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The Concurrency support provided with this COBOL system enables several programs to be active at the same time to achieve better use of the Central Processing Unit (CPU) and more sophisticated application programming techniques.
The Server Express run-time system provides support for the concurrent operation of run-units. A run-unit consists of your main COBOL program and all the programs or subprograms called by the main program, directly or indirectly. A run-unit, therefore, can be a single program or a suite of programs.
These run-units can communicate with each other using library routines to access a shared memory area. Descriptions of these routines are contained at the end of this chapter.
The concurrency support facility is mapped onto an operating system's process.
When dealing with run-units, you might find it useful to understand the following terminology and concepts:
On creation, the child inherits certain attributes from its parent. The attributes inherited are copies of the parent's attributes at the exact time of the call; any changes made to the parent's attributes after the call are not reflected in the child's attributes. Similarly, any changes made to the child's attributes are not reflected in the parent's attributes.
You should be aware of the following when using run-units:
There are a number of COBOL library routines provided for managing run-units:
| Routine
|
Description
|
|---|---|
| CBL_EXEC_RUN_UNIT | Creates an asynchronous COBOL run-unit. |
| CBL_CULL_RUN_UNITS | Clears any dead run-units. |
| CBL_YIELD_RUN_UNIT | Yields the remainder of a run-unit's time-slice. |
Full descriptions of these routines are contained in the chapter Library Routines.
Named values and dynamically allocated shared memory provide two means by which run-units can communicate with one another.
Named memory provides a way of passing pointers between different run-units using a name defined at compile time. Named values can be read simultaneously by all units in the coru because the run-time system protects and serializes any updates. The maximum number of named values that can be defined is dependent upon the amount of memory your machine has.
The pointer value could, for example, be a pointer to a block of dynamically allocated memory. Any currently executing run-unit can access this block of memory by simply fetching its address using the name previously assigned. If many run-units wish to alter data in that block of memory, you will need some kind of semaphoring to maintain the integrity of the data.
Shared memory is owned by the coru, so it is freed when the coru terminates. Conversely, non-shared memory is owned by the program that created it, so it is freed when the program is canceled.
The following routines are provided to enable run-units to make use of shared and/or dynamically allocated memory using named values.
| Routine
|
Description
|
|---|---|
| CBL_PUT_SHMEM_PTR | Creates or updates a named value. |
| CBL_GET_SHMEM_PTR | Reads a named value. |
| CBL_ALLOC_MEM | Dynamically allocates memory. |
| CBL_FREE_MEM | Frees dynamically allocated memory. |
Note: Data is not shared between run-units unless explicitly specified using CBL_GET_SHMEM_PTR and CBL_PUT_SHMEM_PTR
The following COBOL system library routines enable you to manage concurrency. See the chapter Library Routines for details of the individual routines.
| CBL_ALLOC_MEM | Dynamic memory allocation |
| CBL_FREE_MEM | Free dynamically allocated memory |
| CBL_CULL_RUN_UNITS | Clear dead run-units |
| CBL_EXEC_RUN_UNIT | Create run-unit |
| CBL_GET_SHMEM_PTR | Read named value |
| CBL_PUT_SHMEM_PTR | Create or update named value |
| CBL_YIELD_RUN_UNIT | Yields the remainder of a run-unit's time-slice. |
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