Calling C and C++ functions from z/OS Debugger

You can perform calls to user and C library functions within z/OS® Debugger, unless your program was compiled with the FORMAT(DWARF) suboption of the DEBUG compiler option.

You can make calls to C library functions at any time. In addition, you can use the C library variables stdin, stdout, stderr, __amrc, and errno in expressions including function calls.

The library function ctdli cannot be called unless it is referenced in a compile unit in the program, either main or a function linked to main.

Calls to user functions can be made, provided z/OS Debugger is able to locate an appropriate definition for the function within the symbol information in the user program. These definitions are created when the program is compiled with TEST(SYM) for C or TEST for C++.

z/OS Debugger performs parameter conversions and parameter-mismatch checking where possible. Parameter checking is performed if:
  • The function is a library function
  • A prototype for the function exists in the current compile unit
  • z/OS Debugger is able to locate a prototype for the function in another compile unit, or the function itself was compiled with TEST(SYM) for C or with TEST for C++.

You can turn off this checking by specifying SET WARNING OFF.

Calls can be made to any user functions that have linkage supported by the C or C++ compiler. However, for C++ calls made to any user function, the function must be declared as: 
extern "C"
For example, use this declaration if you want to debug an application signal handler. When a condition occurs, control passes to z/OS Debugger which then passes control to the signal handler.

z/OS Debugger attempts linkage checking, and does not perform the function call if it determines there is a linkage mismatch. A linkage mismatch occurs when the target program has one linkage but the source program believes it has a different linkage.

It is important to note the following regarding function calls:
  • The evaluation order of function arguments can vary between the C and C++ program and z/OS Debugger. No discernible difference exists if the evaluation of arguments does not have side effects.
  • z/OS Debugger knows about the function return value, and all the necessary conversions are performed when the return value is used in an expression.
  • The functions cannot be in XPLINK applications.
  • The functions must have debug information available.

Refer to the following topics for more information related to the material discussed in this topic.

  • Related references
  • z/OS XL C/C++ Language Reference