.TH IM_AND 3 "30 October 1992"
.SH NAME
im_add_close_callback, im_add_eval_callback, im_malloc, im_free,
im_add_evalend_callback, im_add_evalstart_callback, im_add_preclose_callback,
im_add_invalidate_callback \- add image callbacks
.SH SYNOPSIS
.B #include <vips/vips.h>

typedef int (*im_callback_fn)( void *, void * );

int im_add_close_callback( IMAGE *, im_callback_fn, void *, void * );

int im_add_preclose_callback( IMAGE *, im_callback_fn, void *, void * );

int im_add_evalstart_callback( IMAGE *, im_callback_fn, void *, void * );

int im_add_eval_callback( IMAGE *, im_callback_fn, void *, void * );

int im_add_evalend_callback( IMAGE *, im_callback_fn, void *, void * );

int im_add_invalidate_callback( IMAGE *, im_callback_fn, void *, void * );

void *im_malloc( IMAGE *, size_t );

int im_free( void * );

.SH DESCRIPTION
These functions attach callbacks to images. Callbacks are functions, with
optional extra arguments a and b, which are remembered by the IMAGE they are
attached to. These functions are triggered at some later stage in reponse to
some event. You can attach as many callbacks as you like; all will be
remembered, and will be called when the event occurs. The most recently added
callback is called first --- this can be important for close callbacks. 

.B im_add_close_callback(3) 
adds a callback which will be triggered when the image
is closed by 
.B im_close(3). 
The callback is expected to return 0 for success and
non-zero for failure. If the function fails, then the whole 
.B im_close(3) 
fails. Close callbacks are guaranteed to be called exactly once, so they are a
good place to release resources.

This function is used by VIPS to implement 
.B im_malloc(3). 
This allocates memory
exactly as the standard 
.B malloc(3) 
function, but memory allocated is local to a
descriptor. When the descriptor is closed, the memory allocated is
automatically freed for you. If you pass NULL for the descriptor, then
.B im_malloc(3) 
acts as 
.B malloc(3). 
On error, 
.B im_malloc(3) 
returns NULL, setting an
error message. See the man pages for 
.B IM_NEW(3) and 
.B im_open_local(3)
for further examples. 

Free memory with 
.B im_free(3).

You may use close callbacks to trigger other 
.B im_close(3) 
operations, and there
may even be circularity in your 
.B im_close(3) 
lists. 

.B im_add_preclose_callback(3) 
adds a callback which will be triggered when the image is closed, but before
any closing has started. Everything is still alive and you can do anything
with the image. Preclose callbacks are guaranteed to be called exactly once.

.B im_add_evalstart_callback(3) 
adds a callback which will be triggered just before image evaluation starts.
It can be called many times. It's a good place to initialize data structures.
Don't allocate resources here.

Eval callbacks are inherited. That is, any images which use your image as
input will inherit your eval callbacks. As a result, if you add an eval
callback to an image, you will be notified if any later image uses your image
for computation.

If a later image adds eval callbacks, then the inheritance is broken, and that
image will receive notification instead.

.B im_add_eval_callback(3) 
adds a callback which will be triggered repeatedly as
the image is evaluated. 

When the callback is triggered, the time field of the descriptor will point to
a 
.B im_time_t 
structure, see vips.h

  typedef struct {
    IMAGE *im;     /* Image we are part of */
    time_t unused; /* For compatibility */
    int run;       /* Time we have been running (secs) */
    int eta;       /* Estimated seconds of computation left */
    gint64 tpels;  /* Number of pels we expect to calculate */
    gint64 npels;  /* Number of pels calculated so far */
    int percent;   /* Percent complete */
    GTimer *start; /* Start time */
  } im_time_t;

These fields are not exact! They should only be used to give approximate
feedback to the user. It is possible to have

  percent > 100
  ntiles > ttiles
  eta == 0

so be careful. Again, the eval callback should return 0 for success and
non-zero for failure. If the callback fails, evaluation is abandoned. This may
be used to provide a `cancel' feature in your user-interface.

  int
  eval_cb( IMAGE *im )
  {
    printf( "%d%% complete ...\\n", im->time->percent );
    return( 0 );
  }

  if( im_add_eval_callback( out, eval_cb, out, NULL ) )
    return( -1 );

  ... now as out is evaluated, we will get %complete 
  ... messages on stdout.

.B im_add_evalend_callback(3) 
adds a callback which will be triggered when VIPS
has finished evaluating the image. If you want to output some diagnostics
from your function (an overflow count, for example), this is the callback to
use. Again, this can be called many times. 

.B im_add_invalidate_callback(3) 
adds a callback which will be triggered when VIPS invalidates the cache on an
image. This is useful for removing images from other, higher-level caches.

.SH RETURN VALUE
All functions return 0 on success and non-zero on error.
.SH SEE ALSO
IM_NEW(3), im_open_local(3).
.SH COPYRIGHT
National Gallery, 1993
.SH AUTHOR
J. Cupitt \- 23/7/93