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 (see "Selecting a frame"); they must be either:

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 use or examine the variable b only while your program is executing inside the block where b is declared.

There's an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point isn't in this file. But it's 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:


Here file or function is the name of the context for the static variable. In the case of filenames, you can use quotes to make sure GDB parses the filename 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.

Note: Occasionally, a local variable may appear to have the wrong value at certain points in a function, such as just after entry to a new scope, and just before exit.

You may see this problem when you're 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're 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.