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Info file gdb, produced by texinfo-format-buffer   -*-Text-*-
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This file documents the GNU debugger GDB.

Copyright (C) 1988 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
sections entitled "Distribution" and "GDB General Public License" are
included exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the sections entitled "Distribution" and "GDB General Public
License" may be included in a translation approved by the author instead
of in the original English.


▶1f◀
File: gdb  Node: Stack, Prev: Stopping, Up: Top, Next: Source

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, the information about
where in the program the call was made from is saved in a block of data
called a "stack frame".  The frame also contains the arguments of the
call and the local variables of the function that was called.  All 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 the program, the value is found in the
selected frame.  There are special GDB commands to select whichever frame
you are interested in.

When the program stops, GDB automatically selects the currently executing
frame and describes it briefly as the `frame' command does
(*Note Info: Frame Info.).

* Menu:

* Frames::          Explanation of stack frames and terminology.
* Backtrace::       Summarizing many frames at once.
* Selection::       How to select a stack frame.
* Info: Frame Info, Commands to print information on stack frames.

▶1f◀
File: gdb  Node: Frames, Prev: Stack, Up: Stack, Next: Backtrace

Stack Frames
============

The call stack is divided up into contiguous pieces called "frames";
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 of those bytes 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 to give you a
way of talking about stack frames in GDB commands.

Many GDB commands refer implicitly to one stack frame.  GDB records a stack
frame that is called the "selected" stack frame; you can select any
frame using one set of GDB commands, and then other commands will operate
on that frame.  When your program stops, GDB automatically selects the
innermost frame.

▶1f◀
File: gdb  Node: Backtrace, Prev: Frames, Up: Stack, Next: Selection

Backtraces
==========

A backtrace is a summary of how the 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 `Control-C'.
     
`backtrace N'     
`bt N'     
     Similar, but stop after N frames.

Each line in a backtrace shows the frame number, the program counter, the
function and its arguments, and the source file name and line number (if
known).  The program counter is omitted if is the beginning of the code for
the source line.  This is the same as the first of the two lines printed
when you select a frame.

▶1f◀
File: gdb  Node: Selection, Prev: Backtrace, Up: Stack, Next: Frame Info

Selecting a Frame
=================

Most commands for examining the stack and other data in the 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'     
     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 `main''s
     frame.
     
`frame 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 the program has multiple stacks and
     options between them.
     
`up N'     
     Select the frame N frames up from the frame previously selected.
     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'     
     Select the frame N frames down from the frame previously
     selected.  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.

All of these commands end by printing some information on the frame that
has been selected: the frame number, the function name, the arguments, the
source file and line number of execution in that frame, and the text of
that source line.  For example:

     #3  main (argc=3, argv=??, env=??) at main.c, line 67
     67        read_input_file (argv[i]);

After such a printout, the `list' command with no arguments will print
ten lines centered on the point of execution in the frame.  *Note List::.

▶1f◀
File: gdb  Node: Frame Info, Prev: Selection, Up: Stack

Information on a Frame
======================

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

`frame'     
     This command prints a brief description of the selected stack frame.
     It can be abbreviated `f'.  With an argument, this command is
     used to select a stack frame; with no argument, it does not change
     which frame is selected, but still prints the same information.
     
`info frame'     
     This command prints a verbose description of the selected stack frame,
     including the address of the frame, the addresses of the next frame in
     (called by this frame) and the next frame out (caller of this frame),
     the address of the frame's arguments, the program counter saved in it
     (the address of execution in the caller frame), and 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'     
     Print a verbose description of the frame at address ADDR,
     without selecting that frame.  The selected frame remains unchanged by
     this command.
     
`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 static or automatic within all
     program blocks that execution in this frame is currently inside of.

▶1f◀
File: gdb  Node: Source, Prev: Stack, Up: Top, Next: Data

Examining Source Files
**********************

GDB knows which source files your program was compiled from, and
can print parts of their text.  When your program stops, GDB
spontaneously prints the line it stopped in.  Likewise, when you
select a stack frame (*Note Selection::), GDB prints the line
which execution in that frame has stopped in.  You can also
print parts of source files by explicit command.

* Menu:

* List::        Using the `list' command to print source files.
* Search::      Commands for searching source files.
* Source Path:: Specifying the directories to search for source files.

▶1f◀
File: gdb  Node: List, Prev: Source, Up: Source, Next: Search

Printing Source Lines
=====================

To print lines from a source file, use the `list' command
(abbreviated `l').  There are several ways to specify what part
of the file you want to print.

Here are the forms of `list' command most commonly used:

`list LINENUM'     
     Print ten lines centered around line number LINENUM in the
     current source file.
     
`list FUNCTION'     
     Print ten lines centered around the beginning of function
     FUNCTION.
     
