SYNOPSIS

#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 * );

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.

im_add_close_callback(3) adds a callback which will be triggered when the image is closed by im_close(3). The callback is expected to return 0 for success and non-zero for failure. If the function fails, then the whole 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 im_malloc(3). This allocates memory exactly as the standard 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 im_malloc(3) acts as malloc(3). On error, im_malloc(3) returns NULL, setting an error message. See the man pages for IM_NEW(3) and im_open_local(3) for further examples.

Free memory with im_free(3).

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

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.

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 initalize 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 recieve notification instead.

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 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.

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.

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.

RETURN VALUE

All functions return 0 on success and non-zero on error.

COPYRIGHT

National Gallery, 1993

AUTHOR

J. Cupitt - 23/7/93