NAME¶
libatomic-stack - Library providing linked stack abstraction
SYNOPSIS¶
#include <atomic_ops_stack.h>
cc ... -latomic_ops_gpl
Note that the AO_stack implementation is licensed under the GPL, unlike the
lower level routines.
void AO_stack_init(AO_stack_t *list);
void AO_stack_push_release(AO_stack_t *list, AO_t *new_element);
AO_t * AO_stack_pop_acquire(volatile AO_stack_t *list);
DESCRIPTION¶
libatomic-stack defines a linked stack abstraction. Stacks may be
accessed by multiple concurrent threads. The implementation is 1-lock-free,
i.e. it will continue to make progress if at most one thread becomes inactive
while operating on the data structure.
This makes it safe to access these data structures from non-reentrant signal
handlers, provided at most one non-signal-handler thread is accessing the data
structure at once. This latter condition can be ensured by acquiring an
ordinary lock around the non-hndler accesses to the data structure.
We use a fully lock-free implementation when the underlying hardware makes that
less expensive, i.e. when we have a double-wide compare-and-swap operation
available. (The fully lock-free implementation uses an AO_t- sized version
count, and assumes it does not wrap during the time any given operation is
active. This seems reasonably safe on 32-bit hardware, and very safe on 64-bit
hardware.) If a fully lock-free implementation is used, the macro
AO_STACK_IS_LOCK_FREE will be defined.
The cleanest way to use these routines is probably to define the stack node type
with an initial
AO_t link field, so that the conversion between the
link-field pointer and the stack element pointer is just a compile-time cast.
But other possibilities exist. (This would be cleaner in C++ with templates.)
A stack is represented by an AO_stack_t structure. (This is normally 2 or 3
times the size of a pointer.) It may be statically initialized by setting it
to
AO_STACK_INITIALIZER , or dynamically initialized to an empty stack
with
AO_stack_init accessing stacks:
- AO_stack_init
- Initalise a stack
- AO_stack_push_release
- Push new element onto the stack.
- AO_stack_pop_acquire
- Pop element off the stack.
We require that the objects pushed as list elements remain addressable as long
as any push or pop operation are in progress. (It is OK for an object to be
"pop"ped off a stack and "deallocated" with a concurrent
"pop" on the same stack still in progress, but only if
"deallocation" leaves the object addressable. The second
"pop" may still read the object, but the value it reads will not
matter.)
We require that the headers (
AO_stack objects) remain allocated and
valid as long as any operations on them are still in-flight.
We also provide macros
AO_REAL_HEAD_PTR that converts an
AO_stack_t to a pointer to the link field in the next element, and
AO_REAL_NEXT_PTR that converts a link field to a real, dereferencable,
pointer to the link field in the next element. This is intended only for
debugging, or to traverse the list after modification has ceased. There is
otherwise no guarantee that walking a stack using this macro will produce any
kind of consistent picture of the data structure.
SEE ALSO¶
libatomic-ops(3),
libatomic-malloc(3)
AUTHOR¶
This manual page was written by Ian Wienand <ianw@gelato.unsw.edu.au>,
based on comments in the source code. It was written for the Debian project
(but may be used by others).