[ViewVC] Diff of: cvs/libeio/eio.pod

Comparing libeio/eio.pod (file contents):
Revision 1.11 by root, Mon Jun 20 07:28:15 2011 UTC vs.
Revision 1.36 by root, Sun Jan 24 16:36:20 2016 UTC

…		…
25	similar functions, as well as less rarely ones such as C<mknod>, C<futime>	25	similar functions, as well as less rarely ones such as C<mknod>, C<futime>
26	or C<readlink>.	26	or C<readlink>.
27		27
28	It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and	28	It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
29	FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with	29	FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
30	emulation elsewhere>).	30	emulation elsewhere).
31		31
32	The goal is to enable you to write fully non-blocking programs. For	32	The goal is to enable you to write fully non-blocking programs. For
33	example, in a game server, you would not want to freeze for a few seconds	33	example, in a game server, you would not want to freeze for a few seconds
34	just because the server is running a backup and you happen to call	34	just because the server is running a backup and you happen to call
35	C<readdir>.	35	C<readdir>.
…		…
45	Unlike the name component C<stamp> might indicate, it is also used for	45	Unlike the name component C<stamp> might indicate, it is also used for
46	time differences throughout libeio.	46	time differences throughout libeio.
47		47
48	=head2 FORK SUPPORT	48	=head2 FORK SUPPORT
49		49
50	Calling C<fork ()> is fully supported by this module. It is implemented in these steps:	50	Usage of pthreads in a program changes the semantics of fork
		51	considerably. Specifically, only async-safe functions can be called after
		52	fork. Libeio uses pthreads, so this applies, and makes using fork hard for
		53	anything but relatively fork + exec uses.
51		54
52	1. wait till all requests in "execute" state have been handled	55	This library only works in the process that initialised it: Forking is
53	(basically requests that are already handed over to the kernel).	56	fully supported, but using libeio in any other process than the one that
54	2. fork	57	called C<eio_init> is not.
55	3. in the parent, continue business as usual, done
56	4. in the child, destroy all ready and pending requests and free the
57	memory used by the worker threads. This gives you a fully empty
58	libeio queue.
59		58
60	Note, however, since libeio does use threads, thr above guarantee doesn't	59	You might get around by not I<using> libeio before (or after) forking in
61	cover your libc, for example, malloc and other libc functions are not	60	the parent, and using it in the child afterwards. You could also try to
62	fork-safe, so there is very little you can do after a fork, and in fatc,	61	call the L<eio_init> function again in the child, which will brutally
63	the above might crash, and thus change.	62	reinitialise all data structures, which isn't POSIX conformant, but
		63	typically works.
		64
		65	Otherwise, the only recommendation you should follow is: treat fork code
		66	the same way you treat signal handlers, and only ever call C<eio_init> in
		67	the process that uses it, and only once ever.
64		68
65	=head1 INITIALISATION/INTEGRATION	69	=head1 INITIALISATION/INTEGRATION
66		70
67	Before you can call any eio functions you first have to initialise the	71	Before you can call any eio functions you first have to initialise the
68	library. The library integrates into any event loop, but can also be used	72	library. The library integrates into any event loop, but can also be used
…		…
77	This function initialises the library. On success it returns C<0>, on	81	This function initialises the library. On success it returns C<0>, on
78	failure it returns C<-1> and sets C<errno> appropriately.	82	failure it returns C<-1> and sets C<errno> appropriately.
79		83
80	It accepts two function pointers specifying callbacks as argument, both of	84	It accepts two function pointers specifying callbacks as argument, both of
81	which can be C<0>, in which case the callback isn't called.	85	which can be C<0>, in which case the callback isn't called.
		86
		87	There is currently no way to change these callbacks later, or to
		88	"uninitialise" the library again.
