| Debugging with GDB: Calling |
|---|
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.
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)