SEND(2) | System Calls Manual | SEND(2) |
send
, sendto
,
sendmsg
, sendmmsg
—
send message(s) from a socket
Standard C Library (libc, -lc)
#include
<sys/socket.h>
ssize_t
send
(int
s, const void *msg,
size_t len,
int flags);
ssize_t
sendto
(int
s, const void *msg,
size_t len,
int flags,
const struct sockaddr
*to, socklen_t
tolen);
ssize_t
sendmsg
(int
s, const struct msghdr
*msg, int
flags);
ssize_t
sendmmsg
(int
s, struct mmsghdr *
restrict msgvec, size_t
vlen, int
flags);
The
send
()
and sendmmsg
() functions, and
sendto
()
and
sendmsg
()
system calls are used to transmit one or more messages (with the
sendmmsg
() call) to another socket. The
send
() function may be used only when the socket is
in a
connected
state, while sendto
(),
sendmsg
() and sendmmsg
() 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.
The
sendmmsg
()
function sends multiple messages at a call. They are given by the
msgvec vector along with vlen
specifying the vector size. The number of octets sent per each message is
placed in the msg_len field of each processed element
of the vector after transmission.
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) system call may be used to determine when it
is possible to send more data.
The flags argument may include one or more of the following:
#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 */
The flag MSG_OOB
is used to send
“out-of-band” data on sockets that support 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 indication be sent at the end of the
specified data; this flag is only implemented for
SOCK_STREAM
sockets in the
PF_INET
protocol family.
MSG_DONTROUTE
is usually used only by diagnostic or
routing programs. MSG_NOSIGNAL
is used to prevent
SIGPIPE
generation when writing a socket that may be
closed.
See recv(2) for a description of the msghdr structure and the mmsghdr structure.
The send
(),
sendto
() and sendmsg
() calls
return the number of octets sent. The sendmmsg
()
call returns the number of messages sent. If an error occurred a value of -1
is returned.
The send
() and
sendmmsg
() functions and
sendto
() and sendmsg
()
system calls fail if:
EBADF
]EACCES
]SO_BROADCAST
has not been set on the socket.ENOTSOCK
]EFAULT
]EMSGSIZE
]EAGAIN
]ENOBUFS
]ENOBUFS
]EHOSTUNREACH
]EISCONN
]ECONNREFUSED
]EHOSTDOWN
]ENETDOWN
]EADDRNOTAVAIL
]SOCK_RAW
socket was jailed and
the source address specified in the IP header did not match the IP address
bound to the prison.EPIPE
]SBS_CANTSENDMORE
has been set on the socket).
This typically means that the socket is not connected.fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2), CMSG_DATA(3)
The send
() function appeared in
4.2BSD. The sendmmsg
()
function appeared in FreeBSD 11.0.
Because sendmsg
() does not necessarily
block until the data has been transferred, it is possible to transfer an
open file descriptor across an AF_UNIX
domain socket
(see recv(2)), then 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.
August 19, 2018 | Debian |