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GLib Reference Manual | |
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#include <glib.h> gint g_atomic_int_get (volatile gint *atomic); void g_atomic_int_set (volatile gint *atomic, gint newval); void g_atomic_int_add (volatile gint *atomic, gint val); gint g_atomic_int_exchange_and_add (volatile gint *atomic, gint val); gboolean g_atomic_int_compare_and_exchange (volatile gint *atomic, gint oldval, gint newval); gpointer g_atomic_pointer_get (volatile gpointer *atomic); void g_atomic_pointer_set (volatile gpointer *atomic, gpointer newval); gboolean g_atomic_pointer_compare_and_exchange (volatile gpointer *atomic, gpointer oldval, gpointer newval); void g_atomic_int_inc (gint *atomic); gboolean g_atomic_int_dec_and_test (gint *atomic);
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.
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.
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.
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().
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.
gint g_atomic_int_get (volatile gint *atomic);
Reads the value of the integer pointed to by atomic. Also acts as
a memory barrier.
atomic : |
a pointer to an integer |
| Returns : | the value of *atomic
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Since 2.4
void g_atomic_int_set (volatile gint *atomic, gint newval);
Sets the value of the integer pointed to by atomic.
Also acts as a memory barrier.
atomic : |
a pointer to an integer |
newval : |
the new value |
Since 2.10
void g_atomic_int_add (volatile gint *atomic, gint val);
Atomically adds val to the integer pointed to by atomic.
Also acts as a memory barrier.
atomic : |
a pointer to an integer. |
val : |
the value to add to *atomic.
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Since 2.4
gint g_atomic_int_exchange_and_add (volatile gint *atomic, gint val);
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.
atomic : |
a pointer to an integer. |
val : |
the value to add to *atomic.
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| Returns : | the value of *atomic before the addition.
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Since 2.4
gboolean g_atomic_int_compare_and_exchange (volatile gint *atomic, gint oldval, gint newval);
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.
atomic : |
a pointer to an integer. |
oldval : |
the assumed old value of *atomic.
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newval : |
the new value of *atomic.
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| Returns : |
TRUE, if *atomic was equal oldval. FALSE otherwise.
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Since 2.4
gpointer g_atomic_pointer_get (volatile gpointer *atomic);
Reads the value of the pointer pointed to by atomic. Also acts as
a memory barrier.
atomic : |
a pointer to a gpointer. |
| Returns : | the value to add to *atomic.
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Since 2.4
void g_atomic_pointer_set (volatile gpointer *atomic, gpointer newval);
Sets the value of the pointer pointed to by atomic.
Also acts as a memory barrier.
atomic : |
a pointer to a gpointer |
newval : |
the new value |
Since 2.10
gboolean g_atomic_pointer_compare_and_exchange (volatile gpointer *atomic, gpointer oldval, gpointer newval);
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.
atomic : |
a pointer to a gpointer. |
oldval : |
the assumed old value of *atomic.
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newval : |
the new value of *atomic.
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| Returns : |
TRUE, if *atomic was equal oldval. FALSE otherwise.
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Since 2.4
void g_atomic_int_inc (gint *atomic);
Atomically increments the integer pointed to by atomic by 1.
atomic : |
a pointer to an integer. |
Since 2.4