# mongoc_write_concern_set_w()

## Synopsis

    void
    mongoc_write_concern_set_w (mongoc_write_concern_t *write_concern, int32_t w);

## Parameters

- `write_concern`: A mongoc_write_concern_t.

- `w`: A positive `int32_t` or zero.

## Description

Sets the `w` value for the write concern. See mongoc_write_concern_t for more information on this setting.

Unacknowledged writes are not causally consistent. If you execute a write operation with a mongoc_write_concern_t on which you have called mongoc_write_concern_set_w() with a value of 0, the write does not participate in causal consistency, even when executed with a mongoc_client_session_t.

Beginning in version 1.9.0, this function can now alter the write concern after it has been used in an operation. Previously, using the struct with an operation would mark it as “frozen” and calling this function would log a warning instead instead of altering the write concern.