Atomic Operations

Atomic Operations — basic atomic integer and pointer operations

Synopsis

#include <glib.h>

gint                g_atomic_int_get                    (volatile gint G_GNUC_MAY_ALIAS *atomic);
void                g_atomic_int_set                    (volatile gint G_GNUC_MAY_ALIAS *atomic,
                                                         gint newval);
void                g_atomic_int_add                    (volatile gint G_GNUC_MAY_ALIAS *atomic,
                                                         gint val);
gint                g_atomic_int_exchange_and_add       (volatile gint G_GNUC_MAY_ALIAS *atomic,
                                                         gint val);
gboolean            g_atomic_int_compare_and_exchange   (volatile gint G_GNUC_MAY_ALIAS *atomic,
                                                         gint oldval,
                                                         gint newval);
gpointer            g_atomic_pointer_get                (volatile gpointer G_GNUC_MAY_ALIAS *atomic);
void                g_atomic_pointer_set                (volatile gpointer G_GNUC_MAY_ALIAS *atomic,
                                                         gpointer newval);
gboolean            g_atomic_pointer_compare_and_exchange
                                                        (volatile gpointer G_GNUC_MAY_ALIAS *atomic,
                                                         gpointer oldval,
                                                         gpointer newval);
void                g_atomic_int_inc                    (gint *atomic);
gboolean            g_atomic_int_dec_and_test           (gint *atomic);

Description

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.

Note

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.

Note

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().

Note

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.

Details

g_atomic_int_get ()

gint                g_atomic_int_get                    (volatile gint G_GNUC_MAY_ALIAS *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

Since 2.4

g_atomic_int_set ()

void                g_atomic_int_set                    (volatile gint G_GNUC_MAY_ALIAS *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

g_atomic_int_add ()

void                g_atomic_int_add                    (volatile gint G_GNUC_MAY_ALIAS *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

Since 2.4

g_atomic_int_exchange_and_add ()

gint                g_atomic_int_exchange_and_add       (volatile gint G_GNUC_MAY_ALIAS *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

Returns :

 the value of *atomic before the addition.

Since 2.4

g_atomic_int_compare_and_exchange ()

gboolean            g_atomic_int_compare_and_exchange   (volatile gint G_GNUC_MAY_ALIAS *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

newval :

 the new value of *atomic

Returns :

 TRUE, if *atomic was equal oldval. FALSE otherwise.

Since 2.4

g_atomic_pointer_get ()

gpointer            g_atomic_pointer_get                (volatile gpointer G_GNUC_MAY_ALIAS *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.

Since 2.4

g_atomic_pointer_set ()

void                g_atomic_pointer_set                (volatile gpointer G_GNUC_MAY_ALIAS *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

g_atomic_pointer_compare_and_exchange ()

gboolean            g_atomic_pointer_compare_and_exchange
                                                        (volatile gpointer G_GNUC_MAY_ALIAS *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

newval :

 the new value of *atomic

Returns :

 TRUE, if *atomic was equal oldval. FALSE otherwise.

Since 2.4

g_atomic_int_inc ()

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

g_atomic_int_dec_and_test ()

gboolean            g_atomic_int_dec_and_test           (gint *atomic);

Atomically decrements the integer pointed to by atomic by 1.

atomic :

 a pointer to an integer

Returns :

 TRUE if the integer pointed to by atomic is 0 after decrementing it

Since 2.4

See Also

GMutex
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