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@c This is part of the Emacs manual.
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@c Copyright (C) 1985-1987, 1993-1995, 1997, 2000-2018 Free Software
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@c Foundation, Inc.
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@c See file emacs.texi for copying conditions.
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@node Building
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@chapter Compiling and Testing Programs
@cindex building programs
@cindex program building
@cindex running Lisp functions

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  The previous chapter discusses the Emacs commands that are useful
for making changes in programs.  This chapter deals with commands that
assist in the process of compiling and testing programs.
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@menu
* Compilation::         Compiling programs in languages other
                          than Lisp (C, Pascal, etc.).
* Compilation Mode::    The mode for visiting compiler errors.
* Compilation Shell::   Customizing your shell properly
                          for use in the compilation buffer.
* Grep Searching::      Searching with grep.
* Flymake::             Finding syntax errors on the fly.
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* Debuggers::           Running symbolic debuggers for non-Lisp programs.
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* Executing Lisp::      Various modes for editing Lisp programs,
                          with different facilities for running
                          the Lisp programs.
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* Libraries: Lisp Libraries.      How Lisp programs are loaded into Emacs.
* Eval: Lisp Eval.      Executing a single Lisp expression in Emacs.
* Interaction: Lisp Interaction.  Executing Lisp in an Emacs buffer.
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* External Lisp::       Communicating through Emacs with a separate Lisp.
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@end menu

@node Compilation
@section Running Compilations under Emacs
@cindex inferior process
@cindex make
@cindex compilation errors
@cindex error log

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  Emacs can run compilers for languages such as C and Fortran, feeding
the compilation log into an Emacs buffer.  It can also parse the error
messages and show you where the errors occurred.
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@table @kbd
@item M-x compile
Run a compiler asynchronously under Emacs, with error messages going to
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the @file{*compilation*} buffer.
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@item M-x recompile
Invoke a compiler with the same command as in the last invocation of
@kbd{M-x compile}.
@item M-x kill-compilation
Kill the running compilation subprocess.
@end table

@findex compile
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  To run @code{make} or another compilation command, type @kbd{M-x
compile}.  This reads a shell command line using the minibuffer, and
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then executes the command by running a shell as a subprocess (or
@dfn{inferior process}) of Emacs.  The output is inserted in a buffer
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named @file{*compilation*}.  The current buffer's default directory is
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used as the working directory for the execution of the command, so by
default compilation takes place in that directory.
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@vindex compile-command
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  The default compilation command is @samp{make -k}, which is usually
correct for programs compiled using the @command{make} utility (the
@samp{-k} flag tells @command{make} to continue compiling as much as
possible after an error).  @xref{Top,, Make, make, GNU Make Manual}.
If you have done @kbd{M-x compile} before, the command that you
specified is automatically stored in the variable
@code{compile-command}; this is used as the default the next time you
type @kbd{M-x compile}.  A file can also specify a file-local value
for @code{compile-command} (@pxref{File Variables}).

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  Starting a compilation displays the @file{*compilation*} buffer in
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another window but does not select it.  While the compilation is
running, the word @samp{run} is shown in the major mode indicator for
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the @file{*compilation*} buffer, and the word @samp{Compiling} appears
in all mode lines.  You do not have to keep the @file{*compilation*}
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buffer visible while compilation is running; it continues in any case.
When the compilation ends, for whatever reason, the mode line of the
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@file{*compilation*} buffer changes to say @samp{exit} (followed by
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the exit code: @samp{[0]} for a normal exit), or @samp{signal} (if a
signal terminated the process).

  If you want to watch the compilation transcript as it appears,
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switch to the @file{*compilation*} buffer and move point to the end of
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the buffer.  When point is at the end, new compilation output is
inserted above point, which remains at the end.  Otherwise, point
remains fixed while compilation output is added at the end of the
buffer.
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  While compilation proceeds, the mode line shows the number of
errors, warnings, and informational messages emitted by the compiler
so far.
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@cindex compilation buffer, keeping point at end
@vindex compilation-scroll-output
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  If you change the variable @code{compilation-scroll-output} to a
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non-@code{nil} value, the @file{*compilation*} buffer scrolls
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automatically to follow the output.  If the value is
@code{first-error}, scrolling stops when the first error appears,
leaving point at that error.  For any other non-@code{nil} value,
scrolling continues until there is no more output.
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@findex recompile
  To rerun the last compilation with the same command, type @kbd{M-x
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recompile}.  This reuses the compilation command from the last
invocation of @kbd{M-x compile}.  It also reuses the
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@file{*compilation*} buffer and starts the compilation in its default
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directory, which is the directory in which the previous compilation
was started.

