gdb.info: Variables

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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
(*note 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,
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

   This use of `::' is very rarely in conflict with the very similar
use of the same notation in C++.  GDB also supports use of the C++
scope resolution operator in GDB expressions.
     _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.  For example, GCC, the GNU C/C++ compiler usually supports the
`-gstabs+' option.  `-gstabs+' produces debug info in a format that is
superior to formats such as COFF.  You may be able to use DWARF 2
(`-gdwarf-2'), which is also an effective form for debug info.  *Note
Options for Debugging Your Program or GNU CC: (gcc.info)Debugging
Options.