libuv/src/eio/eio.pod

=head1 NAME

libeio - truly asynchronous POSIX I/O

=head1 SYNOPSIS

  #include <eio.h>

=head1 DESCRIPTION

The newest version of this document is also available as an html-formatted
web page you might find easier to navigate when reading it for the first
time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.

Note that this library is a by-product of the C<IO::AIO> perl
module, and many of the subtler points regarding requests lifetime
and so on are only documented in its documentation at the
moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.

=head2 FEATURES

This library provides fully asynchronous versions of most POSIX functions
dealing with I/O. Unlike most asynchronous libraries, this not only
includes C<read> and C<write>, but also C<open>, C<stat>, C<unlink> and
similar functions, as well as less rarely ones such as C<mknod>, C<futime>
or C<readlink>.

It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
emulation elsewhere>).

The goal is to enable you to write fully non-blocking programs. For
example, in a game server, you would not want to freeze for a few seconds
just because the server is running a backup and you happen to call
C<readdir>.

=head2 TIME REPRESENTATION

Libeio represents time as a single floating point number, representing the
(fractional) number of seconds since the (POSIX) epoch (somewhere near
the beginning of 1970, details are complicated, don't ask). This type is
called C<eio_tstamp>, but it is guaranteed to be of type C<double> (or
better), so you can freely use C<double> yourself.

Unlike the name component C<stamp> might indicate, it is also used for
time differences throughout libeio.

=head2 FORK SUPPORT

Usage of pthreads in a program changes the semantics of fork
considerably. Specifically, only async-safe functions can be called after
fork. Libeio uses pthreads, so this applies, and makes using fork hard for
anything but relatively fork + exec uses.

This library only works in the process that initialised it: Forking is
fully supported, but using libeio in any other process than the one that
called C<eio_init> is not.

You might get around by not I<using> libeio before (or after) forking in
the parent, and using it in the child afterwards. You could also try to
call the L<eio_init> function again in the child, which will brutally
reinitialise all data structures, which isn't POSIX conformant, but
typically works.

Otherwise, the only recommendation you should follow is: treat fork code
the same way you treat signal handlers, and only ever call C<eio_init> in
the process that uses it, and only once ever.

=head1 INITIALISATION/INTEGRATION

Before you can call any eio functions you first have to initialise the
library. The library integrates into any event loop, but can also be used
without one, including in polling mode.

You have to provide the necessary glue yourself, however.

=over 4

=item int eio_init (void (*want_poll)(void), void (*done_poll)(void))

This function initialises the library. On success it returns C<0>, on
failure it returns C<-1> and sets C<errno> appropriately.

It accepts two function pointers specifying callbacks as argument, both of
which can be C<0>, in which case the callback isn't called.

There is currently no way to change these callbacks later, or to
"uninitialise" the library again.

=item want_poll callback

The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
that is, it will only be called once when eio wants attention, until all
pending requests have been handled.

This callback is called while locks are being held, so I<you must
not call any libeio functions inside this callback>. That includes
C<eio_poll>. What you should do is notify some other thread, or wake up
your event loop, and then call C<eio_poll>.

=item done_poll callback

This callback is invoked when libeio detects that all pending requests
have been handled. It is "edge-triggered", that is, it will only be
called once after C<want_poll>. To put it differently, C<want_poll> and
C<done_poll> are invoked in pairs: after C<want_poll> you have to call
C<eio_poll ()> until either C<eio_poll> indicates that everything has been
handled or C<done_poll> has been called, which signals the same.

Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
have been called again, so watch out for races in your code.

As with C<want_poll>, this callback is called while locks are being held,
so you I<must not call any libeio functions form within this callback>.

=item int eio_poll ()

This function has to be called whenever there are pending requests that
need finishing. You usually call this after C<want_poll> has indicated
that you should do so, but you can also call this function regularly to
poll for new results.

If any request invocation returns a non-zero value, then C<eio_poll ()>
immediately returns with that value as return value.

