A user exit is a

subroutine In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times. Callable units provide a ...

invoked by a

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

package for a predefined event in the execution of the package. In some cases the exit is specified by the installation when configuring the package while in other cases the users of the package can substitute their own subroutines in place of the default ones provided by the package vendor to provide customized functionality. In some cases security controls restrict exits to authorized users, e.g., EXCP

appendages An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

MVS Multiple Virtual Storage, more commonly called MVS, is the most commonly used operating system on the System/370, System/390 and IBM Z IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unr ...

. The earliest use of this term involved operating systems to let customized code temporarily take control when a pre-designated event occurred.IBM term: OS/MFT, MVT A more typical use is replacing the user exits provided by a sort/merge package, whereby the user program provides its own subroutines for comparing records. The procedures provided by the user take the place of the default routines (usually stubs that do nothing but

return Return may refer to: In business, economics, and finance * Return on investment (ROI), the financial gain after an expense. * Rate of return, the financial term for the profit or loss derived from an investment * Tax return, a blank document or t ...

to their caller) provided by the package vendor. Procedures provided as user exits are typically compiled into a

static library A static library or statically linked library contains functions and data that can be included in a consuming computer program at build-time such that the library does not need to be accessible in a separate file at run-time. If all libraries a ...

and linked directly with the package to produce an

executable program In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a da ...

. Another approach employs dynamic libraries to accomplish the same thing. Finally, user exits may be external programs executed at specific points in the execution of a host package.

History

Historically, this term is commonly used in IBM mainframe vernacular. * Title: z/OS V1R10 DFSMS Installation Exits Document Number: SC26-7396-11 * Title: z/OS V1R10.0 JES2 Installation Exits Document Number: SA22-7534-10 * Title: z/OS V1R10.0 MVS Installation Exits Document Number: SA22-7593-14

Restrictions

If the user site specific code is substituted for the software vendor provided default exit it must interface to the software package using the defined parameters as documented for the default exit. User exits are important because while they enable site specific customization they isolate such installation specific customization to defined and supported points enabling the site to upgrade to follow-on releases of the software package without adverse impact to preexisting customized functionality. Some references to IBM user exit manuals are given below. Other vendors such as SAP, Oracle, IFS, HP, Macro4, Compuware, CA all employ user exits in some of their software products.

Example

In SAP, A 'COMMIT WORK' must never be used inside a User Exit, as it may affect the program processing. Furthermore, error messages may not be issued from inside an exit, as they halt the processing of the code that follows the message.

Enforcing standards

If files containing user-created content are meant to follow a standard, the (file) ''Open'' command of a word-processing document can invoke a user exit that does validation and, if necessary, returns an error code.Sample: Opinion.AnalystID.StockSymbol.DateTime - Opinion.Q347.IBM.201907031321 - alternatively, the date/time portion may be filled in by the user exit.

Applications

Some applications that provide user exits: *

Apache Subversion Apache Subversion (often abbreviated SVN, after its command name ''svn'') is a version control system distributed as open source under the Apache License. Software developers use Subversion to maintain current and historical versions of files su ...

allows "hooks", which are "scripts that run when an action is performed" * IBM

CICS IBM CICS (Customer Information Control System) is a family of mixed-language application servers that provide online business transaction management, transaction management and connectivity for applications on IBM mainframe systems under z/OS ...

IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...

CMVC user exits in the form of

Kornshell KornShell (ksh) is a Unix shell which was developed by David Korn (computer scientist), David Korn at Bell Labs in the early 1980s and announced at USENIX Annual Technical Conference, USENIX on July 14, 1983. The initial development was base ...

scripts * IBM JES 2 and 3 * IBM MVS,

SMS Short Message Service, commonly abbreviated as SMS, is a text messaging service component of most telephone, Internet and mobile device systems. It uses standardized communication protocols that let mobile phones exchange short text messages, t ...

z/OS z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest: ...

and dozens of sub-components such as

RACF Resource Access Control Facility (RACF) is an IBM software security product that provides access control and auditing functions for the z/OS and z/VM operating systems. RACF was introduced in 1976. Originally called RACF it was renamed to z/OS S ...

, SMF, etc. *

sort/merge package * Oracle CC&B *

SAP Sap is a fluid transported in the xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, or cell sap; it is a s ...

References