Calling (Debugging with ROCGDB)

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### 17.5 Calling Program Functions

`print expr`

  Evaluate the expression expr and display the resulting value. The expression may include calls to functions in the program being debugged.

`call expr`

  Evaluate the expression expr without displaying `void` returned values.

  You can use this variant of the `print` command if you want to execute a function from your program that does not return anything (a.k.a. a void function), but without cluttering the output with `void` returned values that ROCGDB will otherwise print. If the result is not void, it is printed and saved in the value history.

It is possible for the function you call via the `print` or `call` command to generate a signal (e.g., if there’s a bug in the function, or if you passed it incorrect arguments). What happens in that case is controlled by the `set unwindonsignal` command.

Similarly, with a C`++` program it is possible for the function you call via the `print` or `call` command to generate an exception that is not handled due to the constraints of the dummy frame. In this case, any exception that is raised in the frame, but has an out-of-frame exception handler will not be found. GDB builds a dummy-frame for the inferior function call, and the unwinder cannot seek for exception handlers outside of this dummy-frame. What happens in that case is controlled by the `set unwind-on-terminating-exception` command.

`set unwindonsignal`

  Set unwinding of the stack if a signal is received while in a function that ROCGDB called in the program being debugged. If set to on, ROCGDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off (the default), ROCGDB stops in the frame where the signal was received.

`show unwindonsignal`

  Show the current setting of stack unwinding in the functions called by ROCGDB.

`set unwind-on-terminating-exception`

  Set unwinding of the stack if a C`++` exception is raised, but left unhandled while in a function that ROCGDB called in the program being debugged. If set to on (the default), ROCGDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off, ROCGDB the exception is delivered to the default C`++` exception handler and the inferior terminated.

`show unwind-on-terminating-exception`

  Show the current setting of stack unwinding in the functions called by ROCGDB.

`set may-call-functions`

  Set permission to call functions in the program. This controls whether ROCGDB will attempt to call functions in the program, such as with expressions in the `print` command. It defaults to `on`.

  To call a function in the program, ROCGDB has to temporarily modify the state of the inferior. This has potentially undesired side effects. Also, having ROCGDB call nested functions is likely to be erroneous and may even crash the program being debugged. You can avoid such hazards by forbidding ROCGDB from calling functions in the program being debugged. If calling functions in the program is forbidden, GDB will throw an error when a command (such as printing an expression) starts a function call in the program.

`show may-call-functions`

  Show permission to call functions in the program.

#### 17.5.1 Calling functions with no debug info

Sometimes, a function you wish to call is missing debug information. In such case, ROCGDB does not know the type of the function, including the types of the function’s parameters. To avoid calling the inferior function incorrectly, which could result in the called function functioning erroneously and even crash, ROCGDB refuses to call the function unless you tell it the type of the function.

For prototyped (i.e. ANSI/ISO style) functions, there are two ways to do that. The simplest is to cast the call to the function’s declared return type. For example:

    (gdb) p getenv ("PATH")
    'getenv' has unknown return type; cast the call to its declared return type
    (gdb) p (char *) getenv ("PATH")
    $1 = 0x7fffffffe7ba "/usr/local/bin:/"...

Casting the return type of a no-debug function is equivalent to casting the function to a pointer to a prototyped function that has a prototype that matches the types of the passed-in arguments, and calling that. I.e., the call above is equivalent to:

    (gdb) p ((char * (*) (const char *)) getenv) ("PATH")

and given this prototyped C or C++ function with float parameters:

    float multiply (float v1, float v2) { return v1 * v2; }

these calls are equivalent:

    (gdb) p (float) multiply (2.0f, 3.0f)
    (gdb) p ((float (*) (float, float)) multiply) (2.0f, 3.0f)

If the function you wish to call is declared as unprototyped (i.e. old K&R style), you must use the cast-to-function-pointer syntax, so that ROCGDB knows that it needs to apply default argument promotions (promote float arguments to double). See float promotion. For example, given this unprototyped C function with float parameters, and no debug info:

    float
    multiply_noproto (v1, v2)
      float v1, v2;
    {
      return v1 * v2;
    }

you call it like this:

      (gdb) p ((float (*) ()) multiply_noproto) (2.0f, 3.0f)

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