82		89
83	=item want_poll callback	90	=item want_poll callback
84		91
85	The C<want_poll> callback is invoked whenever libeio wants attention (i.e.	92	The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
86	it wants to be polled by calling C<eio_poll>). It is "edge-triggered",	93	it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
…		…
97	This callback is invoked when libeio detects that all pending requests	104	This callback is invoked when libeio detects that all pending requests
98	have been handled. It is "edge-triggered", that is, it will only be	105	have been handled. It is "edge-triggered", that is, it will only be
99	called once after C<want_poll>. To put it differently, C<want_poll> and	106	called once after C<want_poll>. To put it differently, C<want_poll> and
100	C<done_poll> are invoked in pairs: after C<want_poll> you have to call	107	C<done_poll> are invoked in pairs: after C<want_poll> you have to call
101	C<eio_poll ()> until either C<eio_poll> indicates that everything has been	108	C<eio_poll ()> until either C<eio_poll> indicates that everything has been
102	handled or C<done_poll> has been called, which signals the same.	109	handled or C<done_poll> has been called, which signals the same - only one
		110	method is needed.
103		111
104	Note that C<eio_poll> might return after C<done_poll> and C<want_poll>	112	Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
105	have been called again, so watch out for races in your code.	113	have been called again, so watch out for races in your code.
106		114
		115	It is quite common to have an empty C<done_call> callback and only use
		116	the return value from C<eio_poll>, or, when C<eio_poll> is configured to
		117	handle all outstanding replies, it's enough to call C<eio_poll> once.
		118
107	As with C<want_poll>, this callback is called while locks are being held,	119	As with C<want_poll>, this callback is called while locks are being held,
108	so you I<must not call any libeio functions form within this callback>.	120	so you I<must not call any libeio functions from within this callback>.
109		121
110	=item int eio_poll ()	122	=item int eio_poll ()
111		123
112	This function has to be called whenever there are pending requests that	124	This function has to be called whenever there are pending requests that
113	need finishing. You usually call this after C<want_poll> has indicated	125	need finishing. You usually call this after C<want_poll> has indicated
…		…
124	=back	136	=back
125		137
126	For libev, you would typically use an C<ev_async> watcher: the	138	For libev, you would typically use an C<ev_async> watcher: the
127	C<want_poll> callback would invoke C<ev_async_send> to wake up the event	139	C<want_poll> callback would invoke C<ev_async_send> to wake up the event
128	loop. Inside the callback set for the watcher, one would call C<eio_poll	140	loop. Inside the callback set for the watcher, one would call C<eio_poll
129	()> (followed by C<ev_async_send> again if C<eio_poll> indicates that not	141	()>.
130	all requests have been handled yet). The race is taken care of because	142
131	libev resets/rearms the async watcher before calling your callback,	143	If C<eio_poll ()> is configured to not handle all results in one go
132	and therefore, before calling C<eio_poll>. This might result in (some)	144	(i.e. it returns C<-1>) then you should start an idle watcher that calls
133	spurious wake-ups, but is generally harmless.	145	C<eio_poll> until it returns something C<!= -1>.
		146
		147	A full-featured connector between libeio and libev would look as follows
		148	(if C<eio_poll> is handling all requests, it can of course be simplified a
		149	lot by removing the idle watcher logic):
		150
		151	static struct ev_loop *loop;
		152	static ev_idle repeat_watcher;
		153	static ev_async ready_watcher;
		154
		155	/* idle watcher callback, only used when eio_poll */
		156	/* didn't handle all results in one call */
		157	static void
		158	repeat (EV_P_ ev_idle *w, int revents)
		159	{
		160	if (eio_poll () != -1)
		161	ev_idle_stop (EV_A_ w);
		162	}
		163
		164	/* eio has some results, process them */
		165	static void
		166	ready (EV_P_ ev_async *w, int revents)
		167	{
		168	if (eio_poll () == -1)
		169	ev_idle_start (EV_A_ &repeat_watcher);
		170	}
		171
		172	/* wake up the event loop */
		173	static void
		174	want_poll (void)
		175	{
		176	ev_async_send (loop, &ready_watcher)
		177	}
		178
		179	void
		180	my_init_eio ()
		181	{
		182	loop = EV_DEFAULT;
		183
		184	ev_idle_init (&repeat_watcher, repeat);
		185	ev_async_init (&ready_watcher, ready);
		186	ev_async_start (loop, &watcher);
		187
		188	eio_init (want_poll, 0);
		189	}
134		190
135	For most other event loops, you would typically use a pipe - the event	191	For most other event loops, you would typically use a pipe - the event
136	loop should be told to wait for read readiness on the read end. In	192	loop should be told to wait for read readiness on the read end. In
137	C<want_poll> you would write a single byte, in C<done_poll> you would try	193	C<want_poll> you would write a single byte, in C<done_poll> you would try
138	to read that byte, and in the callback for the read end, you would call	194	to read that byte, and in the callback for the read end, you would call