@findex kill-compilation
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@vindex compilation-always-kill
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  Starting a new compilation also kills any compilation already
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running in @file{*compilation*}, as the buffer can only handle one
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compilation at any time.  However, @kbd{M-x compile} asks for
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confirmation before actually killing a compilation that is running; to
always automatically kill the compilation without asking, change the
variable @code{compilation-always-kill} to @code{t}.  You can also
kill a compilation process with the command @kbd{M-x
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kill-compilation}.

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  To run two compilations at once, start the first one, then rename
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the @file{*compilation*} buffer (perhaps using @code{rename-uniquely};
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@pxref{Misc Buffer}), then switch buffers and start the other
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compilation.  This will create a new @file{*compilation*} buffer.
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@vindex compilation-environment
  You can control the environment passed to the compilation command
with the variable @code{compilation-environment}.  Its value is a list
of environment variable settings; each element should be a string of
the form @code{"@var{envvarname}=@var{value}"}.  These environment
variable settings override the usual ones.

@node Compilation Mode
@section Compilation Mode

@cindex Compilation mode
@cindex mode, Compilation
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@cindex locus
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  The @file{*compilation*} buffer uses a major mode called Compilation
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mode.  Compilation mode turns each error message in the buffer into a
hyperlink; you can move point to it and type @key{RET}, or click on it
with the mouse (@pxref{Mouse References}), to visit the @dfn{locus} of
the error message in a separate window.  The locus is the specific
position in a file where that error occurred.

@findex compile-goto-error
@vindex compilation-auto-jump-to-first-error
  If you change the variable
@code{compilation-auto-jump-to-first-error} to a non-@code{nil} value,
Emacs automatically visits the locus of the first error message that
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appears in the @file{*compilation*} buffer.
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  Compilation mode provides the following additional commands.  These
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commands can also be used in @file{*grep*} buffers, where the
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hyperlinks are search matches rather than error messages (@pxref{Grep
Searching}).
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@table @kbd
@item M-g M-n
@itemx M-g n
@itemx C-x `
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Visit the locus of the next error message or match (@code{next-error}).
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@item M-g M-p
@itemx M-g p
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Visit the locus of the previous error message or match
(@code{previous-error}).
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@item M-n
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Move point to the next error message or match, without visiting its
locus (@code{compilation-next-error}).
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@item M-p
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Move point to the previous error message or match, without visiting
its locus (@code{compilation-previous-error}).
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@item M-@}
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Move point to the next error message or match occurring in a different
file (@code{compilation-next-file}).
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@item M-@{
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Move point to the previous error message or match occurring in a
different file (@code{compilation-previous-file}).
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@item C-c C-f
Toggle Next Error Follow minor mode, which makes cursor motion in the
compilation buffer produce automatic source display.
@end table

@kindex M-g M-n
@kindex M-g n
@kindex C-x `
@findex next-error
@vindex next-error-highlight
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  To visit errors sequentially, type @w{@kbd{C-x `}}
(@code{next-error}), or equivalently @kbd{M-g M-n} or @kbd{M-g n}.
This command can be invoked from any buffer, not just a Compilation
mode buffer.  The first time you invoke it after a compilation, it
visits the locus of the first error message.  Each subsequent
@w{@kbd{C-x `}} visits the next error, in a similar fashion.  If you
visit a specific error with @key{RET} or a mouse click in the
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@file{*compilation*} buffer, subsequent @w{@kbd{C-x `}} commands
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advance from there.  When @w{@kbd{C-x `}} finds no more error messages
to visit, it signals an error.  @w{@kbd{C-u C-x `}} starts again from
the beginning of the compilation buffer, and visits the first locus.

  @kbd{M-g M-p} or @kbd{M-g p} (@code{previous-error}) iterates
through errors in the opposite direction.

  The @code{next-error} and @code{previous-error} commands don't just
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act on the errors or matches listed in @file{*compilation*} and
@file{*grep*} buffers; they also know how to iterate through error or
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match lists produced by other commands, such as @kbd{M-x occur}
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(@pxref{Other Repeating Search}).  If the current buffer contains
error messages or matches, these commands will iterate through them;
otherwise, Emacs looks for a buffer containing error messages or
matches amongst the windows of the selected frame, then for any buffer
that @code{next-error} or @code{previous-error} previously visited,
and finally all other buffers.  Any buffer these commands iterate
through that is not currently displayed in a window will be displayed.
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@vindex compilation-skip-threshold
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  By default, the @code{next-error} and @code{previous-error} commands
skip less important messages.  The variable
@code{compilation-skip-threshold} controls this.  The default value,
1, means to skip anything less important than a warning.  A value of 2
means to skip anything less important than an error, while 0 means not
to skip any messages.