Otherwise, if all requests could be handled, it returns C<0>. If for some
reason not all requests have been handled, i.e. some are still pending, it
returns C<-1>.

=back

For libev, you would typically use an C<ev_async> watcher: the
C<want_poll> callback would invoke C<ev_async_send> to wake up the event
loop. Inside the callback set for the watcher, one would call C<eio_poll
()>.

If C<eio_poll ()> is configured to not handle all results in one go
(i.e. it returns C<-1>) then you should start an idle watcher that calls
C<eio_poll> until it returns something C<!= -1>.

A full-featured connector between libeio and libev would look as follows
(if C<eio_poll> is handling all requests, it can of course be simplified a
lot by removing the idle watcher logic):

  static struct ev_loop *loop;
  static ev_idle repeat_watcher;
  static ev_async ready_watcher;

  /* idle watcher callback, only used when eio_poll */
  /* didn't handle all results in one call */
  static void
  repeat (EV_P_ ev_idle *w, int revents)
  {
    if (eio_poll () != -1)
      ev_idle_stop (EV_A_ w);
  }

  /* eio has some results, process them */
  static void
  ready (EV_P_ ev_async *w, int revents)
  {
    if (eio_poll () == -1)
      ev_idle_start (EV_A_ &repeat_watcher);
  }

  /* wake up the event loop */
  static void
  want_poll (void)
  {
    ev_async_send (loop, &ready_watcher)
  }

  void
  my_init_eio ()
  {
    loop = EV_DEFAULT;

    ev_idle_init (&repeat_watcher, repeat);
    ev_async_init (&ready_watcher, ready);
    ev_async_start (loop &watcher);

    eio_init (want_poll, 0);
  }

For most other event loops, you would typically use a pipe - the event
loop should be told to wait for read readiness on the read end. In
C<want_poll> you would write a single byte, in C<done_poll> you would try
to read that byte, and in the callback for the read end, you would call
C<eio_poll>.

You don't have to take special care in the case C<eio_poll> doesn't handle
all requests, as the done callback will not be invoked, so the event loop
will still signal readiness for the pipe until I<all> results have been
processed.


=head1 HIGH LEVEL REQUEST API

Libeio has both a high-level API, which consists of calling a request
function with a callback to be called on completion, and a low-level API
where you fill out request structures and submit them.

This section describes the high-level API.

=head2 REQUEST SUBMISSION AND RESULT PROCESSING

You submit a request by calling the relevant C<eio_TYPE> function with the
required parameters, a callback of type C<int (*eio_cb)(eio_req *req)>
(called C<eio_cb> below) and a freely usable C<void *data> argument.

The return value will either be 0, in case something went really wrong
(which can basically only happen on very fatal errors, such as C<malloc>
returning 0, which is rather unlikely), or a pointer to the newly-created
and submitted C<eio_req *>.

The callback will be called with an C<eio_req *> which contains the
results of the request. The members you can access inside that structure
vary from request to request, except for:

=over 4

=item C<ssize_t result>

This contains the result value from the call (usually the same as the
syscall of the same name).

=item C<int errorno>

This contains the value of C<errno> after the call.

=item C<void *data>

The C<void *data> member simply stores the value of the C<data> argument.

=back

The return value of the callback is normally C<0>, which tells libeio to
continue normally. If a callback returns a nonzero value, libeio will
stop processing results (in C<eio_poll>) and will return the value to its
caller.

Memory areas passed to libeio must stay valid as long as a request
executes, with the exception of paths, which are being copied
internally. Any memory libeio itself allocates will be freed after the
finish callback has been called. If you want to manage all memory passed
to libeio yourself you can use the low-level API.

