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Revision 1.23 by root, Wed Jul 13 21:31:40 2011 UTC vs.
Revision 1.30 by root, Wed Jan 11 05:13:22 2012 UTC

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45Unlike the name component C<stamp> might indicate, it is also used for 45Unlike the name component C<stamp> might indicate, it is also used for
46time differences throughout libeio. 46time differences throughout libeio.
47 47
48=head2 FORK SUPPORT 48=head2 FORK SUPPORT
49 49
50Calling C<fork ()> is fully supported by this module - but you must not 50Usage of pthreads in a program changes the semantics of fork
51rely on this. It is currently implemented in these steps: 51considerably. Specifically, only async-safe functions can be called after
52fork. Libeio uses pthreads, so this applies, and makes using fork hard for
53anything but relatively fork + exec uses.
52 54
53 1. wait till all requests in "execute" state have been handled 55This library only works in the process that initialised it: Forking is
54 (basically requests that are already handed over to the kernel). 56fully supported, but using libeio in any other process than the one that
55 2. fork 57called C<eio_init> is not.
56 3. in the parent, continue business as usual, done
57 4. in the child, destroy all ready and pending requests and free the
58 memory used by the worker threads. This gives you a fully empty
59 libeio queue.
60 58
61Note, however, since libeio does use threads, the above guarantee doesn't 59You might get around by not I<using> libeio before (or after) forking in
62cover your libc, for example, malloc and other libc functions are not 60the parent, and using it in the child afterwards. You could also try to
63fork-safe, so there is very little you can do after a fork, and in fact, 61call the L<eio_init> function again in the child, which will brutally
64the above might crash, and thus change. 62reinitialise all data structures, which isn't POSIX conformant, but
63typically works.
64
65Otherwise, the only recommendation you should follow is: treat fork code
66the same way you treat signal handlers, and only ever call C<eio_init> in
67the process that uses it, and only once ever.
65 68
66=head1 INITIALISATION/INTEGRATION 69=head1 INITIALISATION/INTEGRATION
67 70
68Before you can call any eio functions you first have to initialise the 71Before you can call any eio functions you first have to initialise the
69library. The library integrates into any event loop, but can also be used 72library. The library integrates into any event loop, but can also be used
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78This function initialises the library. On success it returns C<0>, on 81This function initialises the library. On success it returns C<0>, on
79failure it returns C<-1> and sets C<errno> appropriately. 82failure it returns C<-1> and sets C<errno> appropriately.
80 83
81It accepts two function pointers specifying callbacks as argument, both of 84It accepts two function pointers specifying callbacks as argument, both of
82which can be C<0>, in which case the callback isn't called. 85which can be C<0>, in which case the callback isn't called.
86
87There is currently no way to change these callbacks later, or to
88"uninitialise" the library again.
83 89
84=item want_poll callback 90=item want_poll callback
85 91
86The C<want_poll> callback is invoked whenever libeio wants attention (i.e. 92The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
87it wants to be polled by calling C<eio_poll>). It is "edge-triggered", 93it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
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227 233
228The C<void *data> member simply stores the value of the C<data> argument. 234The C<void *data> member simply stores the value of the C<data> argument.
229 235
230=back 236=back
231 237
238Members not explicitly described as accessible must not be
239accessed. Specifically, there is no guarantee that any members will still
240have the value they had when the request was submitted.
241
232The return value of the callback is normally C<0>, which tells libeio to 242The return value of the callback is normally C<0>, which tells libeio to
233continue normally. If a callback returns a nonzero value, libeio will 243continue normally. If a callback returns a nonzero value, libeio will
234stop processing results (in C<eio_poll>) and will return the value to its 244stop processing results (in C<eio_poll>) and will return the value to its
235caller. 245caller.
236 246
237Memory areas passed to libeio must stay valid as long as a request 247Memory areas passed to libeio wrappers must stay valid as long as a
238executes, with the exception of paths, which are being copied 248request executes, with the exception of paths, which are being copied
239internally. Any memory libeio itself allocates will be freed after the 249internally. Any memory libeio itself allocates will be freed after the
240finish callback has been called. If you want to manage all memory passed 250finish callback has been called. If you want to manage all memory passed
241to libeio yourself you can use the low-level API. 251to libeio yourself you can use the low-level API.
242 252
243For example, to open a file, you could do this: 253For example, to open a file, you could do this:
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586=item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data) 596=item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data)
587 597
588Calls C<readahead(2)>. If the syscall is missing, then the call is 598Calls C<readahead(2)>. If the syscall is missing, then the call is
589emulated by simply reading the data (currently in 64kiB chunks). 599emulated by simply reading the data (currently in 64kiB chunks).