`list'     
     Print ten more lines.  If the last lines printed were printed with a
     `list' command, this prints ten lines following the last lines
     printed; however, if the last line printed was a solitary line printed
     as part of displaying a stack frame (*Note Stack::), this prints ten
     lines centered around that line.
     
`list -'     
     Print ten lines just before the lines last printed.

Repeating a `list' command with RET discards the argument,
so it is equivalent to typing just `list'.  This is more useful
than listing the same lines again.  An exception is made for an
argument of `-'; that argument is preserved in repetition so that
each repetition moves up in the file.

In general, the `list' command expects you to supply zero, one or two
"linespecs".  Linespecs specify source lines; there are several ways
of writing them but the effect is always to specify some source line.
Here is a complete description of the possible arguments for `list':

`list LINESPEC'     
     Print ten lines centered around the line specified by LINESPEC.
     
`list FIRST,LAST'     
     Print lines from FIRST to LAST.  Both arguments are
     linespecs.
     
`list ,LAST'     
     Print ten lines ending with LAST.
     
`list FIRST,'     
     Print ten lines starting with FIRST.
     
`list +'     
     Print ten lines just after the lines last printed.
     
`list -'     
     Print ten lines just before the lines last printed.
     
`list'     
     As described in the preceding table.

Here are the ways of specifying a single source line---all the
kinds of linespec.

LINENUM     
     Specifies line LINENUM of the current source file.
     When a `list' command has two linespecs, this refers to
     the same source file as the first linespec.
     
+OFFSET     
     Specifies the line OFFSET lines after the last line printed.
     When used as the second linespec in a `list' command that has
     two, this specifies the line OFFSET lines down from the
     first linespec.
     
-OFFSET     
     Specifies the line OFFSET lines before the last line printed.
     
FILENAME:LINENUM     
     Specifies line LINENUM in the source file FILENAME.
     
FUNCTION     
     Specifies the line of the open-brace that begins the body of the
     function FUNCTION.
     
FILENAME:FUNCTION     
     Specifies the line of the open-brace that begins the body of the
     function FUNCTION in the file FILENAME.  The file name is
     needed with a function name only for disambiguation of identically
     named functions in different source files.
     
*ADDRESS     
     Specifies the line containing the program address ADDRESS.
     ADDRESS may be any expression.

One other command is used to map source lines to program addresses.

`info line LINENUM'     
     Print the starting and ending addresses of the compiled code for
     source line LINENUM.
     
     The default examine address for the `x' command is changed to the
     starting address of the line, so that `x/i' is sufficient to
     begin examining the machine code (*Note Memory::).  Also, this address
     is saved as the value of the convenience variable `$_'
     (*Note Convenience Vars::).

▶1f◀
File: gdb  Node: Search, Prev: List, Up: Source, Next: Source Path

Searching Source Files
======================

There are two commands for searching through the current source file for a
regular expression.

The command `forward-search REGEXP' checks each line, starting
with the one following the last line listed, for a match for REGEXP.
It lists the line that is found.  You can abbreviate the command name
as `fo'.

The command `reverse-search REGEXP' checks each line, starting
with the one before the last line listed and going backward, for a match
for REGEXP.  It lists the line that is found.  You can abbreviate
this command with as little as `rev'.

▶1f◀
File: gdb  Node: Source Path, Prev: Search, Up: Source

Specifying Source Directories
=============================

Executable programs do not record the directories of the source files they
were compiled from, just the names.  GDB remembers a list of directories to
search for source files; this is called the "source path".  Each time
GDB wants a source file, it tries all the directories in the list, in the
order they are present in the list, until it finds a file with the desired
name.

When you start GDB, its source path contains just the current working
directory.  To add other directories, use the `directory' command.
Note that the search path for executable files and the working directory
are not used for finding source files.

`directory DIRNAME'     
     Add directory DIRNAME to the end of the source path.
     
`directory'     
     Reset the source path to just the current working directory of GDB.
     This requires confirmation.
     
     `directory' with no argument can cause source files previously
     found by GDB to be found in a different directory.  To make this work
     correctly, this command also clears out the tables GDB maintains
     about the source files it has already found.
     
`info directories'     
     Print the source path: show which directories it contains.

Because the `directory' command adds to the end of the source path,
it does not affect any file that GDB has already found.  If the source
path contains directories that you do not want, and these directories
contain misleading files with names matching your source files, the
way to correct the situation is as follows:

  1. Choose the directory you want at the beginning of the source path.
     Use the `cd' command to make that the current working directory.
     
