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<title>VIPS and threads: VIPS Reference Manual</title>
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<div class="refentry">
<a name="using-threads"></a><div class="titlepage"></div>
<div class="refnamediv"><table width="100%"><tr>
<td valign="top">
<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>
</tr></table></div>
<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="VipsOperation.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 <a class="link" href="VipsRegion.html" title="VipsRegion"><span class="type">VipsRegion</span></a> between threads</h4>
<p>
<a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> objects are immutable and can be shared between
threads very simply.
However the lower-level <a class="link" href="VipsRegion.html" title="VipsRegion"><span class="type">VipsRegion</span></a> object used to implement <a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a>
(see <a class="link" href="extending.html" title="Extending VIPS">Extending VIPS</a>) is mutable and you
can only use a <a class="link" href="VipsRegion.html" title="VipsRegion"><span class="type">VipsRegion</span></a> from one thread at once.
</p>
<p>
In fact it's worse than that: to reduce locking, <a class="link" href="VipsRegion.html" title="VipsRegion"><span class="type">VipsRegion</span></a> 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">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
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<td class="listing_code"><pre class="programlisting"><span class="comment">/* Read from many threads. </span>
<span class="comment"> *</span>
<span class="comment"> * Compile with:</span>
<span class="comment"> *</span>
<span class="comment"> * gcc -g -Wall soak.c `pkg-config vips --cflags --libs`</span>
<span class="comment"> *</span>
<span class="comment"> * Run with:</span>
<span class="comment"> *</span>
<span class="comment"> * rm -rf x</span>
<span class="comment"> * mkdir x</span>
<span class="comment"> * for i in {0..10}; do ./a.out ~/pics/k2.jpg; done</span>
<span class="comment"> *</span>
<span class="comment"> */</span>
<span class="gtkdoc ppc">#include <stdio.h></span>
<span class="gtkdoc ppc">#include <glib.h></span>
<span class="gtkdoc ppc">#include <vips/vips.h></span>
<span class="comment">/* How many pings we run at once.</span>
<span class="comment"> */</span>
<span class="gtkdoc ppc">#define NUM_IN_PARALLEL (50)</span>
<span class="comment">/* Number of tests we do in total.</span>
<span class="comment"> */</span>
<span class="gtkdoc ppc">#define TOTAL_TESTS (NUM_IN_PARALLEL * 20)</span>
<span class="comment">/* Workers queue up on this.</span>
<span class="comment"> */</span>
GMutex allocation_lock<span class="gtkdoc opt">;</span>
<span class="comment">/* Our set of threads.</span>
<span class="comment"> */</span>
GThread <span class="gtkdoc opt">*</span>workers<span class="gtkdoc opt">[</span>NUM_IN_PARALLEL<span class="gtkdoc opt">];</span>
<span class="comment">/* Number of calls so far.</span>
<span class="comment"> */</span>
<span class="gtkdoc kwb">int</span> n_calls <span class="gtkdoc opt">=</span> <span class="number">0</span><span class="gtkdoc opt">;</span>
<span class="comment">/* Our test function. This is called by NUM_IN_PARALLEL threads a total of</span>
<span class="comment"> * TOTAL_TESTS times. </span>
<span class="comment"> */</span>
<span class="gtkdoc kwb">static int</span>
<span class="function">test</span> <span class="gtkdoc opt">(</span><span class="gtkdoc kwb">const char</span> <span class="gtkdoc opt">*</span>filename<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
VipsImage <span class="gtkdoc opt">*</span>im<span class="gtkdoc opt">, *</span>x<span class="gtkdoc opt">;</span>
<span class="gtkdoc kwb">char</span> output_file<span class="gtkdoc opt">[</span><span class="number">256</span><span class="gtkdoc opt">];</span>
<span class="function">snprintf</span> <span class="gtkdoc opt">(</span>output_file<span class="gtkdoc opt">,</span> <span class="number">256</span><span class="gtkdoc opt">,</span> <span class="string">"x/tmp-%p.jpg"</span><span class="gtkdoc opt">,</span> <span class="function">g_thread_self</span> <span class="gtkdoc opt">());</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(!(</span>im <span class="gtkdoc opt">=</span> <span class="function"><a href="VipsImage.html#vips-image-new-from-file">vips_image_new_from_file</a></span> <span class="gtkdoc opt">(</span>filename<span class="gtkdoc opt">,</span>
<span class="string">"access"</span><span class="gtkdoc opt">,</span>
VIPS_ACCESS_SEQUENTIAL_UNBUFFERED<span class="gtkdoc opt">,</span>
NULL<span class="gtkdoc opt">)))</span>
<span class="keyword">return</span> <span class="gtkdoc opt">(-</span><span class="number">1</span><span class="gtkdoc opt">);</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span><span class="function"><a href="libvips-resample.html#vips-resize">vips_resize</a></span> <span class="gtkdoc opt">(</span>im<span class="gtkdoc opt">, &</span>x<span class="gtkdoc opt">,</span> <span class="number">0.1</span><span class="gtkdoc opt">,</span> NULL<span class="gtkdoc opt">))</span>
<span class="gtkdoc opt">{</span>
