1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Chapter 8. Atomic Operations</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="Unreliable Guide To Hacking The Linux Kernel"><link rel="up" href="index.html" title="Unreliable Guide To Hacking The Linux Kernel"><link rel="prev" href="queue-waking.html" title="Waking Up Queued Tasks"><link rel="next" href="symbols.html" title="Chapter 9. Symbols"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 8. Atomic Operations</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="queue-waking.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="symbols.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="atomic-ops"></a>Chapter 8. Atomic Operations</h1></div></div></div><p> 2 Certain operations are guaranteed atomic on all platforms. The 3 first class of operations work on <span class="type">atomic_t</span> 4 5 <code class="filename">include/asm/atomic.h</code>; this 6 contains a signed integer (at least 32 bits long), and you must use 7 these functions to manipulate or read atomic_t variables. 8 <code class="function">atomic_read()</code> and 9 <code class="function">atomic_set()</code> get and set the counter, 10 <code class="function">atomic_add()</code>, 11 <code class="function">atomic_sub()</code>, 12 <code class="function">atomic_inc()</code>, 13 <code class="function">atomic_dec()</code>, and 14 <code class="function">atomic_dec_and_test()</code> (returns 15 <span class="returnvalue">true</span> if it was decremented to zero). 16 </p><p> 17 Yes. It returns <span class="returnvalue">true</span> (i.e. != 0) if the 18 atomic variable is zero. 19 </p><p> 20 Note that these functions are slower than normal arithmetic, and 21 so should not be used unnecessarily. 22 </p><p> 23 The second class of atomic operations is atomic bit operations on an 24 <span class="type">unsigned long</span>, defined in 25 26 <code class="filename">include/linux/bitops.h</code>. These 27 operations generally take a pointer to the bit pattern, and a bit 28 number: 0 is the least significant bit. 29 <code class="function">set_bit()</code>, <code class="function">clear_bit()</code> 30 and <code class="function">change_bit()</code> set, clear, and flip the 31 given bit. <code class="function">test_and_set_bit()</code>, 32 <code class="function">test_and_clear_bit()</code> and 33 <code class="function">test_and_change_bit()</code> do the same thing, 34 except return true if the bit was previously set; these are 35 particularly useful for atomically setting flags. 36 </p><p> 37 It is possible to call these operations with bit indices greater 38 than BITS_PER_LONG. The resulting behavior is strange on big-endian 39 platforms though so it is a good idea not to do this. 40 </p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="queue-waking.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="symbols.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Waking Up Queued Tasks </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 9. Symbols</td></tr></table></div></body></html> 41