  2. Use `directory' with no argument to reset the source path to just
     that directory.
     
  3. Use `directory' with suitable arguments to add any other
     directories you want in the source path.

▶1f◀
File: gdb  Node: Data, Prev: Source, Up: Top, Next: Symbols

Examining Data
**************

The usual way of examining data in your program is with the `print'
command (abbreviated `p').  It evaluates and prints the value of any
valid expression of the language the program is written in (for now, C).
You type

     print EXP

where EXP is any valid expression, and the value of EXP
is printed in a format appropriate to its data type.

A more low-level way of examining data is with the `x' command.
It examines data in memory at a specified address and prints it in a
specified format.

* Menu:

* Expressions::      Expressions that can be computed and printed.
* Variables::        Using your program's variables in expressions.
* Assignment::       Setting your program's variables.
* Arrays::           Examining part of memory as an array.
* Formats::          Specifying formats for printing values.
* Memory::           Examining memory explicitly.
* Auto Display::     Printing certain expressions whenever program stops.
* Value History::    Referring to values previously printed.
* Convenience Vars:: Giving names to values for future reference.
* Registers::        Referring to and storing in machine registers.

▶1f◀
File: gdb  Node: Expressions, Prev: Data, Up: Data, Next: Variables

Expressions
===========

Many different GDB commands accept an expression and compute its value.
Any kind of constant, variable or operator defined by the programming
language you are using is legal in an expression in GDB.  This includes
conditional expressions, function calls, casts and string constants.

Casts are supported in all languages, not just in C, because it is so
useful to cast a number into a pointer so as to examine a structure
at that address in memory.

GDB supports three kinds of operator in addition to those of programming
languages:

`@'     
     `@' is a binary operator for treating parts of memory as arrays.
     *Note Arrays::, for more information.
     
`::'     
     `::' allows you to specify a variable in terms of the file or
     function it is defined in.  *Note Variables::.
     
`{TYPE} ADDR'     
     Refers to an object of type TYPE stored at address ADDR in memory.
     ADDR may be any expression whose value is an integer or pointer (but
     parentheses are required around nonunary operators, just as in a
     cast).  This construct is allowed regardless of what kind of data is
     officially supposed to reside at ADDR.

▶1f◀
File: gdb  Node: Variables, Prev: Expressions, Up: Data, Next: Arrays

Program Variables
=================

The most common kind of expression to use is the name of a variable
in your program.

Variables in expressions are understood in the selected stack frame
(*Note Selection::); they must either be global (or static) or be visible
according to the scope rules of the programming language from the point of
execution in that frame.  This means that in the function

     foo (a)
          int a;
     {
       bar (a);
       {
         int b = test ();
         bar (b);
       }
     }

the variable `a' is usable whenever the program is executing
within the function `foo', but the variable `b' is visible
only while the program is executing inside the block in which `b'
is declared.

▶1f◀
File: gdb  Node: Arrays, Prev: Variables, Up: Data, Next: Formats

Artificial Arrays
=================

It is often useful to print out several successive objects of the
same type in memory; a section of an array, or an array of
dynamically determined size for which only a pointer exists in the
program.

This can be done by constructing an "artificial array" with the
binary operator `@'.  The left operand of `@' should be
the first element of the desired array, as an individual object.
The right operand should be the length of the array.  The result is
an array value whose elements are all of the type of the left argument.
The first element is actually the left argument; the second element
comes from bytes of memory immediately following those that hold the
first element, and so on.  Here is an example.  If a program says

     int *array = (int *) malloc (len * sizeof (int));

you can print the contents of `array' with

     p *array@len

The left operand of `@' must reside in memory.  Array values made
with `@' in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
(It would probably appear in an expression via the value history,
after you had printed it out.)

▶1f◀
File: gdb  Node: Formats, Prev: Arrays, Up: Data, Next: Memory

Formats
=======

GDB normally prints all values according to their data types.  Sometimes
this is not what you want.  For example, you might want to print a number
in hex, or a pointer in decimal.  Or you might want to view data in memory
at a certain address as a character string or an instruction.  These things
can be done with "output formats".

The simplest use of output formats is to say how to print a value
already computed.  This is done by starting the arguments of the
`print' command with a slash and a format letter.  The format
letters supported are:

`x'     
     Regard the bits of the value as an integer, and print the integer in
     hexadecimal.
     
`d'     
     Print as integer in signed decimal.
     
`u'     
     Print as integer in unsigned decimal.
     
`o'     
     Print as integer in octal.
     
`a'     
     Print as an address, both absolute in hex and then relative
     to a symbol defined as an address below it.
     
`c'     
     Regard as an integer and print it as a character constant.
     
`f'     
     Regard the bits of the value as a floating point number and print
     using typical floating point syntax.

For example, to print the program counter in hex (*Note Registers::), type

     p/x $pc

Note that no space is required before the slash; this is because command
names in GDB cannot contain a slash.