139	C<eio_poll>. The race is avoided here because the event loop should invoke	195	C<eio_poll>.
140	your callback again and again until the byte has been read (as the pipe	196
141	read callback does not read it, only C<done_poll>).	197	You don't have to take special care in the case C<eio_poll> doesn't handle
		198	all requests, as the done callback will not be invoked, so the event loop
		199	will still signal readiness for the pipe until I<all> results have been
		200	processed.
142		201
143		202
144	=head1 HIGH LEVEL REQUEST API	203	=head1 HIGH LEVEL REQUEST API
145		204
146	Libeio has both a high-level API, which consists of calling a request	205	Libeio has both a high-level API, which consists of calling a request
…		…
153		212
154	You submit a request by calling the relevant C<eio_TYPE> function with the	213	You submit a request by calling the relevant C<eio_TYPE> function with the
155	required parameters, a callback of type C<int (eio_cb)(eio_req req)>	214	required parameters, a callback of type C<int (eio_cb)(eio_req req)>
156	(called C<eio_cb> below) and a freely usable C<void *data> argument.	215	(called C<eio_cb> below) and a freely usable C<void *data> argument.
157		216
158	The return value will either be 0	217	The return value will either be 0, in case something went really wrong
		218	(which can basically only happen on very fatal errors, such as C<malloc>
		219	returning 0, which is rather unlikely), or a pointer to the newly-created
		220	and submitted C<eio_req *>.
159		221
160	The callback will be called with an C<eio_req *> which contains the	222	The callback will be called with an C<eio_req *> which contains the
161	results of the request. The members you can access inside that structure	223	results of the request. The members you can access inside that structure
162	vary from request to request, except for:	224	vary from request to request, except for:
163		225
…		…
175	=item C<void *data>	237	=item C<void *data>
176		238
177	The C<void *data> member simply stores the value of the C<data> argument.	239	The C<void *data> member simply stores the value of the C<data> argument.
178		240
179	=back	241	=back
		242
		243	Members not explicitly described as accessible must not be
		244	accessed. Specifically, there is no guarantee that any members will still
		245	have the value they had when the request was submitted.
180		246
181	The return value of the callback is normally C<0>, which tells libeio to	247	The return value of the callback is normally C<0>, which tells libeio to
182	continue normally. If a callback returns a nonzero value, libeio will	248	continue normally. If a callback returns a nonzero value, libeio will
183	stop processing results (in C<eio_poll>) and will return the value to its	249	stop processing results (in C<eio_poll>) and will return the value to its
184	caller.	250	caller.
185		251
186	Memory areas passed to libeio must stay valid as long as a request	252	Memory areas passed to libeio wrappers must stay valid as long as a
187	executes, with the exception of paths, which are being copied	253	request executes, with the exception of paths, which are being copied
188	internally. Any memory libeio itself allocates will be freed after the	254	internally. Any memory libeio itself allocates will be freed after the
189	finish callback has been called. If you want to manage all memory passed	255	finish callback has been called. If you want to manage all memory passed
190	to libeio yourself you can use the low-level API.	256	to libeio yourself you can use the low-level API.
191		257
192	For example, to open a file, you could do this:	258	For example, to open a file, you could do this:
…		…
210	}	276	}
211		277
212	/* the first three arguments are passed to open(2) */	278	/* the first three arguments are passed to open(2) */
213	/* the remaining are priority, callback and data */	279	/* the remaining are priority, callback and data */
214	if (!eio_open ("/etc/passwd", O_RDONLY, 0, 0, file_open_done, 0))	280	if (!eio_open ("/etc/passwd", O_RDONLY, 0, 0, file_open_done, 0))
215	abort (); /* something ent wrong, we will all die!!! */	281	abort (); /* something went wrong, we will all die!!! */
216		282
217	Note that you additionally need to call C<eio_poll> when the C<want_cb>	283	Note that you additionally need to call C<eio_poll> when the C<want_cb>
218	indicates that requests are ready to be processed.	284	indicates that requests are ready to be processed.
