Tracing functions in your code

You can instruct the compiler to insert calls to user-defined tracing functions to aid in debugging or timing the execution of other functions.

Using tracing functions in your program requires the following steps:
  1. Writing tracing functions.
  2. Specifying which functions to trace with the -qfunctrace option.
Using the -qfunctrace option causes the compiler to insert calls to these tracing functions at key points in the function body; however you are responsible for defining these tracing functions. The following list describes at which points the tracing functions are called: You can use the -qnofunctrace compiler option or the #pragma nofunctrace pragma to disable function tracing.

How to write tracing functions

To trace functions in your code, define the following tracing functions:
  • __func_trace_enter is the entry point tracing function.
  • __func_trace_exit is the exit point tracing function.
The prototypes of these functions are as follows:
  • void __func_trace_enter(const char *const function_name, const char *const file_name, int line_number, void **const user_data);
  • void __func_trace_exit(const char *const function_name, const char *const file_name, int line_number, void **const user_data);
In the preceding tracing functions, the descriptions for their variables are as follows:
  • function_name is the name of the function you want to trace.
  • file_name is the name of the file.
  • line_number is the line number at entry or exit point of the function. This is a 4-byte number.
  • user_data is the address of a static pointer variable. The static pointer variable is generated by the compiler and initialized to NULL; in addition, because the pointer variable is static, its address is the same for all instrumentation calls inside the same function.
Notes:
  • The exit function is not called if the function has an abnormal exit. The abnormal exit can be caused by actions such as raising a signal or calling the exit() or abort() functions.
  • The -qfunctrace option does not support setjmp and longjmp. For example, a call to longjmp() that leaves function1 and returns from setjmp() in function2 will have a missing call to __func_trace_exit in function1 and a missing a call to __func_trace_enter in function2.
  • To define tracing functions in C++ programs, use the extern "C" linkage directive before your function definition.
  • The function calls are only inserted into the function definition, and if a function is inlined, no tracing is done within the inlined code.
  • If you develop a multithreaded program, make sure the tracing functions have the proper synchronization and do not cause deadlock. Calls to the tracing functions are not thread-safe.
  • If you specify a function that does not exist with the option, the function is ignored.

Rules

The following rules apply when you trace functions in your code:
  • When optimization is enabled, line numbers might not be accurate.
  • The tracing function must not call any instrumented function; otherwise an infinite loop might occur.
  • If you instruct the compiler to trace recursive functions, make sure that your tracing functions can handle recursion.
  • Inlined functions are not instrumented.
  • Tracing functions are not instrumented.
  • Compiler-generated functions are not instrumented, except for the outlined functions generated by optimization such as OpenMP. In those cases, the name of the outlined function contains the name of the original user function as prefix.

Examples

The following C example shows how you can trace functions in your code using function prototypes. Assume you want to trace the entry and exit points of function1 and function2, as well as how much time it takes the compiler to trace them in the following code:

Main program file: t1.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#ifdef __cplusplus
extern "C"
#endif
void __func_trace_enter(const char *function_name, const char *file_name,
                        int line_number, void** const user_data){
   if((*user_data)==NULL)
       (*user_data)=(time_t *)malloc(sizeof(time_t));
   (*(time_t *)*user_data)=time(NULL);
   printf("begin function: name=%s file=%s line=%d\n",function_name,file_name,
          line_number);
}
#ifdef __cplusplus
extern "C"
#endif
void __func_trace_exit(const char *function_name, const char*file_name, 
                       int line_number, void** const user_data){
   printf("end function: name=%s file=%s line=%d. It took %g seconds\n",
          function_name,file_name,line_number, difftime(time(NULL),
          *(time_t *)*user_data));
}
void function2(void){
   sleep(3);
}
void function1(void){
   sleep(5);
   function2();
}
int main(){
   function1();
}
Compile the main program source file as follows:
xlc t1.c -qfunctrace+function1:function2
Run executable a.out to output function trace results: 
begin function: name=function1 file=t.c line=27
begin function: name=function2 file=t.c line=24
end function: name=function2 file=t.c line=25. It took 3 seconds
end function: name=function1 file=t.c line=29. It took 8 seconds
As you see from the preceding example, the user_data parameter is defined to use the system time as basis for time calculation. The following steps explain how user_data is defined to achieve this goal:
  1. The function reserves a memory area for storing the value of user_data.
  2. The system time is used as the value for user_data.
  3. In the __func_trace_exit function, the difftime function uses user_data to calculate time differences. The result is displayed in the form of It took %g seconds in the output.
Main program file: t2.cpp
#include <iostream>
#include <vector>
#include <stdexcept>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
extern "C"
void __func_trace_enter(const char *function_name, const char *file_name,
                        int line_number, void** const user_data){
  if((*user_data)==NULL)
    (*user_data)=(time_t *)malloc(sizeof(time_t));
  (*(time_t *)*user_data)=time(NULL);
  printf("enter function: name=%s file=%s line=%d\n",function_name,file_name,
         line_number);
}
extern "C"
void __func_trace_exit(const char *function_name, const char*file_name,
                       int line_number, void** const user_data){
  printf("exit function: name=%s file=%s line=%d. It took %g seconds\n",
         function_name, file_name, line_number, difftime(time(NULL),
         *(time_t *)*user_data));
}
extern "C"
void __func_trace_catch(const char *function_name, const char*file_name,
                        int line_number, void** const user_data){
  printf("catch function: name=%s file=%s line=%d. It took %g seconds\n",
         function_name, file_name,line_number, difftime(time(NULL),
         *(time_t *)*user_data));
}

template <typename T> class myStack{
  private:
  std::vector<T> elements;
  public:
  void push(T const&);
  void pop();
};

template <typename T>
void myStack<T>::push(T const& value){
  sleep(3);
  std::cout<< "\tpush(" << value << ")" <<std::endl;
  elements.push_back(value);
}
template <typename T>
void myStack<T>::pop(){
  sleep(5);
  std::cout<< "\tpop()" <<std::endl;
  if(elements.empty()){
    throw std::out_of_range("myStack is empty");
  } 
  elements.pop_back();
}
void foo(){
  myStack<int> intValues;
  myStack<float> floatValues;
  myStack<double> doubleValues;
  intValues.push(4);
  floatValues.push(5.5f);
  try{
    intValues.pop();
    floatValues.pop();
    doubleValues.pop(); // cause exception
  } catch(std::exception const& e){
    std::cout<<"\tException: "<<e.what()<<std::endl;
  }
  std::cout<<"\tdone"<<std::endl;
}
#pragma nofunctrace(main)
int main(){
  foo();
}
Compile the main program source file as follows:
xlC t2.cpp -qfunctrace+myStack:foo

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