To reprint the last value in the value history with a different format,
you can use the `print' command with just a format and no
expression.  For example, `p/x' reprints the last value in hex.

▶1f◀
File: gdb  Node: Memory, Prev: Formats, Up: Data, Next: Auto Display

Examining Memory
----------------

The command `x' (for `examine') can be used to examine memory under
explicit control of formats, without reference to the program's data types.

`x' is followed by a slash and an output format specification,
followed by an expression for an address.  The expression need not have
a pointer value (though it may); it is used as an integer, as the
address of a byte of memory.

The output format in this case specifies both how big a unit of memory
to examine and how to print the contents of that unit.  It is done
with one or two of the following letters:

These letters specify just the size of unit to examine:

`b'     
     Examine individual bytes.
     
`h'     
     Examine halfwords (two bytes each).
     
`w'     
     Examine words (four bytes each).
     
     Many assemblers and cpu designers still use `word' for a 16-bit quantity,
     as a holdover from specific predecessor machines of the 1970's that really
     did use two-byte words.  But more generally the term `word' has always
     referred to the size of quantity that a machine normally operates on and
     stores in its registers.  This is 32 bits for all the machines that GNU
     runs on.
     
`g'     
     Examine giant words (8 bytes).

These letters specify just the way to print the contents:

`x'     
     Print as integers in unsigned hexadecimal.
     
`d'     
     Print as integers in signed decimal.
     
`u'     
     Print as integers in unsigned decimal.
     
`o'     
     Print as integers in unsigned octal.
     
`a'     
     Print as an address, both absolute in hex and then relative
     to a symbol defined as an address below it.
     
`c'     
     Print as character constants.
     
`f'     
     Print as floating point.  This works only with sizes `w' and
     `g'.
     
`s'     
     Print a null-terminated string of characters.  The specified unit size
     is ignored; instead, the unit is however many bytes it takes to reach
     a null character (including the null character).
     
`i'     
     Print a machine instruction in assembler syntax (or nearly).  The
     specified unit size is ignored; the number of bytes in an instruction
     varies depending on the type of machine, the opcode and the addressing
     modes used.

If either the manner of printing or the size of unit fails to be specified,
the default is to use the same one that was used last.  If you don't want
to use any letters after the slash, you can omit the slash as well.

You can also omit the address to examine.  Then the address used is
just after the last unit examined.  This is why string and instruction
formats actually compute a unit-size based on the data: so that the
next string or instruction examined will start in the right place.
The `print' command sometimes sets the default address for
the `x' command; when the value printed resides in memory, the
default is set to examine the same location.  `info line' also
sets the default for `x', to the address of the start of the
machine code for the specified line and `info breakpoints' sets
it to the address of the last breakpoint listed.

When you use RET to repeat an `x' command, it does not repeat
exactly the same: the address specified previously (if any) is ignored, so
that the repeated command examines the successive locations in memory
rather than the same ones.

You can examine several consecutive units of memory with one command by
writing a repeat-count after the slash (before the format letters, if any).
The repeat count must be a decimal integer.  It has the same effect as
repeating the `x' command that many times except that the output may
be more compact with several units per line.

     x/10i $pc

Prints ten instructions starting with the one to be executed next in the
selected frame.  After doing this, you could print another ten following
instructions with

     x/10

in which the format and address are allowed to default.

The addresses and contents printed by the `x' command are not put in
the value history because there is often too much of them and they would
get in the way.  Instead, GDB makes these values available for subsequent
use in expressions as values of the convenience variables `$_' and
`$__'.

After an `x' command, the last address examined is available for use
in expressions in the convenience variable `$_'.  The contents of that
address, as examined, are available in the convenience variable `$__'.

If the `x' command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.

▶1f◀
File: gdb  Node: Auto Display, Prev: Memory, Up: Data, Next: Value History

Automatic Display
=================

If you find that you want to print the value of an expression frequently
(to see how it changes), you might want to add it to the "automatic
display list" so that GDB will print its value each time the program stops.
Each expression added to the list is given a number to identify it;
to remove an expression from the list, you specify that number.
The automatic display looks like this:

     2: foo = 38
     3: bar[5] = (struct hack *) 0x3804

showing item numbers, expressions and their current values.

`display EXP'     
     Add the expression EXP to the list of expressions to display
     each time the program stops.
     
`display/FMT EXP'     
     For FMT specifying only a display format and not a size or
     count, add the expression EXP to the auto-display list but
     arranges to display it each time in the specified format FMT.
     
`display/FMT ADDR'     
     For FMT `i' or `s', or including a unit-size or a
     number of units, add the expression ADDR as a memory address to
     be examined each time the program stops.  Examining means in effect
     doing `x/FMT ADDR'.  *Note Memory::.
     
