I'm using Linux 2.6.38 (fc14). What is the ioctl flag to get the amount of free space on a socket file descriptor (say, a TCP socket)? I found NetBSD has FIONREAD, FIONWRITE and FIONSPACE for such related purposes. But, I could only use FIONREAD in Linux.
SIOCOUTQ is the Linux equivalent of FIONWRITE. I don't believe there is a direct FIONSPACE equivalent: instead, you can subtract the value returned by SIOCOUTQ from the socket send buffer size, which can be obtained with getsockopt(s, SOL_SOCKET, SO_SNDBUF, ...).
For information, about what #HKK says, found in man socket(7):
SO_SNDBUF
Sets or gets the maximum socket send buffer in bytes. The
kernel doubles this value (to allow space for bookkeeping overhead)
when it is set using setsockopt(2), and this doubled value is
returned by getsockopt(2). The default value is set by the
/proc/sys/net/core/wmem_default file and the maximum allowed value is
set by the /proc/sys/net/core/wmem_max file. The minimum (doubled)
value for this option is 2048.
Related
If I use write in this way: write (fd, buf, 10000000 /* 10MB */) where fd is a socket and uses blocking I/O, will the kernel tries to flush as many bytes as possible so that only one call is enough? Or I have to call write several times according to its return value? If that happens, does it mean something is wrong with fd?
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Thanks for all the answers. Furthermore, if I put fd into poll and it returns successfully with POLLOUT, so call to write cannot be blocked and writes all the data unless something is wrong with fd?
In blocking mode, write(2) will only return if specified number of bytes are written. If it can not write it'll wait.
In non-blocking (O_NONBLOCK) mode it'll not wait. It'll return right then. If it can write all of them it'll be a success other wise it'll set errno accordingly. Then you have check the errno if its EWOULDBLOCK or EAGAIN you have to invoke same write agian.
From manual of write(2)
The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource
limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).)
So yes, there can be something wrong with fd.
Also note this
A successful return from write() does not make any guarantee that data has been committed to disk. In fact, on some buggy implementations, it does not even guar‐
antee that space has successfully been reserved for the data. The only way to be sure is to call fsync(2) after you are done writing all your data.
/etc/sysctl.conf is used in Linux to set parameters for the TCP protocol, which is what I assume you mean by a socket. There may be a lot of parameters there, but when you dig through it, basically there is a limit to the amount of data the TCP buffers can hold at one time.
So if you tried to write 10 MB of data at one go, write would return a ssize_t value equal to that value. Always check the return value of the write() system call. If the system allowed 10MB then write would return that value.
The value is
net.core.wmem_max = [some number]
If you change some number to a value large enough to allow 10MB you can write that much. DON'T do that! You could cause other problems. Research settings before you do anything. Changing settings can decrease performance. Be careful.
http://linux.die.net/man/7/tcp
has basic C information for TCP settings. Also check out /proc/sys/net on your box.
One other point - TCP is a two way door, so just because you can send a zillion bytes at one time does not mean the other side can read it or even handle it. You socket may just block for a while. And possibly your write() return value may be less than you hoped for.
To see how well I'm doing in processing incoming data, I'd like to measure the queue length at my TCP and UDP sockets.
I know that I can get the queue size via SO_RCVBUF socket option, and that ioctl(<sockfd>, SIOCINQ, &<some_int>) tells me the information for TCP sockets. But for UDP the SIOCINQ/FIONREAD ioctl returns only the size of next pending datagram. Is there a way how to get queue size for UDP, without having to parse system tables such as /proc/net/udp?
FWIW, I did some experiments to map out the behavior of FIONREAD on different platforms.
Platforms where FIONREAD returns all the data pending in a SOCK_DGRAM socket:
Mac OS X, NetBSD, FreeBSD, Solaris, HP-UX, AIX, Windows
Platforms where FIONREAD returns only the bytes for the first pending datagram:
Linux
It might also be worth noting that some implementations include headers or other overhead bytes in the count, while others only count the payload bytes. Linux appears to return the payload size, not including IP headers.
