Environment

No user environment is generally available to STREAMS module procedures and drivers.

Exceptions are the module and driver open and close routines, both of which have access to the u_area of the calling process and both of which can sleep. Otherwise, a STREAMS driver, module put procedure, and module service procedure have no user context and can neither sleep nor access the u_area.

Multiple streams can use a copy of the same module (that is, the same fmodsw), each containing the same processing procedures. Therefore, modules must be reentrant, and care must be exercised when using global data in a module. Put and service procedures are always passed the address of the QUEUE (for example, in the Stream Detail diagram (Figure 1), Au calls the Bu put procedure with Bu as a parameter). The processing procedure establishes its environment solely from the QUEUE contents, which is typically the private data (for example, state information).