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Debugging C++

17

This chapter describes how dbx handles C++ exceptions and debugging C++ templates, including a summary of commands used when completing these tasks and examples with code samples.

This chapter is organized into the following sections:

Using dbx with C++

page 191

Exception Handling in dbx

page 192

Debugging With C++ Templates

page 194

Command Reference

page 196

Using dbx with C++

Although this chapter concentrates on two specific aspects of debugging C++, dbx does allow you full functionality when debugging your C++ programs. You can:

For more information on any of these topics, please look in the index under C++.

Exception Handling in dbx

A program stops running if an exception occurs. Exceptions signal programming anomalies, such as division by zero or array overflow. To deal with exceptions, you can set up blocks to catch exceptions raised by expressions elsewhere in the code.

While debugging a program, dbx enables you to:

If you step after stopping at a throw point, control is returned at the start of the first destructor executed during stack unwinding. If you step out of a destructor executed during stack unwinding, control is returned at the start of the next destructor. When all destructors have been executed, step brings you to the catch block handling the throw.

This example demonstrates how exception handling is done in dbx using a sample program containing exceptions. An exception of type int is thrown in the function bar and is caught in the following catch block.

1  #include <stdio.h>

2

3  class c {

4      int x;

5    public:

6      c(int i) { x = i; }

7      ~c() { 

8			printf("destructor for c(%d)\n", x); 

9		}




10  };

11

12  void bar() {

13      c c1(3);

14      throw(99);

15  }

16

17  int main() {

18      try {

19          c c2(5);

20          bar();

21          return 0;

22      }

23      catch (int i) {

24          printf("caught exception %d\n", i);

25      }

26  }

The following transcript from the example program shows the exception handling features in dbx.

(dbx) intercept int

(dbx) intercept

int   

(dbx) stop in bar

(2) stop in bar(void)

(dbx) run

Running: a.out 

(process id 304)

stopped in bar at line 13 in file "foo.cc"

   13       c c1(3);

(dbx) whocatches int

int is caught at line 24, in function main (frame number 2)

(dbx) whocatches c

dbx: no runtime type info for class c (never thrown or caught)

(dbx) cont

Exception of type int is caught at line 24, in function 		main 
(frame number 4)

stopped in _ex_dbg_will_throw at 0xef76dac8

_ex_dbg_will_throw:     save    %sp, -96, %sp

Current function is bar

  14       throw(99);




(dbx) step

stopped in c::~c at line 8 in file "foo.cc"

    8         printf("destructor for c(%d)\n", x); 

(dbx) step

destructor for c(3)

stopped in c::~c at line 9 in file "foo.cc"

    9       }

(dbx) step

stopped in c::~c at line 8 in file "foo.cc"

    8        printf("destructor for c(%d)\n", x); 

(dbx)  step

destructor for c(5)

stopped in c::~c at line 9 in file "foo.cc"

      9        }

(dbx) step

stopped in main at line 24 in file "foo.cc"

   24           printf("caught exception %d\n", i);

(dbx) step

caught exception 99

stopped in main at line 26 in file "foo.cc"

   26   }

Debugging With C++ Templates

dbx supports C++ templates. You can load programs containing class and function templates into dbx and invoke any of the dbx commands on a template that you would use on a class or function, such as:

Template Example

The following code example shows the class template Array and its instantiations and the function template square and its instantiations.

In the following example:

Command Reference

You can invoke any of these exception handling commands when debugging a program.

Commands for Handling Exceptions

exception [-d | +d]

Use this command to display an exception's type. This variable can be used at any time during debugging. With -d, the derived type is shown; otherwise, the static type is shown. This command overwrites the dbxenv variable output_dynamic_type setting.

intercept [-a | -x | typename]

You can intercept, or catch, exceptions of a specific type before the stack has been unwound. Use this command with no arguments to list the types that are being intercepted. Use -a to intercepted all throws. Use typename to add a type to the intercept list. Use -x to exclude a particular type from being intercepted.

For example, to intercept all types except int, you could enter:

(dbx) intercept -a

(dbx) intercept -x int

unintercept [-a | -x | typename]

Use this command to remove exception types from the intercept list. Use this command with no arguments to list the types that are being intercepted (same as intercept). Use -a to remove all intercepted types from the list. Use typename to remove a type from the intercept list. Use -x to stop excluding a particular type from being intercepted.

whocatches typename

This command reports where, if at all, an exception of typename would be caught if thrown at the current point of execution. Use this command to find out what would happen if an exception were thrown from the top frame of the stack.

The line number, function name, and frame number of the catch clause that would catch typename is displayed.

Commands for C++ Exceptions

Use these commands on templates and template instantiations. Once you know the class or type definitions, you can print values, display source listings, or set breakpoints.

whereis name

Use whereis to print a list of all occurrences of function or class instantiations for a function or class template.

For a class template:

(dbx) whereis Array

class template instance: a.out`Array<int> 

class template instance: a.out`Array<double> 

class template:	a.out`template_doc_2.cc`Array

For a function template:

(dbx) whereis square

function template instance: a.out`square(double, double*)

function template instance: a.out`square(int, int*)

function template: a.out`square

whatis name

Use whatis to print the definitions of function and class templates and instantiated functions and classes

For a class template:

(dbx) whatis Array

template<class T> class Array 

To get the full template declaration, try `whatis -t Array<int>';

For a function template:

(dbx) whatis square

More than one identifier `square'.

Select one of the following names:

 0) Cancel

 1) function template instance: `square(int, int*)

 2) function template instance: `square(double, double*)

 3) `a.out`square

> 3

template<class C> void square(C num, C *result);

For a class template instantiation:

(dbx) whatis -t Array<double> 

class Array<double> {

public:     

	int Array<double>::getlength();     

	double &Array<double>::operator [](int i);  

	Array<double>::Array<double>(int l);     		

	Array<double>::~Array<double>(); 

private:     

	int length;     

	double *array; 

};

For a function template instantiation:

(dbx) whatis square(int, int*)

void square(int num, int *result);

stop inclass classname

To stop in all member functions of a template class:

(dbx)stop inclass Array

(2) stop inclass Array

Use stop inclass to set breakpoints at all member functions of a particular template class:

(dbx) stop inclass Array<int> 

(2) stop inclass Array<int>

stop infunction name

Use stop infunction to set breakpoints at all instances of the specified function template:

(dbx) stop infunction square

(9) stop infunction square

stop in function

Use stop in to set a breakpoint at a member function of a template class or at a template function.

For a class template instantiation:

(dbx) stop in Array<int>::Array<int>(int l)

(2) stop in Array<int>::Array<int>(int)

For a function instantiation:

(dbx) stop in square(double, double*)

(6) stop in square(double, double*)

call process [parameters]

Use call to explicitly call a function instantiation or a member function of a class template, provided you are stopped in scope. If dbx is unable to choose the correct instance, a menu allows you to choose the correct instance.

print Expressions

Use print to evaluate a function instantiation or a member function of a class template:

(dbx) print iarray.getlength() 

iarray.getlength() = 5

Use print to evaluate the this pointer:

(dbx) whatis this 

class Array<int> *this; 

(dbx) print *this 

*this = {   

	length = 5     

	array   = 0x21608 

}

list expressions

Use list to print the source listing for the specified function instantiation:

(dbx) list square(int, int*)



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