You will find here the environment variables that change the default behavior of the NATS C Client library.

They are retrieved in nats_Open but will not take precendence over explicit configuration by the application.

In other words, if the environment variable NATS_DEFAULT_LIB_WRITE_DEADLINE is set to say 2000 (in milliseconds, so 2 seconds), then any connection that is created with an option object that has not been explicitly configured with natsOptions_SetWriteDeadline, will use a 2 seconds TCP write deadline, otherwise the value specified by the natsOptions_SetWriteDeadline API will be used.

Note: Using nats_OpenWithConfig prevents the use of the environment variables because the library would not be able to distinguish between the user wanting to explicitly disable a feature or set a value to 0 from the normal C type initialization. In other words, suppose that the environment variable NATS_DEFAULT_TO_LIB_MSG_DELIVERY is set and the user calls nats_OpenWithConfig, the library would not know if natsClientConfig.DefaultToThreadPool is set to false because the user wanted to disable thread pool or because it is initialized to false when the natsClientConfig structure is initialized on the stack.

The primary intent of these environment variables is to be able to change the behavior of the library without needing to modify the code, which is very valuable for the test suite that can be run with the same code base and simply setting the variable before the run.

Name	Effect
`NATS_DEFAULT_TO_LIB_MSG_DELIVERY`	If set (to any value), message delivery uses a global thread pool (see natsOptions_UseGlobalMessageDelivery)
`NATS_DEFAULT_LIB_MSG_DELIVERY_POOL_SIZE`	Sets the message delivery thread pool size (see nats_SetMessageDeliveryPoolSize)
`NATS_DEFAULT_LIB_WRITE_DEADLINE`	Sets the write deadline (in milliseconds). See natsOptions_SetWriteDeadline