RFC-0265：计数器：用于跨进程同步的简单 Zircon 对象

// Asserted when a counter's value is less than or equal to zero.
#define ZX_COUNTER_NON_POSITIVE __ZX_OBJECT_SIGNAL_4

// Asserted when a counter's value is greater than zero.
#define ZX_COUNTER_POSITIVE __ZX_OBJECT_SIGNAL_5

## Summary

Create a counter.

## Declaration

zx_status_t zx_counter_create(uint32_t options, zx_handle_t* out);

## Description

zx_counter_create() creates a counter, which is an object that encapsulates
a signed 64-bit integer value that can be incremented, decremented, read, or
written.

When the value is greater than zero, the signal ZX_COUNTER_POSITIVE is
asserted.  Otherwise ZX_COUNTER_NON_POSITIVE is asserted.  Exactly one of
these two signals is always asserted, and never both at once.

The newly-created handle will have rights ZX_RIGHTS_BASIC, ZX_RIGHTS_IO, and
ZX_RIGHT_SIGNAL.  The value will be zero and the signal
ZX_COUNTER_NON_POSITIVE will be asserted on the newly-created object.

## Rights

Caller job policy must allow ZX_POL_NEW_COUNTER.

## Return value

zx_counter_create() returns ZX_OK and a valid event handle (via *out*) on
success.

On failure, an error value is returned.

## Errors

ZX_ERR_INVALID_ARGS  *out* is an invalid pointer, or *options* is non-zero.

ZX_ERR_NO_MEMORY  Failure due to lack of memory.
There is no good way for userspace to handle this (unlikely) error.
In a future build this error will no longer occur.

## Summary

Add an amount to a counter.

## Declaration

zx_status_t zx_counter_add(zx_handle_t handle, int64_t amount);

## Description

zx_counter_add() adds amount to the counter referenced by handle.

After the result of the addition, if the counter's value is:

* less than or equal to zero - ZX_COUNTER_NON_POSITIVE will be asserted
and ZX_COUNTER_POSITIVE will be deasserted.

* greater than zero - ZX_COUNTER_POSITIVE will be asserted and
ZX_COUNTER_NON_POSITIVE will be deasserted.

## Rights

handle must have both ZX_RIGHT_READ and ZX_RIGHT_WRITE.  Because a counter's
value could be determined by checking for ZX_ERR_OUT_OF_RANGE on a series of
carefully crafted zx_counter_add calls, there is no way to create a counter
that cannot be read, but which can be added.

## Return value

zx_counter_add() returns ZX_OK on success.

On failure, an error value is returned.

## Errors

ZX_ERR_WRONG_TYPE  if handle is not a counter handle.

ZX_ERR_ACCESS_DENIED  if handle does not have ZX_RIGHT_WRITE.

ZX_ERR_OUT_OF_RANGE  if the result of the addition would overflow or underflow.

## Summary

Read the value of a counter.

## Declaration

zx_status_t zx_counter_read(zx_handle_t handle, int64_t* value);

## Description

zx_counter_read() reads the value of the counter referenced by handle into the
integer value points at.

## Rights

handle must have ZX_RIGHT_READ.

## Return value

zx_counter_read() returns ZX_OK on success.

On failure, an error value is returned.

## Errors

ZX_ERR_WRONG_TYPE  if handle is not a counter handle.

ZX_ERR_ACCESS_DENIED  if handle does not have ZX_RIGHT_READ.

ZX_ERR_INVALID_ARGS  if value is an invalid pointer.

## Summary

Write the value to a counter.

## Declaration

zx_status_t zx_counter_write(zx_handle_t handle, int64_t value);

## Description

zx_counter_write() writes value to the counter referenced by handle,
asserting/deasserting signals as necessary.

Because concurrent operations on a counter may be interleaved with one another,
an implementation of a "counting semaphore" synchronization protocol should use
zx_counter_add() instead of a sequence of zx_counter_read(), modify,
zx_counter_write().

## Rights

handle must have ZX_RIGHT_WRITE.

## Return value

zx_counter_write() returns ZX_OK on success.

On failure, an error value is returned.

## Errors

ZX_ERR_WRONG_TYPE  if handle is not a counter handle.

ZX_ERR_ACCESS_DENIED  if handle does not have ZX_RIGHT_WRITE.

ZX_ERR_INVALID_ARGS  if value is an invalid pointer.