send, sendto, sendmsg - send a message from a socket
int send(int s, const void *msg, int len, unsigned int
int sendto(int s, const void *msg, int len, unsigned int
flags, const struct sockaddr *to, int tolen);
int sendmsg(int s, const struct msghdr *msg, unsigned int
Send, sendto, and sendmsg are used to transmit a message
to another socket. Send may be used only when the socket
is in a connected state, while sendto and sendmsg may be
used at any time.
The address of the target is given by to with tolen speci
fying its size. The length of the message is given by
len. If the message is too long to pass atomically
through the underlying protocol, the error EMSGSIZE is
returned, and the message is not transmitted.
No indication of failure to deliver is implicit in a send.
Locally detected errors are indicated by a return value of
When the message does not fit into the send buffer of the
socket, send normally blocks, unless the socket has been
placed in non-blocking I/O mode. In non-blocking mode it
would return EAGAIN in this case. The select(2) call may
be used to determine when it is possible to send more
The flags parameter may include one or more of the follow
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_DONTROUTE 0x4 /* bypass routing, use direct interface */
#define MSG_DONTWAIT 0x40 /* don't block */
#define MSG_NOSIGNAL 0x2000 /* don't raise SIGPIPE */
Sends out-of-band data on sockets that support this
notion (e.g. SOCK_STREAM); the underlying protocol
must also support out-of-band data.
Bypasses the usual routing table lookup and sends
the packet directly to the interface described by
Enables non-blocking operation; if the operation
would block, EAGAIN is returned.
Requests not to send SIGPIPE on errors on stream
oriented sockets when the other end breaks the con
nection. The EPIPE error is still returned.
See recv(2) for a description of the msghdr structure. You
may send control information using the msg_control and
msg_controllen members. The maximum control buffer length
the kernel can process is limited by the net.core.opt
mem_max sysctl; see socket(4).
The calls return the number of characters sent, or -1 if
an error occurred.
These are some standard errors generated by the socket
layer. Additional errors may be generated and returned
from the underlying protocol modules; see their respective
EBADF An invalid descriptor was specified.
sendfile - transfer data between file descriptors
This call copies data between file descriptor and another
file descriptor or socket. in_fd should be a file
descriptor opened for reading. out_fd should be a
descriptor opened for writing or a connected socket. off
set is a pointer to variable holding the input file
pointer position from which sendfile(2) will start reading
data. After sendfile finished this variable will be set to
the offset of the byte following the last byte that was
read. count is the number of bytes to copy between file
Sendfile does not modify the current file pointer of
in_fd, but does for out_fd.
If you plan to use sendfile for sending files to a TCP
socket, but need to send some header data in front of the
file contents, please refer to the TCP_COR