Examining The Stack; Stack Frames - Red Hat ENTERPRISE LINUX 3 - DEBUGGING WITH GDB Manual

Chapter 8.

Examining the Stack

When your program has stopped, the first thing you need to know is where it stopped and how it got
there.
Each time your program performs a function call, information about the call is generated. That in-
formation includes the location of the call in your program, the arguments of the call, and the local
variables of the function being called. The information is saved in a block of data called a stack frame.
The stack frames are allocated in a region of memory called the call stack.
When your program stops, the gdb commands for examining the stack allow you to see all of this
information.
One of the stack frames is selected by gdb and many gdb commands refer implicitly to the selected
frame. In particular, whenever you ask gdb for the value of a variable in your program, the value
is found in the selected frame. There are special gdb commands to select whichever frame you are
interested in. Refer to Section 8.3 Selecting a frame.
When your program stops, gdb automatically selects the currently executing frame and describes it
briefly, similar to the command (refer to Section 8.4 Information about a frame).
frame

8.1. 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 . This is called
main
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 commands.
Some compilers provide a way to compile functions so that they operate without stack frames. (For
example, the gcc option
-fomit-frame-pointer
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.