The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing.
When your program is started, the stack has only one frame, that of the
main. This is called the initial frame or the
outermost frame. Each time a function is called, a new frame is
made. Each time a function returns, the frame for that function invocation
is eliminated. If a function is recursive, there can be many frames for
the same function. The frame for the function in which execution is
actually occurring is called the innermost frame. This is the most
recently created of all the stack frames that still exist.
Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the frame pointer register (see $fp) while execution is going on in that frame.
gdb assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by gdb to give you a way of designating stack frames in gdb commands.
Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the gcc option
generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. gdb has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, gdb nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, gdb has no provision for frameless functions elsewhere in the stack.