For example, to open a file, you could do this:

  static int
  file_open_done (eio_req *req)
  {
    if (req->result < 0)
      {
        /* open() returned -1 */
        errno = req->errorno;
        perror ("open");
      }
    else
      {
        int fd = req->result;
        /* now we have the new fd in fd */
      }

    return 0;
  }

  /* the first three arguments are passed to open(2) */
  /* the remaining are priority, callback and data */
  if (!eio_open ("/etc/passwd", O_RDONLY, 0, 0, file_open_done, 0))
    abort (); /* something went wrong, we will all die!!! */

Note that you additionally need to call C<eio_poll> when the C<want_cb>
indicates that requests are ready to be processed.

=head2 CANCELLING REQUESTS

Sometimes the need for a request goes away before the request is
finished. In that case, one can cancel the request by a call to
C<eio_cancel>:

=over 4

=item eio_cancel (eio_req *req)

Cancel the request (and all its subrequests). If the request is currently
executing it might still continue to execute, and in other cases it might
still take a while till the request is cancelled.

Even if cancelled, the finish callback will still be invoked - the
callbacks of all cancellable requests need to check whether the request
has been cancelled by calling C<EIO_CANCELLED (req)>:

  static int
  my_eio_cb (eio_req *req)
  {
    if (EIO_CANCELLED (req))
      return 0;
  }

In addition, cancelled requests will I<either> have C<< req->result >>
set to C<-1> and C<errno> to C<ECANCELED>, or I<otherwise> they were
successfully executed, despite being cancelled (e.g. when they have
already been executed at the time they were cancelled).

C<EIO_CANCELLED> is still true for requests that have successfully
executed, as long as C<eio_cancel> was called on them at some point.

=back

=head2 AVAILABLE REQUESTS

The following request functions are available. I<All> of them return the
C<eio_req *> on success and C<0> on failure, and I<all> of them have the
same three trailing arguments: C<pri>, C<cb> and C<data>. The C<cb> is
mandatory, but in most cases, you pass in C<0> as C<pri> and C<0> or some
custom data value as C<data>.

=head3 POSIX API WRAPPERS

These requests simply wrap the POSIX call of the same name, with the same
arguments. If a function is not implemented by the OS and cannot be emulated
in some way, then all of these return C<-1> and set C<errorno> to C<ENOSYS>.

=over 4

=item eio_open      (const char *path, int flags, mode_t mode, int pri, eio_cb cb, void *data)

=item eio_truncate  (const char *path, off_t offset, int pri, eio_cb cb, void *data)

=item eio_chown     (const char *path, uid_t uid, gid_t gid, int pri, eio_cb cb, void *data)

=item eio_chmod     (const char *path, mode_t mode, int pri, eio_cb cb, void *data)

=item eio_mkdir     (const char *path, mode_t mode, int pri, eio_cb cb, void *data)

=item eio_rmdir     (const char *path, int pri, eio_cb cb, void *data)

=item eio_unlink    (const char *path, int pri, eio_cb cb, void *data)

=item eio_utime     (const char *path, eio_tstamp atime, eio_tstamp mtime, int pri, eio_cb cb, void *data)

=item eio_mknod     (const char *path, mode_t mode, dev_t dev, int pri, eio_cb cb, void *data)

=item eio_link      (const char *path, const char *new_path, int pri, eio_cb cb, void *data)

=item eio_symlink   (const char *path, const char *new_path, int pri, eio_cb cb, void *data)

=item eio_rename    (const char *path, const char *new_path, int pri, eio_cb cb, void *data)

=item eio_mlock     (void *addr, size_t length, int pri, eio_cb cb, void *data)

=item eio_close     (int fd, int pri, eio_cb cb, void *data)

=item eio_sync      (int pri, eio_cb cb, void *data)

=item eio_fsync     (int fd, int pri, eio_cb cb, void *data)

=item eio_fdatasync (int fd, int pri, eio_cb cb, void *data)

=item eio_futime    (int fd, eio_tstamp atime, eio_tstamp mtime, int pri, eio_cb cb, void *data)

=item eio_ftruncate (int fd, off_t offset, int pri, eio_cb cb, void *data)

=item eio_fchmod    (int fd, mode_t mode, int pri, eio_cb cb, void *data)

=item eio_fchown    (int fd, uid_t uid, gid_t gid, int pri, eio_cb cb, void *data)

=item eio_dup2      (int fd, int fd2, int pri, eio_cb cb, void *data)

These have the same semantics as the syscall of the same name, their
return value is available as C<< req->result >> later.

