SEND(2) | System Calls Manual | SEND(2) |
send
, sendto
,
sendmsg
, sendmmsg
—
#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);
int
sendmmsg
(int
s, struct mmsghdr
*mmsg, unsigned int
vlen, unsigned int
flags);
send
(), sendto
(),
sendmsg
(), and sendmmsg
() are
used to transmit a message to another socket. send
()
may be used only when the socket is in a connected state,
while sendto
(), sendmsg
() and
sendmmsg
() may be used at any time.
The sendmmsg
() call can be used to send
multiple messages in the same call using an array of
mmsghdr elements with the following form, as defined
in ⟨sys/socket.h⟩:
struct mmsghdr { struct msghdr msg_hdr; /* the message to be sent */ unsigned int msg_len; /* number of bytes transmitted */ };
The msg_len member contains the number of
bytes sent for each msg_hdr member. The array has
vlen elements, which is limited to
1024
. If there is an error, a number fewer than
vlen may be returned, and the error may be retrieved
using getsockopt(2) with
SO_ERROR
.
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) or
poll(2) call may be used to
determine when it is possible to send more data. Unfortunately this does not
work when the interface queue which is used to send the message is full, and
the call returns ENOBUFS
.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x0001 /* process out-of-band data */ #define MSG_PEEK 0x0002 /* peek at incoming message */ #define MSG_DONTROUTE 0x0004 /* bypass routing, use direct interface */ #define MSG_EOR 0x0008 /* data completes record */ #define MSG_NOSIGNAL 0x0400 /* 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_DONTROUTE
is usually used only by diagnostic or
routing programs.
See recv(2) for a
description of the msghdr structure.
MSG_NOSIGNAL
is used to prevent
SIGPIPE
generation when writing a socket that may be
closed.
send
(), sendto
(), and
sendmsg
() calls return the number of characters sent,
or -1 if an error occurred. The sendmmsg
() call
returns the number of messages sent, or -1 if an error occured.
send
(), sendto
(),
sendmsg
(), and sendmmsg
() fail
if:
EACCES
]EAFNOSUPPORT
]EAGAIN|EWOULDBLOCK
]EBADF
]EDSTADDRREQ
]EFAULT
]EHOSTDOWN
]EHOSTUNREACH
]EINVAL
]EMSGSIZE
]ENOBUFS
]An alternative reason: 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.
ENOTSOCK
]EPIPE
]sendto
() will also fail if:
EISCONN
]sendmsg
() and
sendmmsg
() will also fail if:
EMSGSIZE
]IOV_MAX
}.send
() function call appeared in
4.2BSD. The sendmmsg
()
function call appeared in Linux 3.0 and NetBSD 7.0.
June 22, 2012 | NetBSD 9.0 |