CODEWITHSUNDEEP
linuxnetwork-programming
old_bear
old_bear

Reputation: 399

linux write(): does it try to write as many bytes as possible?

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?

============================== EDITED ================================

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?

Upvotes: 1

Views: 864

Answers (2)

jim mcnamara
jim mcnamara

Reputation: 16379

/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.

Upvotes: 0

Shiplu Mokaddim
Shiplu Mokaddim

Reputation: 57650

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.

Upvotes: 3

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