Sun Studio 12: Debugging a Program With dbx

Stepping Through a Program

dbx supports two basic single-step commands: next and step, plus two variants of the step command, calledstep up and step to. Both the next command and the step command let the program execute one source line before stopping again.

If the line executed contains a function call, the next command allows the call to be executed and stops at the following line (“steps over” the call). The step command stops at the first line in a called function (“steps into” the call).

The step up command returns the program to the caller function after you have stepped into a function.

The step to command attempts to step into a specified function in the current source line, or if no function is specified, into the last function called as determined by the assembly code for the current source line. The function call may not occur due to a conditional branch, or there may be no function called in the current source line. In these cases, step to steps over the current source line.

Single Stepping

To single step a specified number of lines of code, use the dbx commands next or step followed by the number of lines [n] of code you want executed.

(dbx) next n


(dbx) step n

For more information on the commands, see next Command and step Command.

The step_granularity environment variable determines the unit by which the step command and next command step through your code (see Setting dbx Environment Variables). The unit can be either statement or line.

The step_events environment variable controls whether breakpoints are enabled during a step (see Setting dbx Environment Variables).

The step_abflow environment variable controls whether dbx stops when it detects that an abnormal control flow change is about to happen (see Setting dbx Environment Variables). Such a control flow change can be caused by a call to siglongjmp() or longjmp() or an exception throw.

Continuing Execution of a Program

To continue a program, use the cont command.

(dbx) cont

The cont command has a variant, cont at line_number, which lets you specify a line other than the current program location line at which to resume program execution. This allows you to skip over one or more lines of code that you know are causing problems, without having to recompile.

To continue a program at a specified line, type:

(dbx) cont at 124

The line number is evaluated relative to the file in which the program is stopped; the line number given must be within the scope of the current function.

Using the cont at line_number command with the assign command, you can avoid executing a line of code that contains a call to a function that might be incorrectly computing the value of some variable.

ProcedureTo Resume Program Execution at a Specific Line

  1. Use the assign command to give the variable a correct value.

  2. Use cont at line_number to skip the line that contains the function call that would have computed the value incorrectly.

    Assume that a program is stopped at line 123. Line 123 calls a function, how_fast(), that computes incorrectly a variable, speed. You know what the value of speed should be, so you assign a value to speed. Then you continue program execution at line 124, skipping the call to how_fast().

    (dbx) assign speed = 180; cont at 124;

    For more information, see cont Command.

    If you use the cont command with a when breakpoint command, the program skips the call to how_fast() each time the program attempts to execute line 123.

    (dbx) when at 123 { assign speed = 180; cont at 124;}

    For more information on the when command, see:

Calling a Function

When a program is stopped, you can call a function using the dbx call command, which accepts values for the parameters that must be passed to the called function.

To call a procedure, type the name of the function and supply its parameters. For example:

(dbx) call change_glyph(1,3)

While the parameters are optional, you must type the parentheses after the function_name. For example:

(dbx) call type_vehicle()

You can call a function explicitly, using the call command, or implicitly, by evaluating an expression containing function calls or using a conditional modifier such as stop in glyph -if animate().

A C++ virtual function can be called like any other function using the print command or call command (see print Command or call Command), or any other command that executes a function call.

If the source file in which the function is defined was compiled with the– g option, or if the prototype declaration is visible at the current scope, dbx checks the number and type of arguments and issues an error message if there is a mismatch. Otherwise, dbx does not check the number of parameters and proceeds with the call.

By default, after every call command, dbx automatically calls fflush(stdout) to ensure that any information stored in the I/O buffer is printed. To turn off automatic flushing, set the dbx environment variable output_auto_flush to off.

For C++, dbx handles the implicit this pointer, default arguments, and function overloading. The C++ overloaded functions are resolved automatically if possible. If any ambiguity remains (for example, functions not compiled with -g), dbx displays a list of the overloaded names.

When you use the call command, dbx behaves as though you used the next command, returning from the called function. However, if the program encounters a breakpoint in the called function, dbx stops the program at the breakpoint and issues a message. If you now type a where command, the stack trace shows that the call originated from dbx command level.

If you continue execution, the call returns normally. If you attempt to kill, run, rerun, or debug, the command aborts as dbx tries to recover from the nesting. You can then re-issue the command. Alternatively, you can use the command pop -cto pop all frames up to the most recent call.

Call Safety

Making calls into the process you are debugging, either by using the call command or by printing expressions that contain calls, has the potential for causing severe non-obvious disruptions. Here are some scenarios to watch out for and how you can extricate yourself from them.

Some calls made by dbx are performed “safely.” If a problem, typically a segmentation fault, is encountered instead of the usual “Stopped with call to ...”, dbx:

dbx uses safe calls for: