06-03-2011
Quote:
Originally Posted by
expl
Sockets are also not available on EVERY system, does not mean you should not use them. The option is available on Windows, Linux and BSDs, haven't checked more.
Go check the
POSIX specification for socket's options. The STANDARD said so.
<snip>
I apologize for my rudeness.
You're forgiven.
SO_RCVTIMEO has been standardized with the Single Unix Specification ("aka Unix98"). The POSIX standard that deals with socket is IEEE Std 1003.1g-2000 ("aka Posix.1g") and AFAICS setting timeout on socket is not mandatory.
A decade ago, select() was considered the most portable way to achieve timeout; as there were many unix-like systems that had sockets, but not SO_RCVTIMEO. Now 13 years after Unix98, I expect most modern system to support this option. Even if you don't work on a "archaic system", you may find this pattern in older code.
Beside this point, there might be situation where select() - or rather the modern version poll() - makes more sense. Like: variable timeout depending on the expected answer length, when the polymorphic aspect of the descriptor is used...
I checked the Linux/FreeBSD kernel source. For Linux, you're right: recv() call the poll() syscall internally. For FreeBSD, both select() and recv() are built on the tsleep() syscall.
Cheers, Loïc
Last edited by Loic Domaigne; 06-03-2011 at 06:51 AM..
Reason: grammar
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LEARN ABOUT REDHAT
sendmsg
SEND(2) Linux Programmer's Manual SEND(2)
NAME
send, sendto, sendmsg - send a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int send(int s, const void *msg, size_t len, int flags);
int sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);
int sendmsg(int s, const struct msghdr *msg, int flags);
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 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.
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 data.
The flags parameter is a flagword and can contain the following flags:
MSG_OOB
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.
MSG_DONTROUTE
Dont't use a gateway to send out the packet, only send to hosts on directly connected networks. This is usually used only by diag-
nostic or routing programs. This is only defined for protocol families that route; packet sockets don't.
MSG_DONTWAIT
Enables non-blocking operation; if the operation would block, EAGAIN is returned (this can also be enabled using the O_NONBLOCK with
the F_SETFL fcntl(2)).
MSG_NOSIGNAL
Requests not to send SIGPIPE on errors on stream oriented sockets when the other end breaks the connection. The EPIPE error is still
returned.
MSG_CONFIRM (Linux 2.3+ only)
Tell the link layer that forward process happened: you got a successful reply from the other side. If the link layer doesn't get
this it'll regularly reprobe the neighbour (e.g. via a unicast ARP). Only valid on SOCK_DGRAM and SOCK_RAW sockets and currently
only implemented for IPv4 and IPv6. See arp(7) for details.
The definition of the msghdr structure follows. See recv(2) and below for an exact description of its fields.
struct msghdr {
void * msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec * msg_iov; /* scatter/gather array */
size_t msg_iovlen; /* # elements in msg_iov */
void * msg_control; /* ancillary data, see below */
socklen_t msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
You may send control information using the msg_control and msg_controllen members. The maximum control buffer length the kernel can process
is limited per socket by the net.core.optmem_max sysctl; see socket(7).
RETURN VALUE
The calls return the number of characters sent, or -1 if an error occurred.
ERRORS
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 manual pages.
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
The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
EAGAIN or EWOULDBLOCK
The socket is marked non-blocking and the requested operation would block.
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. (This cannot occur in Linux, packets are just silently dropped when a device queue overflows.)
EINTR A signal occurred.
ENOMEM No memory available.
EINVAL Invalid argument passed.
EPIPE The local end has been shut down on a connection oriented socket. In this case the process will also receive a SIGPIPE unless
MSG_NOSIGNAL is set.
CONFORMING TO
4.4BSD, SVr4, POSIX 1003.1g draft (these function calls appeared in 4.2BSD).
MSG_CONFIRM is a Linux extension.
NOTE
The prototypes given above follow the Single Unix Specification, as glibc2 also does; the flags argument was `int' in BSD 4.*, but
`unsigned int' in libc4 and libc5; the len argument was `int' in BSD 4.* and libc4, but `size_t' in libc5; the tolen argument was `int' in
BSD 4.* and libc4 and libc5. See also accept(2).
SEE ALSO
fcntl(2), recv(2), select(2), getsockopt(2), sendfile(2), socket(2), write(2), socket(7), ip(7), tcp(7), udp(7)
Linux Man Page 1999-07 SEND(2)