SEND(2) System Calls SEND(2)
send, sendto, sendmsg - send a message from a socket
int send (int s, const void *msg, size_t len, unsigned int flags);
int sendto (int s, const void *msg, size_t len, int flags, const struct
sockaddr *to, unsigned int tolen);
int sendmsg (int s, const struct msghdr *msg, unsigned int flags);
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 specifying 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 -1.
If no messages space is available at the socket to hold the message to
be transmitted, then send normally blocks, unless the socket has been
placed in non-blocking I/O mode. The select(2) call may be used to
determine when it is possible to send more data.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_PEEK 0x2 /* peek at incoming message */
#define MSG_DONTROUTE 0x4 /* bypass routing, use direct interface */
#define MSG_EOR 0x8 /* data completes record */
#define MSG_EOF 0x100 /* data completes transaction */
The flag MSG_OOB is used to send out-of-band data on sockets that sup-
port this notion (e.g. SOCK_STREAM); the underlying protocol must also
support out-of-band data. MSG_EOR is used to indicate a record mark
for protocols which support the concept. MSG_EOF requests that the
sender side of a socket be shut down, and that an appropriate indica-
tion be sent at the end of the specified data; this flag is only imple-
mented for SOCK_STREAM sockets in the PF_INET protocol family, and is
used to implement Transaction TCP (see ttcp(4)). MSG_DONTROUTE is usu-
ally used only by diagnostic or routing programs.
See recv(2) for a description of the msghdr structure.
The call returns the number of characters sent, or -1 if an error
Send, sendto, and sendmsg fail if:
EBADF An invalid descriptor was specified.
The argument s is not a socket.
EFAULT An invalid user space address was specified for a parame-
The socket requires that message be sent atomically, and
the size of the message to be sent made this impossible.
EAGAIN The socket is marked non-blocking and the requested oper-
ation would block.
The system was unable to allocate an internal buffer.
The operation may succeed when buffers become available.
The output queue for a network interface was full. This
generally indicates that the interface has stopped send-
ing, but may be caused by transient congestion.
The GNO prototypes of these routines differ slightly from that of
fcntl(2), recv(2), select(2), getsockopt(2), socket(2), write(2)
The sendmsg function call appeared in 4.2BSD. The first appearance in
GNO was in v2.0.5.
GNO 16 January 1997 SEND(2)
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