Controlling external static in C programs

Certain program variables with the extern storage class may be constant and never written. If this is the case, every user does not need to have a separate copy of these variables. In addition, there may be a need to share constant program variables between C and another language.

You can force an external variable to be the part of the program that includes executable code and constant data by using the #pragma variable(varname, NORENT) directive. The program fragment in Figure 1 illustrates how this is accomplished.

Figure 1. Controlling external static

#pragma options(RENT)

#pragma variable(rates, NORENT)
extern float rates[5] = { 3.2, 83.3, 13.4, 3.6, 5.0 };

extern float totals[5];

int main(void) {
    /* ... */
}

In this example, the source file is compiled with the RENT option. The external variable rates are included in the executable code because #pragma variable(rates, NORENT) is specified. The variable totals are included with the writable static. Each user has a copy of the array totals, and the array rates are shared among all users of the program.

The #pragma variable(varname, NORENT) does not apply to, and has no effect on, program variables with the static storage class. Program variables with the static storage class are always included in the writable static. An informational message will appear if you do try to write to a non-reentrant variable when you specify the CHECKOUT compiler option.

When specifying #pragma variable(varname, NORENT), ensure that this variable is never written; if it is written, program exceptions or unpredictable program behavior may result. In addition, you must include #pragma variable(varname, NORENT) in every source file where the variable is referenced or defined. It is good practice to put these pragmas in a common header file.

Note: You can also use the keyword const to ensure that a variable is not written. See the const type qualifier in z/OS XL C/C++ Language Reference for more information.

The ROCONST compiler option has the same effect as specifying the #pragma variable (var_name, NORENT) for all constant variables (i.e. const qualified variables). The option gives the compiler the choice of allocating const variables outside of the Writable Static Area (WSA). For more information, see ROCONST | NORCONST in z/OS XL C/C++ User's Guide.