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 GDB 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 GDB called in the program being debugged. If set to on, GDB 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), GDB stops in the frame where the signal was received.

show unwindonsignal

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

set unwind-on-terminating-exception

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

set may-call-functions

Set permission to call functions in the program. This controls whether GDB 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, GDB has to temporarily modify the state of the inferior. This has potentially undesired side effects. Also, having GDB call nested functions is likely to be erroneous and may even crash the program being debugged. You can avoid such hazards by forbidding GDB 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, GDB 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, GDB 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 GDB 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)