		285
		286	=head2 CANCELLING REQUESTS
		287
		288	Sometimes the need for a request goes away before the request is
		289	finished. In that case, one can cancel the request by a call to
		290	C<eio_cancel>:
		291
		292	=over 4
		293
		294	=item eio_cancel (eio_req *req)
		295
		296	Cancel the request (and all its subrequests). If the request is currently
		297	executing it might still continue to execute, and in other cases it might
		298	still take a while till the request is cancelled.
		299
		300	When cancelled, the finish callback will not be invoked.
		301
		302	C<EIO_CANCELLED> is still true for requests that have successfully
		303	executed, as long as C<eio_cancel> was called on them at some point.
		304
		305	=back
219		306
220	=head2 AVAILABLE REQUESTS	307	=head2 AVAILABLE REQUESTS
221		308
222	The following request functions are available. I<All> of them return the	309	The following request functions are available. I<All> of them return the
223	C<eio_req *> on success and C<0> on failure, and I<all> of them have the	310	C<eio_req *> on success and C<0> on failure, and I<all> of them have the
…		…
317	char target = strndup ((char )req->ptr2, req->result);	404	char target = strndup ((char )req->ptr2, req->result);
318		405
319	free (target);	406	free (target);
320	}	407	}
321		408
		409	=item eio_realpath (const char path, int pri, eio_cb cb, void data)
		410
		411	Similar to the realpath libc function, but unlike that one, C<<
		412	req->result >> is C<-1> on failure. On success, the result is the length
		413	of the returned path in C<ptr2> (which is I<NOT> 0-terminated) - this is
		414	similar to readlink.
		415
322	=item eio_stat (const char path, int pri, eio_cb cb, void data)	416	=item eio_stat (const char path, int pri, eio_cb cb, void data)
323		417
324	=item eio_lstat (const char path, int pri, eio_cb cb, void data)	418	=item eio_lstat (const char path, int pri, eio_cb cb, void data)
325		419
326	=item eio_fstat (int fd, int pri, eio_cb cb, void *data)	420	=item eio_fstat (int fd, int pri, eio_cb cb, void *data)
327		421
328	Stats a file - if C<< req->result >> indicates success, then you can	422	Stats a file - if C<< req->result >> indicates success, then you can
329	access the C<struct stat>-like structure via C<< req->ptr2 >>:	423	access the C<struct stat>-like structure via C<< req->ptr2 >>:
330		424
331	EIO_STRUCT_STAT statdata = (EIO_STRUCT_STAT )req->ptr2;	425	EIO_STRUCT_STAT statdata = (EIO_STRUCT_STAT )req->ptr2;
332		426
333	=item eio_statvfs (const char path, int pri, eio_cb cb, void data)	427	=item eio_statvfs (const char path, int pri, eio_cb cb, void data)
334		428
335	=item eio_fstatvfs (int fd, int pri, eio_cb cb, void *data)	429	=item eio_fstatvfs (int fd, int pri, eio_cb cb, void *data)
336		430
337	Stats a filesystem - if C<< req->result >> indicates success, then you can	431	Stats a filesystem - if C<< req->result >> indicates success, then you can
338	access the C<struct statvfs>-like structure via C<< req->ptr2 >>:	432	access the C<struct statvfs>-like structure via C<< req->ptr2 >>:
339		433
340	EIO_STRUCT_STATVFS statdata = (EIO_STRUCT_STATVFS )req->ptr2;	434	EIO_STRUCT_STATVFS statdata = (EIO_STRUCT_STATVFS )req->ptr2;
341		435
342	=back	436	=back
343		437
344	=head3 READING DIRECTORIES	438	=head3 READING DIRECTORIES
345		439
346	Reading directories sounds simple, but can be rather demanding, especially	440	Reading directories sounds simple, but can be rather demanding, especially
347	if you want to do stuff such as traversing a diretcory hierarchy or	441	if you want to do stuff such as traversing a directory hierarchy or
348	processing all files in a directory. Libeio can assist thess complex tasks	442	processing all files in a directory. Libeio can assist these complex tasks
349	with it's C<eio_readdir> call.	443	with it's C<eio_readdir> call.