590 600
601=item eio_syncfs (int fd, int pri, eio_cb cb, void *data)
602
603Calls Linux' C<syncfs> syscall, if available. Returns C<-1> and sets
604C<errno> to C<ENOSYS> if the call is missing I<but still calls sync()>,
605if the C<fd> is C<< >= 0 >>, so you can probe for the availability of the
606syscall with a negative C<fd> argument and checking for C<-1/ENOSYS>.
607
591=item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data) 608=item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data)
592 609
593Calls C<sync_file_range>. If the syscall is missing, then this is the same 610Calls C<sync_file_range>. If the syscall is missing, then this is the same
594as calling C<fdatasync>. 611as calling C<fdatasync>.
595 612
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707=head4 GROUP REQUEST LIFETIME 724=head4 GROUP REQUEST LIFETIME
708 725
709Left alone, a group request will instantly move to the pending state and 726Left alone, a group request will instantly move to the pending state and
710will be finished at the next call of C<eio_poll>. 727will be finished at the next call of C<eio_poll>.
711 728
712There usefulness stems from the fact that, if a subrequest is added to a 729The usefulness stems from the fact that, if a subrequest is added to a
713group I<before> a call to C<eio_poll>, via C<eio_grp_add>, then the group 730group I<before> a call to C<eio_poll>, via C<eio_grp_add>, then the group
714will not finish until all the subrequests have finished. 731will not finish until all the subrequests have finished.
715 732
716So the usage cycle of a group request is like this: after it is created, 733So the usage cycle of a group request is like this: after it is created,
717you normally instantly add a subrequest. If none is added, the group 734you normally instantly add a subrequest. If none is added, the group
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738can return as "the load request". 755can return as "the load request".
739 756
740=item 2) open the file, maybe 757=item 2) open the file, maybe
741 758
742Next, open the file with C<eio_open> and add the request to the group 759Next, open the file with C<eio_open> and add the request to the group
743request and you are finished steting up the request. 760request and you are finished setting up the request.
744 761
745If, for some reason, you cannot C<eio_open> (path is a null ptr?) you 762If, for some reason, you cannot C<eio_open> (path is a null ptr?) you
746cna set C<< grp->result >> to C<-1> to signal an error and let the gorup 763can set C<< grp->result >> to C<-1> to signal an error and let the group
747request finish on its own. 764request finish on its own.
748 765
749=item 3) open callback adds more requests 766=item 3) open callback adds more requests
750 767
751In the open callback, if the open was not successful, copy C<< 768In the open callback, if the open was not successful, copy C<<
752req->errorno >> to C<< grp->errorno >> and set C<< grp->errorno >> to 769req->errorno >> to C<< grp->errorno >> and set C<< grp->result >> to
753C<-1> to signal an error. 770C<-1> to signal an error.
754 771
755Otherwise, malloc some memory or so and issue a read request, adding the 772Otherwise, malloc some memory or so and issue a read request, adding the
756read request to the group. 773read request to the group.
757 774
758=item 4) continue issuign requests till finished 775=item 4) continue issuing requests till finished
759 776
760In the real callback, check for errors and possibly continue with 777In the read callback, check for errors and possibly continue with
761C<eio_close> or any other eio request in the same way. 778C<eio_close> or any other eio request in the same way.
762 779
763As soon as no new requests are added the group request will finish. Make 780As soon as no new requests are added, the group request will finish. Make
764sure you I<always> set C<< grp->result >> to some sensible value. 781sure you I<always> set C<< grp->result >> to some sensible value.
765 782
766=back 783=back
767 784
768=head4 REQUEST LIMITING 785=head4 REQUEST LIMITING
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909This symbol governs the stack size for each eio thread. Libeio itself 926This symbol governs the stack size for each eio thread. Libeio itself
910was written to use very little stackspace, but when using C<EIO_CUSTOM> 927was written to use very little stackspace, but when using C<EIO_CUSTOM>
911requests, you might want to increase this. 928requests, you might want to increase this.
912 929
913If this symbol is undefined (the default) then libeio will use its default 930If this symbol is undefined (the default) then libeio will use its default
914stack size (C<sizeof (long) * 4096> currently). If it is defined, but 931stack size (C<sizeof (void *) * 4096> currently). If it is defined, but
915C<0>, then the default operating system stack size will be used. In all 932C<0>, then the default operating system stack size will be used. In all
916other cases, the value must be an expression that evaluates to the desired 933other cases, the value must be an expression that evaluates to the desired
917stack size. 934stack size.
918 935
919=back 936=back

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