atomic_operations.sgml

This GLib version 2.16.1. GLib is the low-level core library that forms the basis for projects such

<!-- ##### SECTION Title ##### -->
Atomic Operations

<!-- ##### SECTION Short_Description ##### -->
basic atomic integer and pointer operations

<!-- ##### SECTION Long_Description ##### -->
<para>
The following functions can be used to atomically access integers and
pointers. They are implemented as inline assembler function on most
platforms and use slower fall-backs otherwise. Using them can sometimes
save you from using a performance-expensive #GMutex to protect the
integer or pointer.  
</para>

<para>
The most important usage is reference counting. Using
g_atomic_int_inc() and g_atomic_int_dec_and_test() makes reference
counting a very fast operation.
</para>

<note>
<para>
You must not directly read integers or pointers concurrently accessed
by multiple threads, but use the atomic accessor functions instead.
That is, always use g_atomic_int_get() and g_atomic_pointer_get() for
read outs.
They provide the neccessary synchonization mechanisms like memory
barriers to access memory locations concurrently.
</para> 
</note>

<note>
<para>
If you are using those functions for anything apart from simple
reference counting, you should really be aware of the implications of
doing that. There are literally thousands of ways to shoot yourself in
the foot. So if in doubt, use a #GMutex. If you don't know, what
memory barriers are, do not use anything but g_atomic_int_inc() and
g_atomic_int_dec_and_test().
</para> 
</note>

<note>
<para>
It is not safe to set an integer or pointer just by assigning to it,
when it is concurrently accessed by other threads with the following
functions. Use g_atomic_int_compare_and_exchange() or
g_atomic_pointer_compare_and_exchange() respectively.
</para> 
</note>

<!-- ##### SECTION See_Also ##### -->
<para>
<variablelist>
 
<varlistentry>
<term>#GMutex</term>
<listitem><para>GLib mutual exclusions.</para></listitem>
</varlistentry>
 
</variablelist>
</para>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### FUNCTION g_atomic_int_get ##### -->
<para>
Reads the value of the integer pointed to by @atomic. Also acts as
a memory barrier.
</para>

@atomic: a pointer to an integer
@Returns: the value of *@atomic
@Since: 2.4


<!-- ##### FUNCTION g_atomic_int_set ##### -->
<para>
Sets the value of the integer pointed to by @atomic. 
Also acts as a memory barrier.
</para>

@atomic: a pointer to an integer
@newval: the new value
@Since: 2.10


<!-- ##### FUNCTION g_atomic_int_add ##### -->
<para>
Atomically adds @val to the integer pointed to by @atomic.
Also acts as a memory barrier.
</para>

@atomic: a pointer to an integer.
@val: the value to add to *@atomic.
@Since: 2.4


<!-- ##### FUNCTION g_atomic_int_exchange_and_add ##### -->
<para>
Atomically adds @val to the integer pointed to by @atomic. It returns
the value of *@atomic just before the addition took place.
Also acts as a memory barrier.
</para>

@atomic: a pointer to an integer.
@val: the value to add to *@atomic.
@Returns: the value of *@atomic before the addition.
@Since: 2.4


<!-- ##### FUNCTION g_atomic_int_compare_and_exchange ##### -->
<para>
Compares @oldval with the integer pointed to by @atomic and
if they are equal, atomically exchanges *@atomic with @newval.
Also acts as a memory barrier.
</para>

@atomic: a pointer to an integer.
@oldval: the assumed old value of *@atomic.
@newval: the new value of *@atomic.
@Returns: %TRUE, if *@atomic was equal @oldval. %FALSE otherwise.
@Since: 2.4


<!-- ##### FUNCTION g_atomic_pointer_get ##### -->
<para>
Reads the value of the pointer pointed to by @atomic. Also acts as
a memory barrier.
</para>

@atomic: a pointer to a #gpointer.
@Returns: the value to add to *@atomic.
@Since: 2.4


<!-- ##### FUNCTION g_atomic_pointer_set ##### -->
<para>
Sets the value of the pointer pointed to by @atomic. 
Also acts as a memory barrier.
</para>

@atomic: a pointer to a #gpointer
@newval: the new value
@Since: 2.10


<!-- ##### FUNCTION g_atomic_pointer_compare_and_exchange ##### -->
<para>
Compares @oldval with the pointer pointed to by @atomic and
if they are equal, atomically exchanges *@atomic with @newval. 
Also acts as a memory barrier.
</para>

@atomic: a pointer to a #gpointer.
@oldval: the assumed old value of *@atomic.
@newval: the new value of *@atomic.
@Returns: %TRUE, if *@atomic was equal @oldval. %FALSE otherwise.
@Since: 2.4


<!-- ##### FUNCTION g_atomic_int_inc ##### -->
<para>
Atomically increments the integer pointed to by @atomic by 1.
</para>

@atomic: a pointer to an integer.
@Since: 2.4


<!-- ##### FUNCTION g_atomic_int_dec_and_test ##### -->
<para>
Atomically decrements the integer pointed to by @atomic by 1.
</para>

@atomic: a pointer to an integer.
@Returns: %TRUE, if the integer pointed to by @atomic is 0 after
decrementing it.
@Since: 2.4