Opening an application program

After a VTAM® application program starts, it issues an OPEN macroinstruction to notify VTAM that it is an active element in the network.

VTAM uses the RACROUTE macroinstruction to verify user authority for opening an application. RACROUTE invokes the System Authorization Facility (SAF) MVS™ router, which conditionally directs control to the security management product. For additional information on RACROUTE, refer to the z/OS Security Server RACF Security Administrator's Guide.

VTAM grants the application access to the network when:

SAF indicates that the application is authorized to become an element in the network. VTAM then bypasses the password checking, and the security management product provides resource access control.
SAF cannot make a decision, and VTAM successfully performs VTAM password verification.

During the OPEN processing, VTAM establishes an SSCP-LU session with the application program logical unit (LU). VTAM can now accept requests for LU-LU sessions with this application program. In addition, the application program can now make further requests of VTAM. Normally, the application program remains open until it issues a CLOSE macroinstruction.

VTAM considers each open access method control block (ACB) to be a separate LU. Therefore, if an application program opens more than one ACB, VTAM sees each open ACB as a different LU, even though the ACBs are related to the same application program.

Opening an application program requires the following:

An ACB defined with an ACB or GENCB macroinstruction
An OPEN macroinstruction

Because an ACB can point to a list of exit routines, defined with an EXLST macroinstruction, an EXLST macroinstruction is usually also required.