=item eio_read      (int fd, void *buf, size_t length, off_t offset, int pri, eio_cb cb, void *data)

=item eio_write     (int fd, void *buf, size_t length, off_t offset, int pri, eio_cb cb, void *data)

These two requests are called C<read> and C<write>, but actually wrap
C<pread> and C<pwrite>. On systems that lack these calls (such as cygwin),
libeio uses lseek/read_or_write/lseek and a mutex to serialise the
requests, so all these requests run serially and do not disturb each
other. However, they still disturb the file offset while they run, so it's
not safe to call these functions concurrently with non-libeio functions on
the same fd on these systems.

Not surprisingly, pread and pwrite are not thread-safe on Darwin (OS/X),
so it is advised not to submit multiple requests on the same fd on this
horrible pile of garbage.

=item eio_mlockall  (int flags, int pri, eio_cb cb, void *data)

Like C<mlockall>, but the flag value constants are called
C<EIO_MCL_CURRENT> and C<EIO_MCL_FUTURE>.

=item eio_msync     (void *addr, size_t length, int flags, int pri, eio_cb cb, void *data)

Just like msync, except that the flag values are called C<EIO_MS_ASYNC>,
C<EIO_MS_INVALIDATE> and C<EIO_MS_SYNC>.

=item eio_readlink  (const char *path, int pri, eio_cb cb, void *data)

If successful, the path read by C<readlink(2)> can be accessed via C<<
req->ptr2 >> and is I<NOT> null-terminated, with the length specified as
C<< req->result >>.

  if (req->result >= 0)
    {
      char *target = strndup ((char *)req->ptr2, req->result);

      free (target);
    }

=item eio_realpath  (const char *path, int pri, eio_cb cb, void *data)

Similar to the realpath libc function, but unlike that one, C<<
req->result >> is C<-1> on failure. On success, the result is the length
of the returned path in C<ptr2> (which is I<NOT> 0-terminated) - this is
similar to readlink.

=item eio_stat      (const char *path, int pri, eio_cb cb, void *data)

=item eio_lstat     (const char *path, int pri, eio_cb cb, void *data)

=item eio_fstat     (int fd, int pri, eio_cb cb, void *data)
   
Stats a file - if C<< req->result >> indicates success, then you can
access the C<struct stat>-like structure via C<< req->ptr2 >>:

  EIO_STRUCT_STAT *statdata = (EIO_STRUCT_STAT *)req->ptr2;

=item eio_statvfs   (const char *path, int pri, eio_cb cb, void *data)

=item eio_fstatvfs  (int fd, int pri, eio_cb cb, void *data)

Stats a filesystem - if C<< req->result >> indicates success, then you can
access the C<struct statvfs>-like structure via C<< req->ptr2 >>:

  EIO_STRUCT_STATVFS *statdata = (EIO_STRUCT_STATVFS *)req->ptr2;

=back

=head3 READING DIRECTORIES

Reading directories sounds simple, but can be rather demanding, especially
if you want to do stuff such as traversing a directory hierarchy or
processing all files in a directory. Libeio can assist these complex tasks
with it's C<eio_readdir> call.

=over 4

=item eio_readdir (const char *path, int flags, int pri, eio_cb cb, void *data)

This is a very complex call. It basically reads through a whole directory
(via the C<opendir>, C<readdir> and C<closedir> calls) and returns either
the names or an array of C<struct eio_dirent>, depending on the C<flags>
argument.

The C<< req->result >> indicates either the number of files found, or
C<-1> on error. On success, null-terminated names can be found as C<< req->ptr2 >>,
and C<struct eio_dirents>, if requested by C<flags>, can be found via C<<
req->ptr1 >>.