<span class="function">g_object_unref</span> <span class="gtkdoc opt">(</span>im<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> <span class="gtkdoc opt">(-</span><span class="number">1</span><span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="function">g_object_unref</span> <span class="gtkdoc opt">(</span>im<span class="gtkdoc opt">);</span>
im <span class="gtkdoc opt">=</span> x<span class="gtkdoc opt">;</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span><span class="function"><a href="VipsImage.html#vips-image-write-to-file">vips_image_write_to_file</a></span> <span class="gtkdoc opt">(</span>im<span class="gtkdoc opt">,</span> output_file<span class="gtkdoc opt">,</span> NULL<span class="gtkdoc opt">))</span>
<span class="gtkdoc opt">{</span>
<span class="function">g_object_unref</span> <span class="gtkdoc opt">(</span>im<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> <span class="gtkdoc opt">(-</span><span class="number">1</span><span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="function">g_object_unref</span> <span class="gtkdoc opt">(</span>im<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> <span class="gtkdoc opt">(</span><span class="number">0</span><span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="comment">/* What we run as a thread.</span>
<span class="comment"> */</span>
<span class="gtkdoc kwb">static void</span> <span class="gtkdoc opt">*</span>
<span class="function">worker</span> <span class="gtkdoc opt">(</span><span class="gtkdoc kwb">void</span> <span class="gtkdoc opt">*</span>data<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
<span class="gtkdoc kwb">const char</span> <span class="gtkdoc opt">*</span>filename <span class="gtkdoc opt">= (</span><span class="gtkdoc kwb">const char</span> <span class="gtkdoc opt">*)</span> data<span class="gtkdoc opt">;</span>
<span class="keyword">for</span> <span class="gtkdoc opt">(;;)</span>
<span class="gtkdoc opt">{</span>
gboolean done<span class="gtkdoc opt">;</span>
done <span class="gtkdoc opt">=</span> FALSE<span class="gtkdoc opt">;</span>
<span class="function">g_mutex_lock</span> <span class="gtkdoc opt">(&</span>allocation_lock<span class="gtkdoc opt">);</span>
n_calls <span class="gtkdoc opt">+=</span> <span class="number">1</span><span class="gtkdoc opt">;</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span>n_calls <span class="gtkdoc opt">></span> TOTAL_TESTS<span class="gtkdoc opt">)</span>
done <span class="gtkdoc opt">=</span> TRUE<span class="gtkdoc opt">;</span>
<span class="function">g_mutex_unlock</span> <span class="gtkdoc opt">(&</span>allocation_lock<span class="gtkdoc opt">);</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span>done<span class="gtkdoc opt">)</span>
<span class="keyword">break</span><span class="gtkdoc opt">;</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span><span class="function">test</span> <span class="gtkdoc opt">(</span>filename<span class="gtkdoc opt">))</span>
<span class="function"><a href="libvips-error.html#vips-error-exit">vips_error_exit</a></span> <span class="gtkdoc opt">(</span>NULL<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="keyword">return</span> <span class="gtkdoc opt">(</span>NULL<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="gtkdoc kwb">int</span>
<span class="function">main</span> <span class="gtkdoc opt">(</span><span class="gtkdoc kwb">int</span> argc<span class="gtkdoc opt">,</span> <span class="gtkdoc kwb">char</span> <span class="gtkdoc opt">**</span>argv<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
<span class="gtkdoc kwb">int</span> i<span class="gtkdoc opt">;</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span><span class="function"><a href="libvips-vips.html#VIPS-INIT:CAPS">VIPS_INIT</a></span> <span class="gtkdoc opt">(</span>argv<span class="gtkdoc opt">[</span><span class="number">0</span><span class="gtkdoc opt">]))</span>
<span class="function"><a href="libvips-error.html#vips-error-exit">vips_error_exit</a></span> <span class="gtkdoc opt">(</span>NULL<span class="gtkdoc opt">);</span>
<span class="function">g_mutex_init</span> <span class="gtkdoc opt">(&</span>allocation_lock<span class="gtkdoc opt">);</span>
<span class="keyword">for</span> <span class="gtkdoc opt">(</span>i <span class="gtkdoc opt">=</span> <span class="number">0</span><span class="gtkdoc opt">;</span> i <span class="gtkdoc opt"><</span> NUM_IN_PARALLEL<span class="gtkdoc opt">;</span> i<span class="gtkdoc opt">++)</span>
workers<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">] =</span> <span class="function">g_thread_new</span> <span class="gtkdoc opt">(</span>NULL<span class="gtkdoc opt">, (</span>GThreadFunc<span class="gtkdoc opt">)</span> worker<span class="gtkdoc opt">,</span> argv<span class="gtkdoc opt">[</span><span class="number">1</span><span class="gtkdoc opt">]);</span>
<span class="keyword">for</span> <span class="gtkdoc opt">(</span>i <span class="gtkdoc opt">=</span> <span class="number">0</span><span class="gtkdoc opt">;</span> i <span class="gtkdoc opt"><</span> NUM_IN_PARALLEL<span class="gtkdoc opt">;</span> i<span class="gtkdoc opt">++)</span>
<span class="function">g_thread_join</span> <span class="gtkdoc opt">(</span>workers<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">]);</span>
<span class="keyword">return</span> <span class="gtkdoc opt">(</span><span class="number">0</span><span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span></pre></td>
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