Program variables

Contents|Index|Previous|Next
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 global (sometimes referred to as static) or visible (according to the scope rules of the programming language from the point of execution in that frame). Consider the following function example.

foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}

}

This means that 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 as in the following example.

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’, use (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.

Top| Contents|Index|Previous|Next