Passing function arguments

When writing code for optimization, it is usually better to pass a value as an argument to a function than to let the function take the value from a global variable. Global variables might have to be stored before a value is read from a pointer or before a function call is made. Global variables might have to be reloaded after function calls, or stored through a pointer. For more information, see Using ANSI aliasing rules and Using variables.

The #pragma isolated_call preprocessor directive lists functions that do not modify global storage. You can use it to improve the runtime performance of optimized code. For more information, see isolated_call in z/OS XL C/C++ Language Reference.

Linkage convention or how arguments are passed is not specified in the C language, but is defined by the platform. Compilers in general follow the calling convention as described by the Application Binary Interface (ABI). An ABI can define more than one linkage due to performance considerations; for example, the XPLINK and non-XPLINK linkages on the z/OS platform. To correctly invoke a function, the arguments passed must match the parameters as defined in the function definition. For example, if you pass a pointer argument to a function expecting an integer, the code generated by the compiler for the call and for the function definition may not match (see the note at the end of this topic).

As the following example shows, you can declare a function without providing information about the number and types of its parameters.

int func();
...
int a;
func(a);
...
int func(p)
  void *p;
{
  ...
}

Because the function declaration has no parameter information, the compiler is not required to diagnose parameter mismatch. You can call this function, passing it any number of arguments of any type, but the compilation will not be guaranteed to work if the function is not defined to receive the arguments as passed, due to differences in linkage conventions. In the worse case, when the z/OS XL C/C++ compiler attempts inlining of such ill-formed function calls, it may get into an unrecoverable condition and the compilation is halted.

To correct the situation, use the CHECKOUT(GEN) option to identify missing function declarations and non-prototype function declarators. Add or change the declarations to prototyped declarations, and proceed with compilation again. Should you receive diagnostic messages regarding incorrect function argument assignment, change the function call to pass the expected parameter type.

Note: Such a mismatch may sometimes turn out not to be an issue, depending on the ABI; for example, if the ABI happens to allow both pointers and integers passed using general purpose registers. Even in this case, there is no guarantee that the optimized code would work as expected due to ambiguous information received by the compiler.