| Debugging with GDB: Returning |
|---|
return
return expression
You can cancel execution of a function call with the return
command. If you give an
expression argument, its value is used as the function’s return
value.
When you use return, GDB discards the selected stack frame
(and all frames within it). You can think of this as making the
discarded frame return prematurely. If you wish to specify a value to
be returned, give that value as the argument to return.
This pops the selected stack frame (see Selecting a Frame), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions.
The return command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned. In contrast, the finish command (see Continuing and Stepping) resumes execution until the
selected stack frame returns naturally.
GDB needs to know how the expression argument should be set for
the inferior. The concrete registers assignment depends on the OS ABI and the
type being returned by the selected stack frame. For example it is common for
OS ABI to return floating point values in FPU registers while integer values in
CPU registers. Still some ABIs return even floating point values in CPU
registers. Larger integer widths (such as long long int) also have
specific placement rules. GDB already knows the OS ABI from its
current target so it needs to find out also the type being returned to make the
assignment into the right register(s).
Normally, the selected stack frame has debug info. GDB will always
use the debug info instead of the implicit type of expression when the
debug info is available. For example, if you type return -1, and the
function in the current stack frame is declared to return a long long
int, GDB transparently converts the implicit int value of -1
into a long long int:
Breakpoint 1, func () at gdb.base/return-nodebug.c:29
29 return 31;
(gdb) return -1
Make func return now? (y or n) y
#0 0x004004f6 in main () at gdb.base/return-nodebug.c:43
43 printf ("result=%lld\n", func ());
(gdb)
However, if the selected stack frame does not have a debug info, e.g., if the
function was compiled without debug info, GDB has to find out the type
to return from user. Specifying a different type by mistake may set the value
in different inferior registers than the caller code expects. For example,
typing return -1 with its implicit type int would set only a part
of a long long int result for a debug info less function (on 32-bit
architectures). Therefore the user is required to specify the return type by
an appropriate cast explicitly:
Breakpoint 2, 0x0040050b in func () (gdb) return -1 Return value type not available for selected stack frame. Please use an explicit cast of the value to return. (gdb) return (long long int) -1 Make selected stack frame return now? (y or n) y #0 0x00400526 in main () (gdb)