Here is an example that prints all the names:

  int i;
  char *names = (char *)req->ptr2;

  for (i = 0; i < req->result; ++i)
    {
      printf ("name #%d: %s\n", i, names);

      /* move to next name */
      names += strlen (names) + 1;
    }

Pseudo-entries such as F<.> and F<..> are never returned by C<eio_readdir>.

C<flags> can be any combination of:

=over 4

=item EIO_READDIR_DENTS

If this flag is specified, then, in addition to the names in C<ptr2>,
also an array of C<struct eio_dirent> is returned, in C<ptr1>. A C<struct
eio_dirent> looks like this:

  struct eio_dirent
  {
    int nameofs; /* offset of null-terminated name string in (char *)req->ptr2 */
    unsigned short namelen; /* size of filename without trailing 0 */
    unsigned char type; /* one of EIO_DT_* */
    signed char score; /* internal use */
    ino_t inode; /* the inode number, if available, otherwise unspecified */
  };

The only members you normally would access are C<nameofs>, which is the
byte-offset from C<ptr2> to the start of the name, C<namelen> and C<type>.

C<type> can be one of:

C<EIO_DT_UNKNOWN> - if the type is not known (very common) and you have to C<stat>
the name yourself if you need to know,
one of the "standard" POSIX file types (C<EIO_DT_REG>, C<EIO_DT_DIR>, C<EIO_DT_LNK>,
C<EIO_DT_FIFO>, C<EIO_DT_SOCK>, C<EIO_DT_CHR>, C<EIO_DT_BLK>)
or some OS-specific type (currently
C<EIO_DT_MPC> - multiplexed char device (v7+coherent),
C<EIO_DT_NAM> - xenix special named file,
C<EIO_DT_MPB> - multiplexed block device (v7+coherent),
C<EIO_DT_NWK> - HP-UX network special,
C<EIO_DT_CMP> - VxFS compressed,
C<EIO_DT_DOOR> - solaris door, or
C<EIO_DT_WHT>).

This example prints all names and their type:

  int i;
  struct eio_dirent *ents = (struct eio_dirent *)req->ptr1;
  char *names = (char *)req->ptr2;

  for (i = 0; i < req->result; ++i)
    {
      struct eio_dirent *ent = ents + i;
      char *name = names + ent->nameofs;

      printf ("name #%d: %s (type %d)\n", i, name, ent->type);
    }

=item EIO_READDIR_DIRS_FIRST

When this flag is specified, then the names will be returned in an order
where likely directories come first, in optimal C<stat> order. This is
useful when you need to quickly find directories, or you want to find all
directories while avoiding to stat() each entry.

If the system returns type information in readdir, then this is used
to find directories directly. Otherwise, likely directories are names
beginning with ".", or otherwise names with no dots, of which names with
short names are tried first.

=item EIO_READDIR_STAT_ORDER

When this flag is specified, then the names will be returned in an order
suitable for stat()'ing each one. That is, when you plan to stat()
all files in the given directory, then the returned order will likely
be fastest.

If both this flag and C<EIO_READDIR_DIRS_FIRST> are specified, then the
likely directories come first, resulting in a less optimal stat order.

=item EIO_READDIR_FOUND_UNKNOWN

This flag should not be specified when calling C<eio_readdir>. Instead,
it is being set by C<eio_readdir> (you can access the C<flags> via C<<
req->int1 >>, when any of the C<type>'s found were C<EIO_DT_UNKNOWN>. The
absence of this flag therefore indicates that all C<type>'s are known,
which can be used to speed up some algorithms.

A typical use case would be to identify all subdirectories within a
directory - you would ask C<eio_readdir> for C<EIO_READDIR_DIRS_FIRST>. If
then this flag is I<NOT> set, then all the entries at the beginning of the
returned array of type C<EIO_DT_DIR> are the directories. Otherwise, you
should start C<stat()>'ing the entries starting at the beginning of the
array, stopping as soon as you found all directories (the count can be
deduced by the link count of the directory).

