Specifying linkage for C or C++ to Assembler

The process for specifying the linkage to assembler differs for C and for C++. In C, a #pragma linkage directive is used, while in C++ a linkage specifier is used.

Under C, a #pragma linkage directive enables the compiler to generate and accept parameter lists, using a linkage convention known as OS linkage. Although functionally different, both calling an assembler routine and being called by one are handled by the same #pragma. Its format is shown below, where identifier is the name of the assembler function to be called from C or the C function to be called from assembler. The #pragma linkage directive must occur before the call to the entry point.
```
#pragma linkage(identifier, OS)
```
Under C++, a linkage specifier enables the compiler to generate and accept parameter lists, using a linkage convention known as OS linkage. Although functionally different, both calling an assembler routine and being called by one are handled by the same linkage specifier. The format of the linkage specifier is shown below, where fnx desc is the name of the OS entry point:
```
extern "OS" {
   fn1 desc;
   fn2 desc;
⋮
}
```
Note: For C and C++, in XPLINK compiled code, the OS_UPSTACK and OS_NOSTACK (or OS31_NOSTACK) linkages are used for declaring the linkage convention of a routine that the C or C++ code is calling. You cannot define C or C++ entry points as having OS_NOSTACK linkage. You define C or C++ entry points with OS_UPSTACK linkage by compiling the translation units containing them with the NOXPLINK compiler option. In NOXPLINK compiled code, the OS_DOWNSTACK linkage is used to declare the linkage convention for a routine that the C or C++ code is calling. You define C or C++ entry points with OS_DOWNSTACK linkage by compiling the translation units containing them with the XPLINK compiler option.

Just as C (or C++) linkage programs can call OS linkage programs, OS linkage programs can call C linkage programs. An example of C linkage calling OS linkage, which in turn calls C linkage (in this case, one of the z/OS® XL C/C++ library functions) is shown in Figure 1.

In general, any type that can be passed between C and assembler can also be passed between C++ and assembler. However, if a C++ class that uses features not available to assembler (such as virtual functions, virtual base classes, private and protected data, or static data members) is passed to assembler, the results will be undefined.

Note: In C++, a structure is just a class declared with the keyword struct. Its members and base classes are public by default. A union is a class declared with the keyword union its members are public by default, and it holds only one member at a time.