`undisplay N'     
     Remove item number N from the list of expressions to display.
     
`display'     
     Display the current values of the expressions on the list, just as is
     done when the program stops.
     
`info display'     
     Print the list of expressions to display automatically, each one
     with its item number, but without showing the values.

▶1f◀
File: gdb  Node: Value History, Prev: Auto Display, Up: Data, Next: Convenience Vars

Value History
=============

Every value printed by the `print' command is saved for the entire
session in GDB's "value history" so that you can refer to it in
other expressions.

The values printed are given "history numbers" for you to refer to them
by.  These are successive integers starting with 1.  `print' shows you
the history number assigned to a value by printing `$N = '
before the value; here N is the history number.

To refer to any previous value, use `$' followed by the value's
history number.  The output printed by `print' is designed to remind
you of this.  Just `$' refers to the most recent value in the history,
and `$$' refers to the value before that.

For example, suppose you have just printed a pointer to a structure and
want to see the contents of the structure.  It suffices to type

     p *$

If you have a chain of structures where the component `next' points
to the next one, you can print the contents of the next one with

     p *$.next

It might be useful to repeat this command many times by typing RET.

Note that the history records values, not expressions.  If the value of
`x' is 4 and you type

     print x
     set x=5

then the value recorded in the value history by the `print' command
remains 4 even though `x''s value has changed.

`info history'     
     Print the last ten values in the value history, with their item
     numbers.  This is like `p $$9' repeated ten times, except that
     `info history' does not change the history.
     
`info history N'     
     Print ten history values centered on history item number N.

▶1f◀
File: gdb  Node: Convenience Vars, Prev: Value History, Up: Data, Next: Registers

Convenience Variables
=====================

GDB provides "convenience variables" that you can use within GDB to
hold on to a value and refer to it later.  These variables exist entirely
within GDB; they are not part of your program, and setting a convenience
variable has no effect on further execution of your program.  That's why
you can use them freely.

Convenience variables have names starting with `$'.  Any name starting
with `$' can be used for a convenience variable, unless it is one of
the predefined set of register names (*Note Registers::).

You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program.  Example:

     set $foo = *object_ptr

would save in `$foo' the value contained in the object pointed to by
`object_ptr'.

Using a convenience variable for the first time creates it; but its value
is `void' until you assign a new value.  You can alter the value with
another assignment at any time.

Convenience variables have no fixed types.  You can assign a convenience
variable any type of value, even if it already has a value of a different
type.  The convenience variable as an expression has whatever type its
current value has.

`info convenience'     
     Print a list of convenience variables used so far, and their values.
     Abbreviated `i con'.

One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced.  For example:

     set $i = 0
     print bar[$i++]->contents
     ...repeat that command by typing RET.

Some convenience variables are created automatically by GDB and given
values likely to be useful.

`$_'     
     The variable `$_' is automatically set by the `x' command to
     the last address examined (*Note Memory::).  Other commands which
     provide a default address for `x' to examine also set `$_'
     to that address; these commands include `info line' and `info
     breakpoint'.
     
`$__'     
     The variable `$__' is automatically set by the `x' command
     to the value found in the last address examined.

▶1f◀
File: gdb  Node: Registers, Prev: Convenience Vars, Up: Data

Registers
=========

Machine register contents can be referred to in expressions as variables
with names starting with `$'.  The names of registers are different
for each machine; use `info registers' to see the names used on your
machine.  The names `$pc' and `$sp' are used on all machines for
the program counter register and the stack pointer.  Often `$fp' is
used for a register that contains a pointer to the current stack frame.

GDB always considers the contents of an ordinary register as an integer
when the register is examined in this way.  Programs can store floating
point values in registers also, but there is currently no GDB command
to examine a specified register in floating point.  (However, if the
variable in your program which is stored in the register is a floating
point variable, you can see the floating point value by examining
the variable.)

Some machines have special floating point registers.  GDB considers these
registers' values as floating point when you examine them explicitly.

Some registers have distinct "raw" and "virtual" data formats.  This
means that the data format in which the register contents are saved by the
operating system is not the same one that your program normally sees.  For
example, the registers of the 68881 floating point coprocessor are always
saved in "extended" format, but all C programs expect to work with
"double" format.  In such cases, GDB normally works with the virtual
format only (the format that makes sense for your program), but the
`info registers' command prints the data in both formats.

Register values are relative to the selected stack frame
(*Note Selection::).  This means that you get the value that the register
would contain if all stack frames farther in were exited and their saved
registers restored.  In order to see the real contents of all registers,
you must select the innermost frame (with `frame 0').

Some registers are never saved (typically those numbered zero or one)
because they are used for returning function values; for these registers,
relativization makes no difference.