=back

=back

=head3 OS-SPECIFIC CALL WRAPPERS

These wrap OS-specific calls (usually Linux ones), and might or might not
be emulated on other operating systems. Calls that are not emulated will
return C<-1> and set C<errno> to C<ENOSYS>.

=over 4

=item eio_sendfile (int out_fd, int in_fd, off_t in_offset, size_t length, int pri, eio_cb cb, void *data)

Wraps the C<sendfile> syscall. The arguments follow the Linux version, but
libeio supports and will use similar calls on FreeBSD, HP/UX, Solaris and
Darwin.

If the OS doesn't support some sendfile-like call, or the call fails,
indicating support for the given file descriptor type (for example,
Linux's sendfile might not support file to file copies), then libeio will
emulate the call in userspace, so there are almost no limitations on its
use.

=item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data)

Calls C<readahead(2)>. If the syscall is missing, then the call is
emulated by simply reading the data (currently in 64kiB chunks).

=item eio_syncfs (int fd, int pri, eio_cb cb, void *data)

Calls Linux' C<syncfs> syscall, if available. Returns C<-1> and sets
C<errno> to C<ENOSYS> if the call is missing I<but still calls sync()>,
if the C<fd> is C<< >= 0 >>, so you can probe for the availability of the
syscall with a negative C<fd> argument and checking for C<-1/ENOSYS>.

=item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data)

Calls C<sync_file_range>. If the syscall is missing, then this is the same
as calling C<fdatasync>.

Flags can be any combination of C<EIO_SYNC_FILE_RANGE_WAIT_BEFORE>,
C<EIO_SYNC_FILE_RANGE_WRITE> and C<EIO_SYNC_FILE_RANGE_WAIT_AFTER>.

=item eio_fallocate (int fd, int mode, off_t offset, off_t len, int pri, eio_cb cb, void *data)

Calls C<fallocate> (note: I<NOT> C<posix_fallocate>!). If the syscall is
missing, then it returns failure and sets C<errno> to C<ENOSYS>.

The C<mode> argument can be C<0> (for behaviour similar to
C<posix_fallocate>), or C<EIO_FALLOC_FL_KEEP_SIZE>, which keeps the size
of the file unchanged (but still preallocates space beyond end of file).

=back

=head3 LIBEIO-SPECIFIC REQUESTS

These requests are specific to libeio and do not correspond to any OS call.

=over 4

=item eio_mtouch (void *addr, size_t length, int flags, int pri, eio_cb cb, void *data)

Reads (C<flags == 0>) or modifies (C<flags == EIO_MT_MODIFY) the given
memory area, page-wise, that is, it reads (or reads and writes back) the
first octet of every page that spans the memory area.

This can be used to page in some mmapped file, or dirty some pages. Note
that dirtying is an unlocked read-write access, so races can ensue when
the some other thread modifies the data stored in that memory area.

=item eio_custom (void (*)(eio_req *) execute, int pri, eio_cb cb, void *data)

Executes a custom request, i.e., a user-specified callback.

The callback gets the C<eio_req *> as parameter and is expected to read
and modify any request-specific members. Specifically, it should set C<<
req->result >> to the result value, just like other requests.

Here is an example that simply calls C<open>, like C<eio_open>, but it
uses the C<data> member as filename and uses a hardcoded C<O_RDONLY>. If
you want to pass more/other parameters, you either need to pass some
struct or so via C<data> or provide your own wrapper using the low-level
API.

  static int
  my_open_done (eio_req *req)
  {
    int fd = req->result;

    return 0;
  }

  static void
  my_open (eio_req *req)
  {
    req->result = open (req->data, O_RDONLY);
  }

  eio_custom (my_open, 0, my_open_done, "/etc/passwd");

=item eio_busy (eio_tstamp delay, int pri, eio_cb cb, void *data)

This is a request that takes C<delay> seconds to execute, but otherwise
does nothing - it simply puts one of the worker threads to sleep for this
long.

