Messages in this thread | | | From | Jeff Moyer <> | Date | Wed, 6 Oct 2004 09:13:38 -0400 | Subject | Re: [patch rfc] towards supporting O_NONBLOCK on regular files |
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==> Regarding Re: [patch rfc] towards supporting O_NONBLOCK on regular files; Marcelo Tosatti <marcelo.tosatti@cyclades.com> adds:
marcelo.tosatti> On Fri, Oct 01, 2004 at 04:57:50PM -0400, Jeff Moyer marcelo.tosatti> wrote: >> This patch makes an attempt at supporting the O_NONBLOCK flag for >> regular files. It's pretty straight-forward. One limitation is that we >> still call into the readahead code, which I believe can block. However, >> if we don't do this, then an application which only uses non-blocking >> reads may never get it's data. >> >> Comments welcome.
marcelo.tosatti> Hi Jeff,
marcelo.tosatti> Curiosity: Is this defined in any UNIX standard?
marcelo.tosatti> Adv. Programming in the UNIX environment says:
marcelo.tosatti> 12.2 Nonblocking I/O
marcelo.tosatti> "Nonblocking I/O lets us issue an I/O operation, such as marcelo.tosatti> open, read, or write and not have it block forever. If the marcelo.tosatti> operation cannot be completed, return is made immediately marcelo.tosatti> with an error noting that the operation would have marcelo.tosatti> blocked."
marcelo.tosatti> He is talking about read's on descriptors (pipe's, marcelo.tosatti> devices, etc) which block in case of no data present, not marcelo.tosatti> about filesystem IO.
marcelo.tosatti> But here we create our own semantics of O_NONBLOCK on marcelo.tosatti> read() of fs IO. As you say page_cache_readahead can block marcelo.tosatti> for one trying to allocate pages, possibly while marcelo.tosatti> submitting IO too.
marcelo.tosatti> The patch is cool - might be nice to check if SuS or marcelo.tosatti> someone else specificies behaviour and try to match if so?
Hi, Marcelo,
The text below was taken from the following standard:
IEEE Std 1003.1, 2004 Edition The Open Group Technical Standard Base Sepcifications, Issue 6 Includes IEEE Std 1003.1-2001, IEEE Std 1003.1-2001/Cor 1-2002
open()
O_NONBLOCK When opening a FIFO with O_RDONLY or O_WRONLY set: o If O_NONBLOCK is set, an open( ) for reading-only shall return without delay. An open( ) for writing-only shall return an error if no process currently has the file open for reading. o If O_NONBLOCK is clear, an open( ) for reading-only shall block the calling thread until a thread opens the file for writing. An open( ) for writing-only shall block the calling thread until a thread opens the file for reading.
When opening a block special or character special file that supports non- blocking opens: o If O_NONBLOCK is set, the open( ) function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific. o If O_NONBLOCK is clear, the open( ) function shall block the calling thread until the device is ready or available before returning.
Otherwise, the behavior of O_NONBLOCK is unspecified.
read()
When attempting to read a file (other than a pipe or FIFO) that supports non-blocking reads and has no data currently available: o If O_NONBLOCK is set, read( ) shall return -1 and set errno to [EAGAIN]. o If O_NONBLOCK is clear, read( ) shall block the calling thread until some data becomes available. o The use of the O_NONBLOCK flag has no effect if there is some data available.
write()
When attempting to write to a file descriptor (other than a pipe or FIFO) that supports non- blocking writes and cannot accept the data immediately: o If the O_NONBLOCK flag is clear, write( ) shall block the calling thread until the data can be accepted. o If the O_NONBLOCK flag is set, write( ) shall not block the thread. If some data can be written without blocking the thread, write( ) shall write what it can and return the number of bytes written. Otherwise, it shall return -1 and set errno to [EAGAIN].
Thanks!
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