350		444
351	=over 4	445	=over 4
352		446
353	=item eio_readdir (const char path, int flags, int pri, eio_cb cb, void data)	447	=item eio_readdir (const char path, int flags, int pri, eio_cb cb, void data)
…		…
385		479
386	If this flag is specified, then, in addition to the names in C<ptr2>,	480	If this flag is specified, then, in addition to the names in C<ptr2>,
387	also an array of C<struct eio_dirent> is returned, in C<ptr1>. A C<struct	481	also an array of C<struct eio_dirent> is returned, in C<ptr1>. A C<struct
388	eio_dirent> looks like this:	482	eio_dirent> looks like this:
389		483
390	struct eio_dirent	484	struct eio_dirent
391	{	485	{
392	int nameofs; /* offset of null-terminated name string in (char )req->ptr2 /	486	int nameofs; /* offset of null-terminated name string in (char )req->ptr2 /
393	unsigned short namelen; /* size of filename without trailing 0 */	487	unsigned short namelen; /* size of filename without trailing 0 */
394	unsigned char type; /* one of EIO_DT_* */	488	unsigned char type; /* one of EIO_DT_* */
395	signed char score; /* internal use */	489	signed char score; /* internal use */
396	ino_t inode; /* the inode number, if available, otherwise unspecified */	490	ino_t inode; /* the inode number, if available, otherwise unspecified */
397	};	491	};
398		492
399	The only members you normally would access are C<nameofs>, which is the	493	The only members you normally would access are C<nameofs>, which is the
400	byte-offset from C<ptr2> to the start of the name, C<namelen> and C<type>.	494	byte-offset from C<ptr2> to the start of the name, C<namelen> and C<type>.
401		495
402	C<type> can be one of:	496	C<type> can be one of:
…		…
445	When this flag is specified, then the names will be returned in an order	539	When this flag is specified, then the names will be returned in an order
446	suitable for stat()'ing each one. That is, when you plan to stat()	540	suitable for stat()'ing each one. That is, when you plan to stat()
447	all files in the given directory, then the returned order will likely	541	all files in the given directory, then the returned order will likely
448	be fastest.	542	be fastest.
449		543
450	If both this flag and C<EIO_READDIR_DIRS_FIRST> are specified, then	544	If both this flag and C<EIO_READDIR_DIRS_FIRST> are specified, then the
451	the likely dirs come first, resulting in a less optimal stat order.	545	likely directories come first, resulting in a less optimal stat order.
452		546
453	=item EIO_READDIR_FOUND_UNKNOWN	547	=item EIO_READDIR_FOUND_UNKNOWN
454		548
455	This flag should not be specified when calling C<eio_readdir>. Instead,	549	This flag should not be specified when calling C<eio_readdir>. Instead,
456	it is being set by C<eio_readdir> (you can access the C<flags> via C<<	550	it is being set by C<eio_readdir> (you can access the C<flags> via C<<
457	req->int1 >>, when any of the C<type>'s found were C<EIO_DT_UNKNOWN>. The	551	req->int1 >>, when any of the C<type>'s found were C<EIO_DT_UNKNOWN>. The
458	absense of this flag therefore indicates that all C<type>'s are known,	552	absence of this flag therefore indicates that all C<type>'s are known,
459	which can be used to speed up some algorithms.	553	which can be used to speed up some algorithms.
460		554
461	A typical use case would be to identify all subdirectories within a	555	A typical use case would be to identify all subdirectories within a
462	directory - you would ask C<eio_readdir> for C<EIO_READDIR_DIRS_FIRST>. If	556	directory - you would ask C<eio_readdir> for C<EIO_READDIR_DIRS_FIRST>. If
463	then this flag is I<NOT> set, then all the entries at the beginning of the	557	then this flag is I<NOT> set, then all the entries at the beginning of the
…		…
493	=item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data)	587	=item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data)
494		588
495	Calls C<readahead(2)>. If the syscall is missing, then the call is	589	Calls C<readahead(2)>. If the syscall is missing, then the call is
496	emulated by simply reading the data (currently in 64kiB chunks).	590	emulated by simply reading the data (currently in 64kiB chunks).