This request can be used to artificially increase load, e.g. for debugging
or benchmarking reasons.

=item eio_nop (int pri, eio_cb cb, void *data)
   
This request does nothing, except go through the whole request cycle. This
can be used to measure latency or in some cases to simplify code, but is
not really of much use.

=back

=head3 GROUPING AND LIMITING REQUESTS

There is one more rather special request, C<eio_grp>. It is a very special
aio request: Instead of doing something, it is a container for other eio
requests.

There are two primary use cases for this: a) bundle many requests into a
single, composite, request with a definite callback and the ability to
cancel the whole request with its subrequests and b) limiting the number
of "active" requests.

Further below you will find more discussion of these topics - first
follows the reference section detailing the request generator and other
methods.

=over 4

=item eio_req *grp = eio_grp (eio_cb cb, void *data)

Creates, submits and returns a group request. Note that it doesn't have a
priority, unlike all other requests.

=item eio_grp_add (eio_req *grp, eio_req *req)

Adds a request to the request group.

=item eio_grp_cancel (eio_req *grp)

Cancels all requests I<in> the group, but I<not> the group request
itself. You can cancel the group request I<and> all subrequests via a
normal C<eio_cancel> call.

=back

=head4 GROUP REQUEST LIFETIME

Left alone, a group request will instantly move to the pending state and
will be finished at the next call of C<eio_poll>.

The usefulness stems from the fact that, if a subrequest is added to a
group I<before> a call to C<eio_poll>, via C<eio_grp_add>, then the group
will not finish until all the subrequests have finished.

So the usage cycle of a group request is like this: after it is created,
you normally instantly add a subrequest. If none is added, the group
request will finish on it's own. As long as subrequests are added before
the group request is finished it will be kept from finishing, that is the
callbacks of any subrequests can, in turn, add more requests to the group,
and as long as any requests are active, the group request itself will not
finish.

=head4 CREATING COMPOSITE REQUESTS

Imagine you wanted to create an C<eio_load> request that opens a file,
reads it and closes it. This means it has to execute at least three eio
requests, but for various reasons it might be nice if that request looked
like any other eio request.

This can be done with groups:

=over 4

=item 1) create the request object

Create a group that contains all further requests. This is the request you
can return as "the load request".

=item 2) open the file, maybe

Next, open the file with C<eio_open> and add the request to the group
request and you are finished setting up the request.

If, for some reason, you cannot C<eio_open> (path is a null ptr?) you
can set C<< grp->result >> to C<-1> to signal an error and let the group
request finish on its own.

=item 3) open callback adds more requests

In the open callback, if the open was not successful, copy C<<
req->errorno >> to C<< grp->errorno >> and set C<< grp->errorno >> to
C<-1> to signal an error.

Otherwise, malloc some memory or so and issue a read request, adding the
read request to the group.

=item 4) continue issuing requests till finished

In the real callback, check for errors and possibly continue with
C<eio_close> or any other eio request in the same way.

As soon as no new requests are added the group request will finish. Make
sure you I<always> set C<< grp->result >> to some sensible value.

=back

=head4 REQUEST LIMITING


#TODO

void eio_grp_limit     (eio_req *grp, int limit);


=back


=head1 LOW LEVEL REQUEST API

#TODO


=head1 ANATOMY AND LIFETIME OF AN EIO REQUEST

A request is represented by a structure of type C<eio_req>. To initialise
it, clear it to all zero bytes:

  eio_req req;

  memset (&req, 0, sizeof (req));

A more common way to initialise a new C<eio_req> is to use C<calloc>:

  eio_req *req = calloc (1, sizeof (*req));

In either case, libeio neither allocates, initialises or frees the
C<eio_req> structure for you - it merely uses it.

zero

#TODO

=head2 CONFIGURATION

The functions in this section can sometimes be useful, but the default
configuration will do in most case, so you should skip this section on
first reading.

