Debugging with GDB - Examining the Stack

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# 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 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.

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. See section Selecting a frame.

When your program stops, GDB automatically selects the currently executing frame and describes it briefly, similar to the `frame` command (see section Information about a frame).

## 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 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.

`frame args`
  The `frame` command allows you to move from one stack frame to another, and to print the stack frame you select. args may be either the address of the frame or the stack frame number. Without an argument, `frame` prints the current stack frame.
`select-frame`
  The `select-frame` command allows you to move from one stack frame to another without printing the frame. This is the silent version of `frame`.

## Backtraces

A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

`backtrace`
`bt`
  Print a backtrace of the entire stack: one line per frame for all frames in the stack. You can stop the backtrace at any time by typing the system interrupt character, normally `C-c`.
`backtrace n`
`bt n`
  Similar, but print only the innermost n frames.
`backtrace -n`
`bt -n`
  Similar, but print only the outermost n frames.

The names `where` and `info stack` (abbreviated `info s`) are additional aliases for `backtrace`.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown--unless you use `set print address off`. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number.

Here is an example of a backtrace. It was made with the command ``bt 3'`, so it shows the innermost three frames.

    #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
        at builtin.c:993
    #1  0x6e38 in expand_macro (sym=0x2b600) at macro.c:242
    #2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
        at macro.c:71
    (More stack frames follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line `993` of `builtin.c`.

## Selecting a frame

Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.

`frame n`
`f n`
  Select frame number n. Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on. The highest-numbered frame is the one for `main`.
`frame addr`
`f addr`
  Select the frame at address addr. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDB to assign numbers properly to all frames. In addition, this can be useful when your program has multiple stacks and switches between them. On the SPARC architecture, `frame` needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer. On the MIPS and Alpha architecture, it needs two addresses: a stack pointer and a program counter. On the 29k architecture, it needs three addresses: a register stack pointer, a program counter, and a memory stack pointer.
`up n`
  Move n frames up the stack. For positive numbers n, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer. n defaults to one.
`down n`
  Move n frames down the stack. For positive numbers n, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently. n defaults to one. You may abbreviate `down` as `do`.

All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line.

For example:

    (gdb) up
    #1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
        at env.c:10
    10              read_input_file (argv[i]);

After such a printout, the `list` command with no arguments prints ten lines centered on the point of execution in the frame. See section Printing source lines.

`up-silently n`
`down-silently n`
  These two commands are variants of `up` and `down`, respectively; they differ in that they do their work silently, without causing display of the new frame. They are intended primarily for use in GDB command scripts, where the output might be unnecessary and distracting.

## Information about a frame

There are several other commands to print information about the selected stack frame.

`frame`
`f`
  When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame. It can be abbreviated `f`. With an argument, this command is used to select a stack frame. See section Selecting a frame.
`info frame`
`info f`
  This command prints a verbose description of the selected stack frame, including:
  - the address of the frame
  - the address of the next frame down (called by this frame)
  - the address of the next frame up (caller of this frame)
  - the language in which the source code corresponding to this frame is written
  - the address of the frame's arguments
  - the program counter saved in it (the address of execution in the caller frame)
  - which registers were saved in the frame
  The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions.
`info frame addr`
`info f addr`
  Print a verbose description of the frame at address addr, without selecting that frame. The selected frame remains unchanged by this command. This requires the same kind of address (more than one for some architectures) that you specify in the `frame` command. See section Selecting a frame.
`info args`
  Print the arguments of the selected frame, each on a separate line.
`info locals`
  Print the local variables of the selected frame, each on a separate line. These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame.
`info catch`
  Print a list of all the exception handlers that are active in the current stack frame at the current point of execution. To see other exception handlers, visit the associated frame (using the `up`, `down`, or `frame` commands); then type `info catch`. See section Setting catchpoints.

## MIPS/Alpha machines and the function stack

Alpha- and MIPS-based computers use an unusual stack frame, which sometimes requires GDB to search backward in the object code to find the beginning of a function.

To improve response time (especially for embedded applications, where GDB may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands:

`set heuristic-fence-post limit`
  Restrict GDB to examining at most limit bytes in its search for the beginning of a function. A value of 0 (the default) means there is no limit. However, except for 0, the larger the limit the more bytes `heuristic-fence-post` must search and therefore the longer it takes to run.
`show heuristic-fence-post`
  Display the current limit.

These commands are available only when GDB is configured for debugging programs on Alpha or MIPS processors.

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