497		591
		592	=item eio_syncfs (int fd, int pri, eio_cb cb, void *data)
		593
		594	Calls Linux' C<syncfs> syscall, if available. Returns C<-1> and sets
		595	C<errno> to C<ENOSYS> if the call is missing I<but still calls sync()>,
		596	if the C<fd> is C<< >= 0 >>, so you can probe for the availability of the
		597	syscall with a negative C<fd> argument and checking for C<-1/ENOSYS>.
		598
498	=item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data)	599	=item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data)
499		600
500	Calls C<sync_file_range>. If the syscall is missing, then this is the same	601	Calls C<sync_file_range>. If the syscall is missing, then this is the same
501	as calling C<fdatasync>.	602	as calling C<fdatasync>.
502		603
503	Flags can be any combination of C<EIO_SYNC_FILE_RANGE_WAIT_BEFORE>,	604	Flags can be any combination of C<EIO_SYNC_FILE_RANGE_WAIT_BEFORE>,
504	C<EIO_SYNC_FILE_RANGE_WRITE> and C<EIO_SYNC_FILE_RANGE_WAIT_AFTER>.	605	C<EIO_SYNC_FILE_RANGE_WRITE> and C<EIO_SYNC_FILE_RANGE_WAIT_AFTER>.
505		606
		607	=item eio_fallocate (int fd, int mode, off_t offset, off_t len, int pri, eio_cb cb, void *data)
		608
		609	Calls C<fallocate> (note: I<NOT> C<posix_fallocate>!). If the syscall is
		610	missing, then it returns failure and sets C<errno> to C<ENOSYS>.
		611
		612	The C<mode> argument can be C<0> (for behaviour similar to
		613	C<posix_fallocate>), or C<EIO_FALLOC_FL_KEEP_SIZE>, which keeps the size
		614	of the file unchanged (but still preallocates space beyond end of file).
		615
506	=back	616	=back
507		617
508	=head3 LIBEIO-SPECIFIC REQUESTS	618	=head3 LIBEIO-SPECIFIC REQUESTS
509		619
510	These requests are specific to libeio and do not correspond to any OS call.	620	These requests are specific to libeio and do not correspond to any OS call.
511		621
512	=over 4	622	=over 4
513		623
514	=item eio_mtouch (void addr, size_t length, int flags, int pri, eio_cb cb, void data)	624	=item eio_mtouch (void addr, size_t length, int flags, int pri, eio_cb cb, void data)
515		625
516	Reads (C<flags == 0>) or modifies (C<flags == EIO_MT_MODIFY) the given	626	Reads (C<flags == 0>) or modifies (C<flags == EIO_MT_MODIFY>) the given
517	memory area, page-wise, that is, it reads (or reads and writes back) the	627	memory area, page-wise, that is, it reads (or reads and writes back) the
518	first octet of every page that spans the memory area.	628	first octet of every page that spans the memory area.
519		629
520	This can be used to page in some mmapped file, or dirty some pages. Note	630	This can be used to page in some mmapped file, or dirty some pages. Note
521	that dirtying is an unlocked read-write access, so races can ensue when	631	that dirtying is an unlocked read-write access, so races can ensue when
…		…
551		661
552	eio_custom (my_open, 0, my_open_done, "/etc/passwd");	662	eio_custom (my_open, 0, my_open_done, "/etc/passwd");
553		663
554	=item eio_busy (eio_tstamp delay, int pri, eio_cb cb, void *data)	664	=item eio_busy (eio_tstamp delay, int pri, eio_cb cb, void *data)
555		665
556	This is a a request that takes C<delay> seconds to execute, but otherwise	666	This is a request that takes C<delay> seconds to execute, but otherwise
557	does nothing - it simply puts one of the worker threads to sleep for this	667	does nothing - it simply puts one of the worker threads to sleep for this
558	long.	668	long.
559		669
560	This request can be used to artificially increase load, e.g. for debugging	670	This request can be used to artificially increase load, e.g. for debugging
561	or benchmarking reasons.	671	or benchmarking reasons.
