gdb: Ambiguous Expressions
10.2 Ambiguous Expressions
==========================
Expressions can sometimes contain some ambiguous elements. For
instance, some programming languages (notably Ada, C++ and Objective-C)
permit a single function name to be defined several times, for
application in different contexts. This is called "overloading".
Another example involving Ada is generics. A "generic package" is
similar to C++ templates and is typically instantiated several times,
resulting in the same function name being defined in different contexts.
In some cases and depending on the language, it is possible to adjust
the expression to remove the ambiguity. For instance in C++, you can
specify the signature of the function you want to break on, as in 'break
FUNCTION(TYPES)'. In Ada, using the fully qualified name of your
function often makes the expression unambiguous as well.
When an ambiguity that needs to be resolved is detected, the debugger
has the capability to display a menu of numbered choices for each
possibility, and then waits for the selection with the prompt '>'. The
first option is always '[0] cancel', and typing '0 <RET>' aborts the
current command. If the command in which the expression was used allows
more than one choice to be selected, the next option in the menu is '[1]
all', and typing '1 <RET>' selects all possible choices.
For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol 'String::after'. We choose three
particular definitions of that function name:
(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
breakpoints.
(gdb)
'set multiple-symbols MODE'
This option allows you to adjust the debugger behavior when an
expression is ambiguous.
By default, MODE is set to 'all'. If the command with which the
expression is used allows more than one choice, then GDB
automatically selects all possible choices. For instance,
inserting a breakpoint on a function using an ambiguous name
results in a breakpoint inserted on each possible match. However,
if a unique choice must be made, then GDB uses the menu to help you
disambiguate the expression. For instance, printing the address of
an overloaded function will result in the use of the menu.
When MODE is set to 'ask', the debugger always uses the menu when
an ambiguity is detected.
Finally, when MODE is set to 'cancel', the debugger reports an
error due to the ambiguity and the command is aborted.
'show multiple-symbols'
Show the current value of the 'multiple-symbols' setting.
Info Catalog
gdb: Expressions
gdb: Data
gdb: Variables