As ldx mentioned, it is not supported through ioctl or getsockopt.
It seems to me that the current implementation of SIOCINQ was aimed to determine how much buffer is needed to read the entire waiting buffer (but I guess it is not so useful for that, as it can change between the read of it to the actual buffer read).
There are many other telemetries which are not supported though such system calls, I guess there is no real need in normal production usage.
You can check the drops/errors through "netstat -su" , or better using SNMP (udpInErrors) if you just want to monitor the machine state.
BTW: You always have the option to hack in the Kernel code and add this value (or others).
Is there any simple functions to check how much data is buffered but unread? FD_ISSET only indicates the presence of data in the buffer. Is possible not to create a second buffer in the program for greater control of buffer?
You could use recv() with the MSG_PEEK and MSG_DONTWAIT flags, but there's no firm guarantee that there aren't more bytes available than recv() returned in that case.
Using a buffer within your program is the normal and accepted way to solve the problem.
I am trying to send text data from one PC to other using Serial cable. One of the PC is running linux and I am sending data from it using write(2) system call. The log size is approx 65K bytes but the write(2) system call returns some 4K bytes (i.e. this much amount of data is getting transferred). I tried breaking the data in chunks of 4K but write(2) returns -1.
My question is that "Is there any buffer limit for writing data on serial port? or can I send data of any size?. Also do I need to continously read data from other PC as I write 4K chunk of data"
Do I need to do any special configuration in termios structure for sending (huge) data?
The transmit buffer is one page (took a look at Linux 2.6.18 sources) - which is 4K in most (if not all) cases.
The other end must read (don't know the size of the receive buffer), but more importantly you should not write faster than the serial port can transmit, if you are using 115200 bps 8-N-1 you can write the 4K chunk approximately 3 times a second. (115200 / 9 / 4096 = 3.125)
Yes, there is a buffer limit - but when you reach that limit, the write() should block.
When write() returns -1, what is errno set to?
Make sure that the receiver is reading.
You should update the current position it your buffer from the write(), and continue the next write from there. (Applies to all writes(), regardless if the fd is a serial port, tcp socket or a file.)
If you get an error back for subsequent writes. Judging by the manpage, its safe to retry the writes for the following errnos: EAGAIN, EINTR, and probably ENOSPC. Use perror() to see what you get. (..and post it, I am curious.)
EFBIG would seem to indicate that you are trying to write using a buffer (or rather count) that is too large, but that is probably much larger than 64k.
If the internal buffer is filled up, because you are writing to fast, try to (nano)sleep a little between the writes. There are several clever ways of doing this (like tcp does), but if the rate is known, just write at a fixed rate.
If you think the receiver is actually reading, but not much happens, have a look at the serial ports flow-control options and if the cable is wired for DTS/RTS.
In Linux, one can specify the system's default receive buffer size for network packets, say UDP, using the following commands:
sysctl -w net.core.rmem_max=<value>
sysctl -w net.core.rmem_default=<value>
But I wonder, is it possible for an application (say, in c) to override system's defaults by specifying the receive buffer size per UDP socket in runtime?
You can increase the value from the default, but you can't increase it beyond the maximum value. Use setsockopt to change the SO_RCVBUF option:
int n = 1024 * 1024;
if (setsockopt(socket, SOL_SOCKET, SO_RCVBUF, &n, sizeof(n)) == -1) {
// deal with failure, or ignore if you can live with the default size
}
Note that this is the portable solution; it should work on any POSIX platform for increasing the receive buffer size. Linux has had autotuning for a while now (since 2.6.7, and with reasonable maximum buffer sizes since 2.6.17), which automatically adjusts the receive buffer size based on load. On kernels with autotuning, it is recommended that you not set the receive buffer size using setsockopt, as that will disable the kernel's autotuning. Using setsockopt to adjust the buffer size may still be necessary on other platforms, however.