gdb: Variables
10.3 Program Variables
======================
The most common kind of expression to use is the name of a variable in
your program.
Variables in expressions are understood in the selected stack frame
(
Selecting a Frame Selection.); they must be either:
* global (or file-static)
or
* visible according to the scope rules of the programming language
from the point of execution in that frame
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable 'a' whenever your program is
executing within the function 'foo', but you can only use or examine the
variable 'b' while your program is executing inside the block where 'b'
is declared.
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file by
using the colon-colon ('::') notation:
FILE::VARIABLE
FUNCTION::VARIABLE
Here FILE or FUNCTION is the name of the context for the static
VARIABLE. In the case of file names, you can use quotes to make sure
GDB parses the file name as a single word--for example, to print a
global value of 'x' defined in 'f2.c':
(gdb) p 'f2.c'::x
The '::' notation is normally used for referring to static variables,
since you typically disambiguate uses of local variables in functions by
selecting the appropriate frame and using the simple name of the
variable. However, you may also use this notation to refer to local
variables in frames enclosing the selected frame:
void
foo (int a)
{
if (a < 10)
bar (a);
else
process (a); /* Stop here */
}
int
bar (int a)
{
foo (a + 5);
}
For example, if there is a breakpoint at the commented line, here is
what you might see when the program stops after executing the call
'bar(0)':
(gdb) p a
$1 = 10
(gdb) p bar::a
$2 = 5
(gdb) up 2
#2 0x080483d0 in foo (a=5) at foobar.c:12
(gdb) p a
$3 = 5
(gdb) p bar::a
$4 = 0
These uses of '::' are very rarely in conflict with the very similar
use of the same notation in C++. When they are in conflict, the C++
meaning takes precedence; however, this can be overridden by quoting the
file or function name with single quotes.
For example, suppose the program is stopped in a method of a class
that has a field named 'includefile', and there is also an include file
named 'includefile' that defines a variable, 'some_global'.
(gdb) p includefile
$1 = 23
(gdb) p includefile::some_global
A syntax error in expression, near `'.
(gdb) p 'includefile'::some_global
$2 = 27
_Warning:_ Occasionally, a local variable may appear to have the
wrong value at certain points in a function--just after entry to a
new scope, and just before exit.
You may see this problem when you are stepping by machine
instructions. This is because, on most machines, it takes more than one
instruction to set up a stack frame (including local variable
definitions); if you are stepping by machine instructions, variables may
appear to have the wrong values until the stack frame is completely
built. On exit, it usually also takes more than one machine instruction
to destroy a stack frame; after you begin stepping through that group of
instructions, local variable definitions may be gone.
This may also happen when the compiler does significant
optimizations. To be sure of always seeing accurate values, turn off
all optimization when compiling.
Another possible effect of compiler optimizations is to optimize
unused variables out of existence, or assign variables to registers (as
opposed to memory addresses). Depending on the support for such cases
offered by the debug info format used by the compiler, GDB might not be
able to display values for such local variables. If that happens, GDB
will print a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or
use a different debug info format, if the compiler supports several such
formats. Compilation, for more information on choosing compiler
options. C and C++ C, for more information about debug info
formats that are best suited to C++ programs.
If you ask to print an object whose contents are unknown to GDB,
e.g., because its data type is not completely specified by the debug
information, GDB will say '<incomplete type>'. incomplete type
Symbols, for more about this.
If you try to examine or use the value of a (global) variable for
which GDB has no type information, e.g., because the program includes no
debug information, GDB displays an error message. unknown type
Symbols, for more about unknown types. If you cast the variable to its
declared type, GDB gets the variable's value using the cast-to type as
the variable's type. For example, in a C program:
(gdb) p var
'var' has unknown type; cast it to its declared type
(gdb) p (float) var
$1 = 3.14
If you append '@entry' string to a function parameter name you get
its value at the time the function got called. If the value is not
available an error message is printed. Entry values are available only
with some compilers. Entry values are normally also printed at the
function parameter list according to set print entry-values.
Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
29 i++;
(gdb) next
30 e (i);
(gdb) print i
$1 = 31
(gdb) print i@entry
$2 = 30
Strings are identified as arrays of 'char' values without specified
signedness. Arrays of either 'signed char' or 'unsigned char' get
printed as arrays of 1 byte sized integers. '-fsigned-char' or
'-funsigned-char' GCC options have no effect as GDB defines literal
string type '"char"' as 'char' without a sign. For program code
char var0[] = "A";
signed char var1[] = "A";
You get during debugging
(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 'A', 0 '\0'}
Info Catalog
gdb: Ambiguous Expressions
gdb: Data
gdb: Arrays