`info registers'     
     Print the names and relativized values of all registers.
     
`info registers REGNAME'     
     Print the relativized value of register REGNAME.  REGNAME
     may be any register name valid on the machine you are using, with
     or without the initial `$'.


Examples
--------

You could print the program counter in hex with

     p/x $pc

or print the instruction to be executed next with

     x/i $pc

or add four to the stack pointer with

     set $sp += 4

The last is a way of removing one word from the stack, on machines where
stacks grow downward in memory (most machines, nowadays).  This assumes
that the innermost stack frame is selected.  Setting `$sp' is
not allowed when other stack frames are selected.

▶1f◀
File: gdb  Node: Symbols, Prev: Data, Up: Top, Next: Altering

Examining the Symbol Table
**************************

The commands described in this section allow you to make inquiries for
information about the symbols (names of variables, functions and types)
defined in your program.  This information is found by GDB in the symbol
table loaded by the `symbol-file' command; it is inherent in the text
of your program and does not change as the program executes.

`whatis EXP'     
     Print the data type of expression EXP.  EXP is not
     actually evaluated, and any side-effecting operations (such as
     assignments or function calls) inside it do not take place.
     
`whatis'     
     Print the data type of `$', the last value in the value history.
     
`info address SYMBOL'     
     Describe where the data for SYMBOL is stored.  For register
     variables, this says which register.  For other automatic variables,
     this prints the stack-frame offset at which the variable is always
     stored.  Note the contrast with `print &SYMBOL', which does
     not work at all for register variables and for automatic variables
     prints the exact address of the current instantiation of the variable.
     
`ptype TYPENAME'     
     Print a description of data type TYPENAME.  TYPENAME may be the name
     of a type, or for C code it may have the form `struct STRUCT-TAG',
     `union UNION-TAG' or `enum ENUM-TAG'.
     
`info sources'     
     Print the names of all source files in the program for which there
     is debugging information.
     
`info functions'     
     Print the names and data types of all defined functions.
     
`info functions REGEXP'     
     Print the names and data types of all defined functions
     whose names contain a match for regular expression REGEXP.
     Thus, `info fun step' finds all functions whose names
     include `step'; `info fun ^step' finds those whose names
     start with `step'.
     
`info variables'     
     Print the names and data types of all variables that are declared
     outside of functions.
     
`info variables REGEXP'     
     Print the names and data types of all variables, declared outside of
     functions, whose names contain a match for regular expression
     REGEXP.
     
`info types'     
     Print all data types that are defined in the program.
     
`info types REGEXP'     
     Print all data types that are defined in the program whose names
     contain a match for regular expression REGEXP.
     
`printsyms FILENAME'     
     Write a complete dump of the debugger's symbol data into the
     file FILENAME.

▶1f◀
File: gdb  Node: Altering, Prev: Symbols, Up: Top, Next: Sequences

Altering Execution
******************

There are several ways to alter the execution of your program with GDB
commands.

* Menu:

* Assignment::    Altering variable values or memory contents.
* Jumping::       Altering control flow.
* Signaling::     Making signals happen in the program.
* Returning::     Making a function return prematurely.

▶1f◀
File: gdb  Node: Assignment, Prev: Altering, Up: Altering, Next: Jumping

Assignment to Variables
=======================

To alter the value of a variable, evaluate an assignment expression.
For example,

     print x=4

would store the value 4 into the variable `x', and then print
the value of the assignment expression (which is 4).

If you are not interested in seeing the value of the assignment, use the
`set' command instead of the `print' command.  `set' is
really the same as `print' except that the expression's value is not
printed and is not put in the value history (*Note Value History::).  The
expression is evaluated only for side effects.

GDB allows more implicit conversions in assignments than C does; you can
freely store an integer value into a pointer variable or vice versa, and
any structure can be converted to any other structure that is the same
length or shorter.

In C, all the other assignment operators such as `+=' and `++'
are supported as well.

To store into arbitrary places in memory, use the `{...}'
construct to generate a value of specified type at a specified address
(*Note Expressions::).  For example,

     set {int}0x83040 = 4

▶1f◀
File: gdb  Node: Jumping, Prev: Assignment, Up: Altering, Next: Signaling

Continuing at a Different Address
=================================

`jump LINENUM'     
     Resume execution at line number LINENUM.  Execution may stop
     immediately if there is a breakpoint there.
     
     The `jump' command does not change the current stack frame, or
     the stack pointer, or the contents of any memory location or any
     register other than the program counter.  If line LINENUM is in
     a different function from the one currently executing, the results may
     be wild if the two functions expect different patterns of arguments or
     of local variables.  For his reason, the `jump' command requests
     confirmation if the specified line is not in the function currently
     executing.  However, even wild results are predictable based on
     changing the program counter.
     