=over 4

=item eio_set_max_poll_time (eio_tstamp nseconds)

This causes C<eio_poll ()> to return after it has detected that it was
running for C<nsecond> seconds or longer (this number can be fractional).

This can be used to limit the amount of time spent handling eio requests,
for example, in interactive programs, you might want to limit this time to
C<0.01> seconds or so.

Note that:

=over 4

=item a) libeio doesn't know how long your request callbacks take, so the
time spent in C<eio_poll> is up to one callback invocation longer then
this interval.

=item b) this is implemented by calling C<gettimeofday> after each
request, which can be costly.

=item c) at least one request will be handled.

=back

=item eio_set_max_poll_reqs (unsigned int nreqs)

When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
C<nreqs> requests per invocation. This is a less costly way to limit the
amount of work done by C<eio_poll> then setting a time limit.

If you know your callbacks are generally fast, you could use this to
encourage interactiveness in your programs by setting it to C<10>, C<100>
or even C<1000>.

=item eio_set_min_parallel (unsigned int nthreads)

Make sure libeio can handle at least this many requests in parallel. It
might be able handle more.

=item eio_set_max_parallel (unsigned int nthreads)

Set the maximum number of threads that libeio will spawn.

=item eio_set_max_idle (unsigned int nthreads)

Libeio uses threads internally to handle most requests, and will start and stop threads on demand.

This call can be used to limit the number of idle threads (threads without
work to do): libeio will keep some threads idle in preparation for more
requests, but never longer than C<nthreads> threads.

In addition to this, libeio will also stop threads when they are idle for
a few seconds, regardless of this setting.

=item unsigned int eio_nthreads ()

Return the number of worker threads currently running.

=item unsigned int eio_nreqs ()

Return the number of requests currently handled by libeio. This is the
total number of requests that have been submitted to libeio, but not yet
destroyed.

=item unsigned int eio_nready ()

Returns the number of ready requests, i.e. requests that have been
submitted but have not yet entered the execution phase.

=item unsigned int eio_npending ()

Returns the number of pending requests, i.e. requests that have been
executed and have results, but have not been finished yet by a call to
C<eio_poll>).

=back

=head1 EMBEDDING

Libeio can be embedded directly into programs. This functionality is not
documented and not (yet) officially supported.

Note that, when including C<libeio.m4>, you are responsible for defining
the compilation environment (C<_LARGEFILE_SOURCE>, C<_GNU_SOURCE> etc.).

If you need to know how, check the C<IO::AIO> perl module, which does
exactly that.


=head1 COMPILETIME CONFIGURATION

These symbols, if used, must be defined when compiling F<eio.c>.

=over 4

=item EIO_STACKSIZE

This symbol governs the stack size for each eio thread. Libeio itself
was written to use very little stackspace, but when using C<EIO_CUSTOM>
requests, you might want to increase this.

If this symbol is undefined (the default) then libeio will use its default
stack size (C<sizeof (void *) * 4096> currently). If it is defined, but
C<0>, then the default operating system stack size will be used. In all
other cases, the value must be an expression that evaluates to the desired
stack size.

=back


=head1 PORTABILITY REQUIREMENTS

In addition to a working ISO-C implementation, libeio relies on a few
additional extensions:

=over 4

=item POSIX threads

To be portable, this module uses threads, specifically, the POSIX threads
library must be available (and working, which partially excludes many xBSD
systems, where C<fork ()> is buggy).

=item POSIX-compatible filesystem API

This is actually a harder portability requirement: The libeio API is quite
demanding regarding POSIX API calls (symlinks, user/group management
etc.).

=item C<double> must hold a time value in seconds with enough accuracy

The type C<double> is used to represent timestamps. It is required to
have at least 51 bits of mantissa (and 9 bits of exponent), which is good
enough for at least into the year 4000. This requirement is fulfilled by
implementations implementing IEEE 754 (basically all existing ones).

=back

If you know of other additional requirements drop me a note.


=head1 AUTHOR

Marc Lehmann <libeio@schmorp.de>.