…		…
568		678
569	=back	679	=back
570		680
571	=head3 GROUPING AND LIMITING REQUESTS	681	=head3 GROUPING AND LIMITING REQUESTS
572		682
		683	There is one more rather special request, C<eio_grp>. It is a very special
		684	aio request: Instead of doing something, it is a container for other eio
		685	requests.
		686
		687	There are two primary use cases for this: a) bundle many requests into a
		688	single, composite, request with a definite callback and the ability to
		689	cancel the whole request with its subrequests and b) limiting the number
		690	of "active" requests.
		691
		692	Further below you will find more discussion of these topics - first
		693	follows the reference section detailing the request generator and other
		694	methods.
		695
		696	=over 4
		697
		698	=item eio_req grp = eio_grp (eio_cb cb, void data)
		699
		700	Creates, submits and returns a group request. Note that it doesn't have a
		701	priority, unlike all other requests.
		702
		703	=item eio_grp_add (eio_req grp, eio_req req)
		704
		705	Adds a request to the request group.
		706
		707	=item eio_grp_cancel (eio_req *grp)
		708
		709	Cancels all requests I<in> the group, but I<not> the group request
		710	itself. You can cancel the group request I<and> all subrequests via a
		711	normal C<eio_cancel> call.
		712
		713	=back
		714
		715	=head4 GROUP REQUEST LIFETIME
		716
		717	Left alone, a group request will instantly move to the pending state and
		718	will be finished at the next call of C<eio_poll>.
		719
		720	The usefulness stems from the fact that, if a subrequest is added to a
		721	group I<before> a call to C<eio_poll>, via C<eio_grp_add>, then the group
		722	will not finish until all the subrequests have finished.
		723
		724	So the usage cycle of a group request is like this: after it is created,
		725	you normally instantly add a subrequest. If none is added, the group
		726	request will finish on it's own. As long as subrequests are added before
		727	the group request is finished it will be kept from finishing, that is the
		728	callbacks of any subrequests can, in turn, add more requests to the group,
		729	and as long as any requests are active, the group request itself will not
		730	finish.
		731
		732	=head4 CREATING COMPOSITE REQUESTS
		733
		734	Imagine you wanted to create an C<eio_load> request that opens a file,
		735	reads it and closes it. This means it has to execute at least three eio
		736	requests, but for various reasons it might be nice if that request looked
		737	like any other eio request.
		738
		739	This can be done with groups:
		740
		741	=over 4
		742
		743	=item 1) create the request object
		744
		745	Create a group that contains all further requests. This is the request you
		746	can return as "the load request".
		747
		748	=item 2) open the file, maybe
		749
		750	Next, open the file with C<eio_open> and add the request to the group
		751	request and you are finished setting up the request.
		752
		753	If, for some reason, you cannot C<eio_open> (path is a null ptr?) you
		754	can set C<< grp->result >> to C<-1> to signal an error and let the group
		755	request finish on its own.
		756
		757	=item 3) open callback adds more requests
		758
		759	In the open callback, if the open was not successful, copy C<<
		760	req->errorno >> to C<< grp->errorno >> and set C<< grp->result >> to
		761	C<-1> to signal an error.
		762
		763	Otherwise, malloc some memory or so and issue a read request, adding the
		764	read request to the group.
		765
		766	=item 4) continue issuing requests till finished
		767
		768	In the read callback, check for errors and possibly continue with
		769	C<eio_close> or any other eio request in the same way.
		770
		771	As soon as no new requests are added, the group request will finish. Make
		772	sure you I<always> set C<< grp->result >> to some sensible value.