`jump *ADDRESS'     
     Resume execution at the instruction at address ADDRESS.

A similar effect can be obtained by storing a new value into the register
`$pc', but not exactly the same.

     set $pc = 0x485

specifies the address at which execution will resume, but does not resume
execution.  That does not happen until you use the `cont' command or a
stepping command (*Note Stepping::).

▶1f◀
File: gdb  Node: Signaling, Prev: Jumping, Up: Altering, Next: Returning

Giving the Program a Signal
===========================

`signal SIGNALNUM'     
     Resume execution where the program stopped, but give it immediately
     the signal number SIGNALNUM.
     
     Alternatively, if SIGNALNUM is zero, continue execution and give
     no signal.  This may be useful when the program has received a signal
     and the `cont' command would allow the program to see that
     signal.

▶1f◀
File: gdb  Node: Returning, Prev: Signaling, Up: Altering

Returning from a Function
=========================

You can make any function call return immediately, using the `return'
command.

First select the stack frame that you wish to return from
(*Note Selection::).  Then type the `return' command.  If you wish to
specify the value to be returned, give that as an argument.

This pops the selected stack frame (and any other frames inside of it),
leaving its caller as the innermost remaining frame.  That frame becomes
selected.  The specified value is stored in the registers used for
returning values of functions.

The `return' command does not resume execution; it leaves the program
stopped in the state that would exist if the function had just returned.
Contrast this with the `finish' command (*Note Stepping::), which
resumes execution until the selected stack frame returns naturally.

▶1f◀
File: gdb  Node: Sequences, Prev: Altering, Up: Top, Next: Emacs

Canned Sequences of Commands
****************************

GDB provides two ways to store sequences of commands for execution as a
unit: user-defined commands and command files.

* Menu:

* Define::         User-defined commands.
* Command Files::  Command files.
* Output::         Controlled output commands useful in
                   user-defined commands and command files.

▶1f◀
File: gdb  Node: Define, Prev: Sequences, Up: Sequences, Next: Command Files

User-Defined Commands
=====================

A "user-defined command" is a sequence of GDB commands to which you
assign a new name as a command.  This is done with the `define'
command.

`define COMMANDNAME'     
     Define a command named COMMANDNAME.  If there is already a command
     by that name, you are asked to confirm that you want to redefine it.
     
     The definition of the command is made up of other GDB command lines,
     which are given following the `define' command.  The end of these
     commands is marked by a line containing `end'.
     
`document COMMANDNAME'     
     Give documentation to the user-defined command COMMANDNAME.  The
     command COMMANDNAME must already be defined.  This command reads
     lines of documentation just as `define' reads the lines of the
     command definition.  After the `document' command is finished,
     `help' on command COMMANDNAME will print the documentation
     you have specified.
     
     You may use the `document' command again to change the
     documentation of a command.  Redefining the command with `define'
     does not change the documentation.

User-defined commands do not take arguments.  When they are executed, the
commands of the definition are not printed.  An error in any command
stops execution of the user-defined command.

Commands that would ask for confirmation if used interactively proceed
without asking when used inside a user-defined command.  Many GDB commands
that normally print messages to say what they are doing omit the messages
when used in user-defined command.

▶1f◀
File: gdb  Node: Command Files, Prev: Define, Up: Sequences, Next: Output

Command Files
=============

A command file for GDB is a file of lines that are GDB commands.  Comments
(lines starting with `#') may also be included.  An empty line in a
command file does nothing; it does not mean to repeat the last command, as
it would from the terminal.

When GDB starts, it automatically executes its "init files", command
files named `.gdbinit'.  GDB reads the init file (if any) in your home
directory and then the init file (if any) in the current working
directory.  (The init files are not executed if the `-nx' option
is given.)  You can also request the execution of a command file with the
`source' command:

`source FILENAME'     
     Execute the command file FILENAME.

The lines in a command file are executed sequentially.  They are not
printed as they are executed.  An error in any command terminates execution
of the command file.

Commands that would ask for confirmation if used interactively proceed
without asking when used in a command file.  Many GDB commands that
normally print messages to say what they are doing omit the messages
when used in a command file.

▶1f◀
File: gdb  Node: Output, Prev: Command Files, Up: Sequences

Commands for Controlled Output
==============================

During the execution of a command file or a user-defined command, the only
output that appears is what is explicitly printed by the commands of the
definition.  This section describes three commands useful for generating
exactly the output you want.

`echo TEXT'     
     Print TEXT.  Nonprinting characters can be included in
     TEXT using C escape sequences, such as `\n' to print a
     newline.  No newline will be printed unless you specify one.
     
