Debugging a Program With dbx

Chapter 4

Controlling Program Execution

The commands used for running, stepping, and continuing (run, rerun, next, step, and cont) are called process control commands. Used together with the event management commands described in Chapter 6, you can control the run-time behavior of a program as it executes under dbx.

This chapter is organized into the following sections:

• Running a Program
• Attaching dbx to a Running Process
• Detaching dbx From a Process
• Stepping Through a Program
• Using Ctrl+C to Stop a Process

Running a Program

When you first load a program into dbx, dbx visits the program's "main" block (main for C, C++, and Fortran 90; MAIN for FORTRAN 77). dbx waits for you to issue further commands; you can visit code or use event management commands.

You can set breakpoints in the program before running it. Use the run command to start program execution.

To run a program in dbx without arguments, type:

(dbx) run

You can optionally add command-line arguments and redirection of input and output

(dbx) run [arguments][ < input_file] [ > output_file]

.

Output from the run command overwrites an existing file even if you have set noclobber for the shell in which you are running dbx.

The run command without arguments restarts the program using the previous arguments and redirection. The rerun command restarts the program and clears the original arguments and redirection. For more information, see "rerun Command" in the Using dbx Commands section of the Sun WorkShop online help.

Attaching dbx to a Running Process

You might need to debug a program that is already running. You would attach to a running process if:

• You wanted to debug a running server, and you did not want to stop or kill it.
• You wanted to debug a running program that has a graphical user interface, and you didn't want to restart it.
• Your program was looping indefinitely, and you want to debug it without killing it.

You can attach dbx to a running program by using the program's pid number as an argument to the dbx debug command.

Once you have debugged the program, you can then use the detach command to take the program out from the control of dbx without terminating the process.

If you quit dbx after attaching it to a running process, dbx implicitly detaches before terminating.

To attach dbx to a program that is running independently of dbx, you can use either the attach command or the debug command.

To attach dbx to a process that is already running, type:

(dbx) debug program_name process_ID or (dbx) attach program_name process_ID

You can substitute a - (dash) for the program_name; dbx automatically finds the program associated with the process_ID and loads it.

For more information, see "debug Command" and "attach Command" in the Using dbx Commands section of the Sun WorkShop online help.

If dbx is not running, start dbx by typing:

% dbx program_name process_ID

After you have attached dbx to a program, the program stops executing. You can examine it as you would any program loaded into dbx. You can use any event management or process control command to debug it.

You can also attach dbx to a running process in the Sun WorkShop Debugging window by choosing Debug Attach Process. For more information, see "Attaching to a Running Process" in the Using the Debugging Window section of the Sun WorkShop online help.

Detaching dbx From a Process

When you have finished debugging the program, use the detach command to detach dbx from the program. The program then resumes running independently of dbx.

To detach a process from running under the control of dbx:

(dbx) detach

For more information, see "detach Command" in the Using dbx Commands section of the Sun WorkShop online help.

You can also detach dbx from a process in the Sun WorkShop Debugging window by choosing Execute Detach Process. For more information, see "Detaching From a Process" in the Using the Debugging Window section of the Sun WorkShop online help.

Stepping Through a Program

dbx supports two basic single-step commands: next and step, plus a variant of step, called step up. 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). You can also step over a function in the Sun WorkShop Debugging window by choosing Execute Step Over or clicking the Step Over button on the tool bar. You can step into a function by choosing Execute Step Into or clicking the Step Into button on the tool bar.

The step up command returns the program to the caller function after you have stepped into a function. You can also step out of a function in the Sun WorkShop Debugging window by choosing Execute Step Out or clicking the Step Out button on the tool bar.

For more information, see "Program Stepping" in the Using the Debugging Window section of the Sun WorkShop online help.

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 or (dbx) step n

The step_granularity environment variable determines the granularity of source line stepping; that is, the number of next commands needed to step through a line of code. For more information, see "step_granularity Environment Variable" in the Using dbx Commands section of the Sun WorkShop online help.

For more information on the commands, see "next Command" and "step Command" in the Using dbx Commands section of the Sun WorkShop online help.

Continuing Execution of a Program

To continue a program, use the cont command.

(dbx) cont

You can also continue execution of a program in the Sun WorkShop Debugging window by choosing Execute Continue or clicking the Continue button on the tool bar.

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 cont at line_number with assign, 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.

To resume program execution at a specific line:

1. Use assign 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" in the Using dbx Commands section of the Sun WorkShop online help.

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:

• Setting a stop Breakpoint at a Line of Source Code
• Setting Breakpoints in Member Functions of Different Classes
• Setting Breakpoints in Member Functions of the Same Class
• Setting Multiple Breakpoints in Nonmember Functions
• "when Command" in the Using dbx Commands section of the Sun WorkShop online help

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" in the Using dbx Commands section of the Sun WorkShop online help), 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_autoflush 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 -c to pop all frames up to the most recent call.

Using Ctrl+C to Stop a Process

You can stop a process running in dbx by pressing Ctrl+C (^C). When you stop a process using ^C, dbx ignores the ^C, but the child process accepts it as a SIGINT and stops. You can then inspect the process as if it had been stopped by a breakpoint.

To resume execution after stopping a program with ^C, use the cont command. You do not need to use the cont optional modifier, sig signal_name, to resume execution. The cont command resumes the child process after cancelling the pending signal.