		773
		774	=back
		775
		776	=head4 REQUEST LIMITING
		777
		778
573	#TODO	779	#TODO
574		780
575	/*****************************************************************************/
576	/* groups */
577
578	eio_req eio_grp (eio_cb cb, void data);
579	void eio_grp_feed (eio_req grp, void (feed)(eio_req *req), int limit);
580	void eio_grp_limit (eio_req *grp, int limit);	781	void eio_grp_limit (eio_req *grp, int limit);
581	void eio_grp_add (eio_req grp, eio_req req);
582	void eio_grp_cancel (eio_req grp); / cancels all sub requests but not the group */
583		782
584
585	=back
586		783
587		784
588	=head1 LOW LEVEL REQUEST API	785	=head1 LOW LEVEL REQUEST API
589		786
590	#TODO	787	#TODO
…		…
593	=head1 ANATOMY AND LIFETIME OF AN EIO REQUEST	790	=head1 ANATOMY AND LIFETIME OF AN EIO REQUEST
594		791
595	A request is represented by a structure of type C<eio_req>. To initialise	792	A request is represented by a structure of type C<eio_req>. To initialise
596	it, clear it to all zero bytes:	793	it, clear it to all zero bytes:
597		794
598	eio_req req;	795	eio_req req;
599		796
600	memset (&req, 0, sizeof (req));	797	memset (&req, 0, sizeof (req));
601		798
602	A more common way to initialise a new C<eio_req> is to use C<calloc>:	799	A more common way to initialise a new C<eio_req> is to use C<calloc>:
603		800
604	eio_req req = calloc (1, sizeof (req));	801	eio_req req = calloc (1, sizeof (req));
605		802
606	In either case, libeio neither allocates, initialises or frees the	803	In either case, libeio neither allocates, initialises or frees the
607	C<eio_req> structure for you - it merely uses it.	804	C<eio_req> structure for you - it merely uses it.
608		805
609	zero	806	zero
…		…
627	for example, in interactive programs, you might want to limit this time to	824	for example, in interactive programs, you might want to limit this time to
628	C<0.01> seconds or so.	825	C<0.01> seconds or so.
629		826
630	Note that:	827	Note that:
631		828
		829	=over 4
		830
632	a) libeio doesn't know how long your request callbacks take, so the time	831	=item a) libeio doesn't know how long your request callbacks take, so the
633	spent in C<eio_poll> is up to one callback invocation longer then this	832	time spent in C<eio_poll> is up to one callback invocation longer then
634	interval.	833	this interval.
635		834
636	b) this is implemented by calling C<gettimeofday> after each request,	835	=item b) this is implemented by calling C<gettimeofday> after each
637	which can be costly.	836	request, which can be costly.
638		837
639	c) at least one request will be handled.	838	=item c) at least one request will be handled.
		839
		840	=back
640		841
641	=item eio_set_max_poll_reqs (unsigned int nreqs)	842	=item eio_set_max_poll_reqs (unsigned int nreqs)
642		843
643	When C<nreqs> is non-zero, then C<eio_poll> will not handle more than	844	When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
644	C<nreqs> requests per invocation. This is a less costly way to limit the	845	C<nreqs> requests per invocation. This is a less costly way to limit the
…		…
714	This symbol governs the stack size for each eio thread. Libeio itself	915	This symbol governs the stack size for each eio thread. Libeio itself
715	was written to use very little stackspace, but when using C<EIO_CUSTOM>	916	was written to use very little stackspace, but when using C<EIO_CUSTOM>
716	requests, you might want to increase this.	917	requests, you might want to increase this.
717		918
718	If this symbol is undefined (the default) then libeio will use its default	919	If this symbol is undefined (the default) then libeio will use its default
719	stack size (C<sizeof (long) * 4096> currently). If it is defined, but	920	stack size (C<sizeof (void ) 4096> currently). In all other cases, the
720	C<0>, then the default operating system stack size will be used. In all
721	other cases, the value must be an expression that evaluates to the desired	921	value must be an expression that evaluates to the desired stack size.
722	stack size.
723		922
724	=back	923	=back
725		924
726		925
727	=head1 PORTABILITY REQUIREMENTS	926	=head1 PORTABILITY REQUIREMENTS

Diff Legend

-–
+Removed lines
-+
+Added lines
-<
+Changed lines
->
+Changed lines

Comparing libeio/eio.pod (file contents): Revision 1.11 by root, Mon Jun 20 07:28:15 2011 UTC vs. Revision 1.36 by root, Sun Jan 24 16:36:20 2016 UTC

Diff Legend

Comparing libeio/eio.pod (file contents):
Revision 1.11 by root, Mon Jun 20 07:28:15 2011 UTC vs.
Revision 1.36 by root, Sun Jan 24 16:36:20 2016 UTC