     A backslash at the end of TEXT is ignored.  It is useful for
     outputting a string ending in spaces, since trailing spaces are
     trimmed from all arguments.  A backslash at the beginning preserves
     leading spaces in the same way, because `\ ' as an escape
     sequence stands for a space.  Thus, to print ` and foo = ', do
     
          echo \ and foo = \
     
`output EXPRESSION'     
     Print the value of EXPRESSION and nothing but that value: no
     newlines, no `$NN = '.  The value is not entered in the
     value history either.
     
`output/FMT EXPRESSION'     
     Print the value of EXPRESSION in format FMT.
     *Note Formats::, for more information.
     
`printf STRING, EXPRESSIONS...'     
     Print the values of the EXPRESSIONS under the control of
     STRING.  The EXPRESSIONS are separated by commas and may
     be either numbers or pointers.  Their values are printed as specified
     by STRING, exactly as if the program were to execute
     
          printf (STRING, EXPRESSIONS...);
     
     For example, you can print two values in hex like this:
     
          printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo
     
     The only backslash-escape sequences that you can use in the string are
     the simple ones that consist of backslash followed by a letter.

▶1f◀
File: gdb  Node: Emacs, Prev: Sequences, Up: Top, Next: Remote

Using GDB under GNU Emacs
*************************

A special interface allows you to use GNU Emacs to view (and
edit) the source files for the program you are debugging with
GDB.

To use this interface, use the command `M-x gdb' in Emacs.
Give the executable file you want to debug as an argument.  This
command starts a GDB process as a subprocess of Emacs, with input
and output through a newly created Emacs buffer.

Using this GDB process is just like using GDB normally except for two things:

   * All "terminal" input and output goes through the Emacs buffer.  This
     applies both to GDB commands and their output, and to the input and
     output done by the program you are debugging.
     
     This is useful because it means that you can copy the text of previous
     commands and input them again; you can even use parts of the output
     in this way.
     
     All the facilities of Emacs's Shell mode are available for this purpose.
     
   * GDB displays source code through Emacs.  Each time GDB displays a
     stack frame, Emacs automatically finds the source file for that frame
     and puts an arrow (`=>') at the left margin of the current line.
     
     Explicit GDB `list' or search commands still produce output as
     usual, but you probably will have no reason to use them.

In the GDB I/O buffer, you can use these special Emacs commands:

`M-s'     
     Execute to another source line, like the GDB `step' command.
     
`M-n'     
     Execute to next source line in this function, skipping all function
     calls, like the GDB `next' command.
     
`M-i'     
     Execute one instruction, like the GDB `stepi' command.
     
`M-u'     
     Move up one stack frame (and display that frame's source file in
     Emacs), like the GDB `up' command.
     
`M-d'     
     Move down one stack frame (and display that frame's source file in
     Emacs), like the GDB `down' command.  (This means that you cannot
     delete words in the usual fashion in the GDB buffer; I am guessing you
     won't often want to do that.)
     
`C-c C-f'     
     Execute until exit from the selected stack frame, like the GDB
     `finish' command.

In any source file, the Emacs command `C-x SPC' (`gdb-break')
tells GDB to set a breakpoint on the source line point is on.

The source files displayed in Emacs are in ordinary Emacs buffers
which are visiting the source files in the usual way.  You can edit
the files with these buffers if you wish; but keep in mind that GDB
communicates with Emacs in terms of line numbers.  If you add or
delete lines from the text, the line numbers that GDB knows will cease
to correspond properly to the code.

▶1f◀
File: gdb  Node: Remote, Prev: Emacs, Up: Top, Next: Commands

Remote Kernel Debugging
***********************

GDB has a special facility for debugging a remote machine via a serial
connection.  This can be used for kernel debugging.

The program to be debugged on the remote machine needs to contain a
debugging device driver which talks to GDB over the serial line using the
protocol described below.  The same version of GDB that is used ordinarily
can be used for this.

* Menu:

* Remote Commands::       Commands used to start and finish remote debugging.

For details of the communication protocol, see the comments in the GDB
source file `remote.c'.

▶1f◀
File: gdb  Node: Remote Commands, Prev: Remote, Up: Remote

Commands for Remote Debugging
=============================

To start remote debugging, first run GDB and specify as an executable file
the program that is running in the remote machine.  This tells GDB how
to find the program's symbols and the contents of its pure text.  Then
establish communication using the `attach' command with a device
name rather than a pid as an argument.  For example:

     attach /dev/ttyd

if the serial line is connected to the device named `/dev/ttyd'.  This
will stop the remote machine if it is not already stopped.

Now you can use all the usual commands to examine and change data and to
step and continue the remote program.

To resume the remote program and stop debugging it, use the `detach'
command.

▶1f◀
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