6.7.1 Stack Frames
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 function `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" 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
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