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From: @(#)send.2 8.2 (Berkeley) 2/21/94
$FreeBSD: releng/10.2/lib/libc/sys/send.2 188144 2009-02-05 14:06:09Z jamie $
.Dd February 5, 2009 .Dt SEND 2 .Os .Sh NAME .Nm send , .Nm sendto , .Nm sendmsg .Nd send a message from a socket .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/types.h n sys/socket.h .Ft ssize_t .Fn send "int s" "const void *msg" "size_t len" "int flags" .Ft ssize_t .Fn sendto "int s" "const void *msg" "size_t len" "int flags" "const struct sockaddr *to" "socklen_t tolen" .Ft ssize_t .Fn sendmsg "int s" "const struct msghdr *msg" "int flags" .Sh DESCRIPTION The .Fn send function, and .Fn sendto and .Fn sendmsg system calls are used to transmit a message to another socket. The .Fn send function may be used only when the socket is in a .Em connected state, while .Fn sendto and .Fn sendmsg may be used at any time.
p The address of the target is given by .Fa to with .Fa tolen specifying its size. The length of the message is given by .Fa len . If the message is too long to pass atomically through the underlying protocol, the error .Er EMSGSIZE is returned, and the message is not transmitted.
p No indication of failure to deliver is implicit in a .Fn send . Locally detected errors are indicated by a return value of -1.
p If no messages space is available at the socket to hold the message to be transmitted, then .Fn send normally blocks, unless the socket has been placed in non-blocking I/O mode. The .Xr select 2 system call may be used to determine when it is possible to send more data.
p The .Fa flags argument may include one or more of the following: d -literal #define MSG_OOB 0x00001 /* process out-of-band data */ #define MSG_DONTROUTE 0x00004 /* bypass routing, use direct interface */ #define MSG_EOR 0x00008 /* data completes record */ #define MSG_EOF 0x00100 /* data completes transaction */ #define MSG_NOSIGNAL 0x20000 /* do not generate SIGPIPE on EOF */ .Ed
p The flag .Dv MSG_OOB is used to send .Dq out-of-band data on sockets that support this notion (e.g.\& .Dv SOCK_STREAM ) ; the underlying protocol must also support .Dq out-of-band data. .Dv MSG_EOR is used to indicate a record mark for protocols which support the concept. .Dv MSG_EOF requests that the sender side of a socket be shut down, and that an appropriate indication be sent at the end of the specified data; this flag is only implemented for .Dv SOCK_STREAM sockets in the .Dv PF_INET protocol family. .Dv MSG_DONTROUTE is usually used only by diagnostic or routing programs. .Dv MSG_NOSIGNAL is used to prevent .Dv SIGPIPE generation when writing a socket that may be closed.
p See .Xr recv 2 for a description of the .Fa msghdr structure. .Sh RETURN VALUES The call returns the number of characters sent, or -1 if an error occurred. .Sh ERRORS The .Fn send function and .Fn sendto and .Fn sendmsg system calls fail if: l -tag -width Er t Bq Er EBADF An invalid descriptor was specified. t Bq Er EACCES The destination address is a broadcast address, and .Dv SO_BROADCAST has not been set on the socket. t Bq Er ENOTSOCK The argument .Fa s is not a socket. t Bq Er EFAULT An invalid user space address was specified for an argument. t Bq Er EMSGSIZE The socket requires that message be sent atomically, and the size of the message to be sent made this impossible. t Bq Er EAGAIN The socket is marked non-blocking and the requested operation would block. t Bq Er ENOBUFS The system was unable to allocate an internal buffer. The operation may succeed when buffers become available. t Bq Er ENOBUFS The output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion. t Bq Er EHOSTUNREACH The remote host was unreachable. t Bq Er EISCONN A destination address was specified and the socket is already connected. t Bq Er ECONNREFUSED The socket received an ICMP destination unreachable message from the last message sent. This typically means that the receiver is not listening on the remote port. t Bq Er EHOSTDOWN The remote host was down. t Bq Er ENETDOWN The remote network was down. t Bq Er EADDRNOTAVAIL The process using a .Dv SOCK_RAW socket was jailed and the source address specified in the IP header did not match the IP address bound to the prison. t Bq Er EPIPE The socket is unable to send anymore data .Dv ( SBS_CANTSENDMORE has been set on the socket). This typically means that the socket is not connected. .El .Sh SEE ALSO .Xr fcntl 2 , .Xr getsockopt 2 , .Xr recv 2 , .Xr select 2 , .Xr socket 2 , .Xr write 2 .Sh HISTORY The .Fn send function appeared in x 4.2 . .Sh BUGS Because .Fn sendmsg does not necessarily block until the data has been transferred, it is possible to transfer an open file descriptor across an .Dv AF_UNIX domain socket (see .Xr recv 2 ) , then .Fn close it before it has actually been sent, the result being that the receiver gets a closed file descriptor. It is left to the application to implement an acknowledgment mechanism to prevent this from happening.