system call may be used to determine when more data ar- rives. The flags argument to a recv() function is formed by or'ing one or more of the values: MSG_OOB process out-of-band data MSG_PEEK peek at incoming message MSG_TRUNC return real packet or datagram length MSG_WAITALL wait for full request or error MSG_DONTWAIT do not block MSG_CMSG_CLOEXEC set received fds close-on-exec MSG_WAITFORONE do not block after receiving the first message (only for recvmmsg() ) The MSG_OOB 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. The MSG_PEEK flag causes the receive opera- tion to return data from the beginning of the receive queue without re- moving that data from the queue. Thus, a subsequent receive call will return the same data. The MSG_TRUNC flag causes the receive operation to return the full length of the packet or datagram even if larger than provided buffer. The flag is supported on SOCK_DGRAM sockets for AF_INET , AF_INET6 and AF_UNIX families. The MSG_WAITALL flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. The MSG_DONTWAIT flag re- quests the call to return when it would block otherwise. If no data is available, errno is set to EAGAIN. This flag is not available in ANSI X3.159-1989 ("ANSI C89") or ISO/IEC 9899:1999 ("ISO C99") compilation mode. The MSG_WAITFORONE flag sets MSG_DONTWAIT after the first mes- sage has been received. This flag is only relevant for recvmmsg(). The recvmsg() system call uses a msghdr structure to minimize the num- ber of directly supplied arguments. This structure has the following form, as defined in <sys/socket.h>: struct msghdr { void *msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ void *msg_control; /* ancillary data, see below */ socklen_t msg_controllen;/* ancillary data buffer len */ int msg_flags; /* flags on received message */ }; Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as a null pointer if no names are desired or required. The msg_iov and msg_iovlen arguments describe scatter gather locations, as discussed in read(2). The msg_control ar- gument, which has length msg_controllen, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form: struct cmsghdr { socklen_t cmsg_len; /* data byte count, including hdr */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by u_char cmsg_data[]; */ }; As an example, the SO_TIMESTAMP socket option returns a reception time- stamp for UDP packets. With AF_UNIX domain sockets, ancillary data can be used to pass file descriptors and process credentials. See unix(4) for details. The msg_flags field is set on return according to the message received. MSG_EOR indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET). MSG_TRUNC indi- cates 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 expe- dited or out-of-band data were received. The recvmmsg() system call uses the mmsghdr structure, defined as fol- lows in the <sys/socket.h> header: struct mmsghdr { struct msghdr msg_hdr; /* message header */ ssize_t msg_len; /* message length */ }; On data reception the msg_len field is updated to the length of the re- ceived message. RETURN VALUES These calls except recvmmsg() return the number of bytes received. recvmmsg() returns the number of messages received. A value of -1 is returned if an error occurred. ERRORS The calls fail if: [EBADF] The argument s is an invalid descriptor. [ECONNRESET] The remote socket end is forcibly closed. [ENOTCONN] The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)). [ENOTSOCK] The argument s does not refer to a socket. [EMFILE] The recvmsg() system call was used to receive rights (file descriptors) that were in flight on the con- nection. However, the receiving program did not have enough free file descriptor slots to accept them. In this case the descriptors are closed, with pending data either discarded in the case of the un- reliable datagram protocol or preserved in the case of a reliable protocol. The pending data can be re- trieved with another call to recvmsg(). [EMSGSIZE] The msg_iovlen member of the msghdr structure pointed to by msg is less than or equal to 0, or is greater than IOV_MAX. [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 were re- ceived. [EINTR] The receive was interrupted by delivery of a signal before any data were available. [EFAULT] The receive buffer pointer(s) point outside the process's address space. SEE ALSO fcntl(2), getsockopt(2), read(2), select(2), socket(2), CMSG_DATA(3), unix(4) HISTORY The recv() function appeared in 4.2BSD. The recvmmsg() function ap- peared in FreeBSD 11.0. FreeBSD 13.2 July 30, 2022 RECV(2)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY
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