main/syscalls/counter_create.md

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# zx_counter_create

## Summary

Create a counter.

## Declaration

```c
#include <zircon/syscalls.h>

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.

Additionally, the signal `ZX_COUNTER_SIGNALED` may be set or cleared on the
counter using [`zx_object_signal()`]. This is independent of the value of the
counter or the state of the other signals.

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

TODO(https://fxbug.dev/387324141): Add/enforce ZX_POL_NEW_COUNTER policy.

Caller job policy must allow `ZX_POL_NEW_COUNTER`.

## Return value

`zx_counter_create()` returns `ZX_OK` and a valid counter 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.

## See also

 - [`zx_handle_close()`]
 - [`zx_handle_duplicate()`]
 - [`zx_handle_replace()`]
 - [`zx_object_wait_async()`]
 - [`zx_object_wait_many()`]
 - [`zx_object_wait_one()`]
 - [`zx_counter_add()`]
 - [`zx_counter_read()`]
 - [`zx_counter_write()`]

[`zx_handle_close()`]: handle_close.md
[`zx_handle_duplicate()`]: handle_duplicate.md
[`zx_handle_replace()`]: handle_replace.md
[`zx_object_signal()`]: object_signal.md
[`zx_object_wait_async()`]: object_wait_async.md
[`zx_object_wait_many()`]: object_wait_many.md
[`zx_object_wait_one()`]: object_wait_one.md
[`zx_counter_add()`]: counter_add.md
[`zx_counter_read()`]: counter_read.md
[`zx_counter_write()`]: counter_write.md