Reentrancy in z/OS® XL C/C++

This information describes the concept of reentrancy. It tells you how to use reentrancy in C programs to help make your programs more efficient, and how C++ achieves constructed reentrancy.

Reentrant programs are structured to allow multiple users to share a single copy of an executable module or to use an executable module repeatedly without reloading. C and C++ achieve reentrancy by splitting your program into two parts, which are maintained in separate areas of memory until the program terminates:
  • The first part, which consists of executable code and constant data, does not change during program execution.
  • The second part contains persistent data that can be altered. This part includes the dynamic storage area (DSA) and a piece of storage known as the writable static area.

    For XPLINK, the writable static area is further logically subdivided into areas called environments. Environments are optional, and each function can have its own environment. When an XPLINK function is called, the caller must load general purpose register 5 with the address of the environment of the called function before control is given to the entry point of the called function.

If the program is installed in the Link Pack Area (LPA) or Extended Link Pack Area (ELPA) of your operating system, only a single copy of the first (constant or reentrant) part exists within a single address space. This occurs regardless of the number of users that are running the program simultaneously. This reentrant part may be shared across address spaces or across sessions. In this case, the executable module is loaded only once. Separate concurrent invocations of the program share or reenter the same copy of the write-protected executable module. If the program is not installed in the LPA or ELPA area, each invocation receives a private copy of the code part, but this copy may not be write-protected.

The modifiable writable static part of the program contains:
  • All program variables with the static storage class
  • All program variables receiving the extern storage class
  • All writable strings
  • All function linkage descriptors for all referenced DLL functions
  • Function linkage descriptors for all referenced DLL functions that are used by multiple compilation units in the program, but are not imported (XPLINK, RENT)
  • All variable pointers for imported variables (non-XPLINK)
  • All function pointers for imported functions (XPLINK, RENT)
  • All variable linkage descriptors to reference imported variables (non-XPLINK)

Each user running the program receives a private copy of the second (data or non-reentrant) part. This part, the data area, is modifiable by each user.

The code part of the program contains:
  • Executable instructions
  • Read-only constants
  • Global objects compiled with the #pragma variable(identifier, NORENT)
    Note: The ROCONST compiler option implicitly inserts a #pragma variable(identifier, NORENT) for const qualified variables.
Parent topic: Coding: Advanced Topics