main/syscalls/counter_write.md

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# zx_counter_write

## Summary

Write a value to a counter.

## Declaration

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

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.  See [`zx_counter_add()`] for
the conditions on which signals are asserted/deasserted.

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.

## See also

 - [`zx_object_wait_async()`]
 - [`zx_object_wait_many()`]
 - [`zx_object_wait_one()`]
 - [`zx_counter_create()`]
 - [`zx_counter_add()`]
 - [`zx_counter_read()`]

[`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_create()`]: counter_create.md
[`zx_counter_add()`]: counter_add.md
[`zx_counter_read()`]: counter_read.md