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recv(2) [bsd man page]

RECV(2) 							System Calls Manual							   RECV(2)

NAME
recv, recvfrom, recvmsg - receive a message from a socket SYNOPSIS
#include <sys/types.h> #include <sys/socket.h> cc = recv(s, buf, len, flags) int cc, s; char *buf; int len, flags; cc = recvfrom(s, buf, len, flags, from, fromlen) int cc, s; char *buf; int len, flags; struct sockaddr *from; int *fromlen; cc = recvmsg(s, msg, flags) int cc, s; struct msghdr msg[]; int flags; DESCRIPTION
Recv, recvfrom, and recvmsg are used to receive messages from a socket. The recv call is normally used only on a connected socket (see connect(2)), while recvfrom and recvmsg may be used to receive data on a socket whether it is in a connected state or not. If from is non-zero, 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 length of the message is returned in cc. 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 nonblocking (see ioctl(2)) in which case a cc of -1 is returned with the external variable errno set to EWOULDBLOCK. The select(2) call may be used to determine when more data arrives. The flags argument to a recv call is formed by or'ing one or more of the values, #define MSG_OOB 0x1 /* process out-of-band data */ #define MSG_PEEK 0x2 /* peek at incoming message */ The recvmsg call uses a msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in <sys/socket.h>: struct msghdr { caddr_t msg_name; /* optional address */ int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_accrights; /* access rights sent/received */ int msg_accrightslen; }; Here msg_name and msg_namelen specify the destination 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 describe the scatter gather locations, as described in read(2). A buffer to receive any access rights sent along with the message is specified in msg_accrights, which has length msg_accrightslen. Access rights are currently limited to file descriptors, which each occupy the size of an int. RETURN VALUE
These calls return the number of bytes received, or -1 if an error occurred. ERRORS
The calls fail if: [EBADF] The argument s is an invalid descriptor. [ENOTSOCK] The argument s is not a socket. [EWOULDBLOCK] The socket is marked non-blocking and the receive operation would block. [EINTR] The receive was interrupted by delivery of a signal before any data was available for the receive. [EFAULT] The data was specified to be received into a non-existent or protected part of the process address space. SEE ALSO
fcntl(2), read(2), send(2), select(2), getsockopt(2), socket(2) 4.2 Berkeley Distribution May 23, 1986 RECV(2)

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RECV(2) 						      BSD System Calls Manual							   RECV(2)

NAME
recv, recvfrom, recvmsg -- receive a message from a socket SYNOPSIS
#include <sys/types.h> #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 *from, int *fromlen); ssize_t recvmsg(int s, struct msghdr *msg, int flags); DESCRIPTION
Recvfrom() and recvmsg() are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connec- tion-oriented. If from is non-nil, and the socket is not connection-oriented, 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 recv() call is normally used only on a connected socket (see connect(2)) and is identical to recvfrom() with a nil from parameter. As it is redundant, it may not be supported in future releases. On successful completion, all three routines return the number of message bytes read. If a message is too long to fit in the supplied buf- fer, 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 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; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2). The select(2) call may be used to determine when more data arrive. The flags argument to a recv call 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_WAITALL wait for full request or error 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 oper- ation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data. 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 recvmsg() call uses a msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in <sys/socket.h>: struct msghdr { caddr_t msg_name; /* optional address */ u_int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ u_int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_control; /* ancillary data, see below */ u_int 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. Msg_iov and msg_iovlen describe scatter gather locations, as discussed in read(2). Msg_control, 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 { u_int 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, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting a recvmsg with no data buffer provided immediately after an accept() call. Open file descriptors are now passed as ancillary data for AF_UNIX domain sockets, with cmsg_level set to SOL_SOCKET and cmsg_type set to SCM_RIGHTS. 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 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. RETURN VALUES
These calls return the number of bytes received, or -1 if an error occurred. ERRORS
The calls fail if: [EBADF] The argument s is an invalid descriptor. [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. [EAGAIN] The socket is marked non-blocking, and the receive operation would block, or a receive timeout had been set, and the time- out expired before data were received. [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), read(2), select(2), getsockopt(2), socket(2) HISTORY
The recv() function call appeared in 4.2BSD. 4.3-Reno Berkeley Distribution February 21, 1994 4.3-Reno Berkeley Distribution
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