SEND(2)SEND(2)NAME
send, sendto, sendmsg - send a message from a socket
C SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int send(int s, const void *msg, int len, int flags);
int sendto(int s, const void *msg, int len, int flags,
const struct sockaddr *to, socklen_t tolen);
int sendmsg(int s, const struct msghdr *msg, int flags);
#if _XOPEN_SOURCE >= 500
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);
#elif _XOPEN_SOURCE < 500
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, size_t tolen;
ssize_t sendmsg(int s, const struct msghdr *msg, int flags);
#endif
DESCRIPTION
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 when the socket is unconnected.
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, then the error EMSGSIZE
is returned, and the message is not transmitted.
Usually no indication of failure to deliver is implicit in a send.
Return values of -1 indicate some locally detected errors. Connected
datagram sockets may receive error indications from a previous send.
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_DONTROUTE 0x4 /* bypass routing,
use direct interface */
#define MSG_DONT_BRKPG 0x100 /* don't break up pages for send */
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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_DONTROUTE is usually used only by
diagnostic or routing programs. MSG_DONT_BRKPG is used to allow the
user, on a per send basis to stop page flipping and the break up of large
memory pages to 16k. This can be done automatically for all large pages
transfers by setting the mtune/bsd/ip_nolgpg_brkup variable. NOTE that
setting this can have a negative effect on performance.
See recv(2) for a description of the msghdr structure.
RETURN VALUE
The call returns the number of characters sent, or -1 if an error
occurred.
ERRORS
[EBADF] An invalid descriptor was specified.
[ENOTSOCK] The argument s is not a socket.
[EFAULT] An invalid user space address was specified for a
parameter.
[EMSGSIZE] (1) The socket requires that message be sent
atomically, and the size of the message to be sent
made this impossible; OR
(2) In the sendmsg() call, the msg_iovlen member of
the msghdr structure pointed to by msg is greater
than MSG_MAXIOVLEN or less than or equal to 0.
[EWOULDBLOCK] The socket is marked non-blocking and the requested
operation would block.
[ENOBUFS] The system was unable to allocate an internal buffer.
The operation may succeed when buffers become
available.
[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.
[ECONNREFUSED] The remote port was invalid when using the send call
on a connected datagram socket.
[EISCONN] A sendto or sendmsg call was used on a connected
socket.
[EACCES] The requested operation specified a broadcast address
as the destination but the SO_BROADCAST socket option
was not enabled (see setsockopt(2)).
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[EHOSTUNREACH] The remote host was unreachable via the network.
[ENETUNREACH] The remote network is unknown to the routing system.
[EHOSTDOWN] The remote host was determined to be down, possibly
due to a failure to resolve its MAC-level address
(see arp(7P)).
SEE ALSOfcntl(2), recv(2), select(2), getsockopt(2), socket(2), write(2)NOTES
ABI-compliant versions of the above call can be obtained from
libsocket.so.
For each of these three functions, send, sendto and sendmsg , there are
three types of functions in n32 and 64 bit C libraries for IRIX 6.5.19
and later versions. One is the normal type when _XOPEN_SOURCE is not
defined; the second is XPG5 type when _XOPEN_SOURCE is set to >= 500; and
the third is XPG4 type when _XOPEN_SOURCE set to < 500.
send function:
1. For the normal case when _XOPEN_SOURCE is not defined, third argument
type, will be an int and the normal send is used.
2. When _XOPEN_SOURCE is >= 500 or < 500, third argument type will be a
size_t and XPG5 or XPG4 send is used.
sendto function:
1. For the normal case when _XOPEN_SOURCE is not defined, third argument
type will be an int and the sixth argument will be a pointer to socklen_t
type, which is actually a pointer to an int, and the normal sendto is
used.
2. When _XOPEN_SOURCE is set to >= 500, third argument type will be a
size_t and the sixth argument will be of socklen_t type, which is
actually a u_int32_t.
3. When _XOPEN_SOURCE is set to < 500, third argument type will be a
size_t and the sixth argument will be of size_t type.
sendmsg function:
1. For the normal case when _XOPEN_SOURCE is not defined, the normal
struct msghdr will be used in the second argument.
2. When _XOPEN_SOURCE is set to >= 500, _XOPEN5 struct msghdr will be
used in the second argument.
3. When _XOPEN_SOURCE is set to < 500, _XOPEN4 struct msghdr will be used
in the second argument.
In the AF_UNIX domain sendmsg function behaves in the following way:
a. If _XOPEN_SOURCE is not defined, the msg_accrights field of struct
msghdr that is used to specify access rights is limited to file
descriptors, which each occupy the size of an int.
b. If _XOPEN_SOURCE is defined, the msg_control field of struct msghdr
that is used to specify ancillary data is limited to a cmsghdr structure
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followed by an array of file descriptors. The fields of cmsghdr
structure are set to:
cmsg_level set to SOL_SOCKET; cmsg_type set to SCM_RIGHTS; and cmsg_len
set to data byte count, including the cmsghdr.
Refer <sys/socket.h> for alternate definitions of socklen_t and struct
msghdr.
XPG5 type functions are not supported in o32 C library.
The XPG5 type send, sendto and sendmsg functions are actually defined as
static inline functions in <sys/socket.h>, and each call a new function
_xpg5_send, _xpg5_sendto or _xpg5_sendmsg which are specific to IRIX
6.5.19 and later. Applications that call any of these XPG5 type functions
should check for the existence of the new symbol as done in the following
example.
#include <sys/socket.h>
#include <optional_sym.h>
if (_MIPS_SYMBOL_PRESENT(_xpg5_send)) {
send(s, &buf, len, flags);
} else {
...
}
Because the static inline functions are defined in each source file that
includes <sys/socket.h>, these static functions will have different
addresses in case the inline expansion is not performed. This may cause
problems if the address of the function is examined in programs. To
avoid this problem, use -D_XPG5_SEND_USER_DEFINED compile option to
disable the static inline definition of send in <sys/socket.h>, and
define a user defined function in the following way:
ssize_t
send(int _s, void *_buf, size_t _len, int _flags)
{
return(_xpg5_send(_s, _buf, _len, _flags));
}
Similarly use -D_XPG5_SENDTO_USER_DEFINED or -D_XPG5_SENDMSG_USER_DEFINED
compile option to disable the xpg5 static inline definition of sendto or
sendmsg in <sys/socket.h>
Use the appropriate compile option always, when a user defined XPG5
function is required.
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