Debugging with GDB Manual HP WDB v6.3 (5900-2180, August 2012)

6 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. The
information 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.
The GDB commands for examining the stack allow you to view all of this information.
6.1 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 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 commands.
One of the stack frames is selected by GDB and the 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. See “Selecting a frame” (page 56).
When your program stops, GDB automatically selects the currently executing frame and describes
it briefly, similar to the frame command (see “Information about a frame” (page 57).
6.2 Stacks Without frames
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.
6.3 Commands for Examining the Stack
The following commands are used for examining the stack:
frame args Select and print a stack frame. With no argument, prints the selected stack
frame. An argument specifies the frame to select. It can be a stack frame
6.1 Stack frames 55