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<a name="using-threads"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">VIPS and threads</span></h2>
<p>Using VIPS — VIPS and threading</p>
</td>
<td class="gallery_image" valign="top" align="right"></td>
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<div class="refsect3">
<a name="using-threads-intro"></a><h4>Introduction</h4>
<p>
      This section tries to summarise the rules for threaded programs using 
      libvips. Generally, libvips is threaded and thread-safe, with a few 
      exceptions.
    </p>
</div>
<div class="refsect3">
<a name="using-threads-images"></a><h4>Images</h4>
<p>
      On startup, you need to call <a class="link" href="libvips-vips.html#VIPS-INIT:CAPS" title="VIPS_INIT()"><code class="function">VIPS_INIT()</code></a> single-threaded. After that,
      you can freely create images in any thread and read them in any other 
      thread. See the example at the end of this chapter. 
      Note that results can also be shared between threads for you by the vips 
      operation cache. 
    </p>
<p>
      The exception is the drawing operators, such as <a class="link" href="libvips-draw.html#vips-draw-circle" title="vips_draw_circle ()"><code class="function">vips_draw_circle()</code></a>. 
      These operations modify their image argument so you can't call them on 
      the same image from more than one thread. Reading from an image while 
      another thread is writing to it with one of the draw operations will 
      obviously also fail.
    </p>
<p>
      When libvips calculates an image, by default it will use as many 
      threads as you have CPU cores. Use <a class="link" href="libvips-VipsThreadState.html#vips-concurrency-set" title="vips_concurrency_set ()"><code class="function">vips_concurrency_set()</code></a> to change this.
    </p>
</div>
<div class="refsect3">
<a name="using-threads-errors"></a><h4>Error handling</h4>
<p>
      libvips has a single error code (-1 or <code class="literal">NULL</code>) returned by all functions 
      on error. Error messages are not returned, instead they are logged  
      in a single global error buffer shared by all threads, see 
      <a class="link" href="libvips-error.html#vips-error-buffer" title="vips_error_buffer ()"><code class="function">vips_error_buffer()</code></a>. 
    </p>
<p>
      This makes error handling very simple but the obvious downside is that
      because error returns and error messages are separate when you 
      detect an error return you can't be 
      sure that what's in the error buffer is the message that matches your
      error. 
    </p>
<p>
      The simplest way to handle this is to present the whole error log to 
      the user on the next interaction and leave it to them to decide what 
      action caused the failure. 
    </p>
</div>
<div class="refsect3">
<a name="using-threads-regions"></a><h4>Using <span class="type">VipsRegion</span> between threads</h4>
<p>
      <span class="type">VipsImage</span> objects are immutable and can be shared between 
      threads very simply.
      However the lower-level <span class="type">VipsRegion</span> object used to implement <span class="type">VipsImage</span> 
      (see <a class="link" href="extending.html" title="Extending VIPS">Extending VIPS</a>) is mutable and you 
      can only use a <span class="type">VipsRegion</span> from one thread at once. 
    </p>
<p>
      In fact it's worse than that: to reduce locking, <span class="type">VipsRegion</span> keeps a 
      lot of state in per-thread storage. If you want to create a region in 
      one thread and use it in another, you have to first tag the region as 
      unowned from the creating thread with <code class="function">vips__region_no_ownership()</code>, then 
      in the recieving thread take ownership with 
      <code class="function">vips__region_take_ownership()</code>. See the source for operations like 
      <a class="link" href="libvips-conversion.html#vips-tilecache" title="vips_tilecache ()"><code class="function">vips_tilecache()</code></a> if you're curious how this works. 
    </p>
<p>
      libvips includes a set of sanity checks for region ownership and will
      fail if you don't pass ownership correctly. 
    </p>
</div>
<div class="refsect3">
<a name="using-threads-example"></a><h4>Example</h4>
<p>
      This example runs many <a class="link" href="libvips-resample.html#vips-resize" title="vips_resize ()"><code class="function">vips_resize()</code></a> in parallel from many threads. 
    </p>
<div class="example">
<a name="id-1.2.11.7.3"></a><p class="title"><b>Example 2. VIPS and threads example</b></p>
<div class="example-contents"><pre class="programlisting">
/* Read from many threads. 
 *
 * Compile with:
 *
 * 	gcc -g -Wall soak.c `pkg-config vips --cflags --libs`
 *
 * Run with:
 *
 * 	rm -rf x
 * 	mkdir x
 * 	for i in {0..10}; do ./a.out ~/pics/k2.jpg; done
 *
 */

#include &lt;stdio.h&gt;
#include &lt;glib.h&gt;

#include &lt;vips/vips.h&gt;

/* How many pings we run at once.
 */
#define NUM_IN_PARALLEL (50)

/* Number of tests we do in total.
 */
#define TOTAL_TESTS (NUM_IN_PARALLEL * 20)

/* Workers queue up on this.
 */
GMutex allocation_lock;

/* Our set of threads.
 */
GThread *workers[NUM_IN_PARALLEL];

/* Number of calls so far.
 */
int n_calls = 0;

/* Our test function. This is called by NUM_IN_PARALLEL threads a total of
 * TOTAL_TESTS times. 
 */
static int
test (const char *filename)
{
  VipsImage *im, *x;
  char output_file[256];

  snprintf (output_file, 256, "x/tmp-%p.jpg", g_thread_self ());

  if (!(im = vips_image_new_from_file (filename,
				       "access",
				       VIPS_ACCESS_SEQUENTIAL_UNBUFFERED,
				       NULL)))
    return (-1);

  if (vips_resize (im, &amp;x, 0.1, NULL))
    {
      g_object_unref (im);
      return (-1);
    }
  g_object_unref (im);
  im = x;

  if (vips_image_write_to_file (im, output_file, NULL))
    {
      g_object_unref (im);
      return (-1);
    }
  g_object_unref (im);

  return (0);
}

/* What we run as a thread.
 */
static void *
worker (void *data)
{
  const char *filename = (const char *) data;

  for (;;)
    {
      gboolean done;

      done = FALSE;
      g_mutex_lock (&amp;allocation_lock);
      n_calls += 1;
      if (n_calls &gt; TOTAL_TESTS)
	done = TRUE;
      g_mutex_unlock (&amp;allocation_lock);

      if (done)
	break;

      if (test (filename))
	vips_error_exit (NULL);
    }

  return (NULL);
}

int
main (int argc, char **argv)
{
  int i;

  if (VIPS_INIT (argv[0]))
    vips_error_exit (NULL);

  g_mutex_init (&amp;allocation_lock);

  for (i = 0; i &lt; NUM_IN_PARALLEL; i++)
    workers[i] = g_thread_new (NULL, (GThreadFunc) worker, argv[1]);

  for (i = 0; i &lt; NUM_IN_PARALLEL; i++)
    g_thread_join (workers[i]);

  return (0);
}
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