  When Emacs visits the locus of an error message, it momentarily
highlights the relevant source line.  The duration of this highlight
is determined by the variable @code{next-error-highlight}.

@vindex compilation-context-lines
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  If the @file{*compilation*} buffer is shown in a window with a left
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fringe (@pxref{Fringes}), the locus-visiting commands put an arrow in
the fringe, pointing to the current error message.  If the window has
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no left fringe, such as on a text terminal, these commands scroll the
window so that the current message is at the top of the window.  If
you change the variable @code{compilation-context-lines} to an integer
value @var{n}, these commands scroll the window so that the current
error message is @var{n} lines from the top, whether or not there is a
fringe; the default value, @code{nil}, gives the behavior described
above.
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@vindex compilation-error-regexp-alist
@vindex grep-regexp-alist
  To parse messages from the compiler, Compilation mode uses the
variable @code{compilation-error-regexp-alist} which lists various
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error message formats and tells Emacs how to extract the locus from
each.  A similar variable, @code{grep-regexp-alist}, tells Emacs how
to parse output from a @code{grep} command (@pxref{Grep Searching}).
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@findex compilation-next-error
@findex compilation-previous-error
@findex compilation-next-file
@findex compilation-previous-file
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  Compilation mode also defines the keys @key{SPC} and @key{DEL} to
scroll by screenfuls; @kbd{M-n} (@code{compilation-next-error}) and
@kbd{M-p} (@code{compilation-previous-error}) to move to the next or
previous error message; and @kbd{M-@{} (@code{compilation-next-file})
and @kbd{M-@}} (@code{compilation-previous-file}) to move to the next
or previous error message for a different source file.
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@cindex Next Error Follow mode
@findex next-error-follow-minor-mode
  You can type @kbd{C-c C-f} to toggle Next Error Follow mode.  In
this minor mode, ordinary cursor motion in the compilation buffer
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automatically updates the source buffer, i.e., moving the cursor over
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an error message causes the locus of that error to be displayed.
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  The features of Compilation mode are also available in a minor mode
called Compilation Minor mode.  This lets you parse error messages in
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any buffer, not just a normal compilation output buffer.  Type
@kbd{M-x compilation-minor-mode} to enable the minor mode.  For
instance, in an Rlogin buffer (@pxref{Remote Host}), Compilation minor
mode automatically accesses remote source files by FTP (@pxref{File
Names}).
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@node Compilation Shell
@section Subshells for Compilation

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  The @kbd{M-x compile} command uses a shell to run the compilation
command, but specifies the option for a noninteractive shell.  This
means, in particular, that the shell should start with no prompt.  If
you find your usual shell prompt making an unsightly appearance in the
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@file{*compilation*} buffer, it means you have made a mistake in your
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shell's init file by setting the prompt unconditionally.  (This init
file may be named @file{.bashrc}, @file{.profile}, @file{.cshrc},
@file{.shrc}, etc., depending on what shell you use.)  The shell init
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file should set the prompt only if there already is a prompt.  Here's
how to do it in bash:

@example
if [ "$@{PS1+set@}" = set ]
then PS1=@dots{}
fi
@end example

@noindent
And here's how to do it in csh:

@example
if ($?prompt) set prompt = @dots{}
@end example

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@vindex TERM, environment variable, in compilation mode
  If you want to customize the value of the @env{TERM} environment
variable passed to the compilation subshell, customize the variable
@code{comint-terminfo-terminal} (@pxref{Shell Options}).

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  Emacs does not expect a compiler process to launch asynchronous
subprocesses; if it does, and they keep running after the main
compiler process has terminated, Emacs may kill them or their output
may not arrive in Emacs.  To avoid this problem, make the main
compilation process wait for its subprocesses to finish.  In a shell
script, you can do this using @samp{$!} and @samp{wait}, like this:

@example
(sleep 10; echo 2nd)& pid=$!  # @r{Record pid of subprocess}
echo first message
wait $pid                     # @r{Wait for subprocess}
@end example

@noindent
If the background process does not output to the compilation buffer,
so you only need to prevent it from being killed when the main
compilation process terminates, this is sufficient:

@example
nohup @var{command}; sleep 1
@end example
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@ifnottex
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  On MS-DOS, asynchronous subprocesses are
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not supported, so @kbd{M-x compile} runs the compilation command
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synchronously (i.e., you must wait until the command finishes before
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you can do anything else in Emacs).  @xref{MS-DOS}.
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@end ifnottex

@node Grep Searching
@section Searching with Grep under Emacs

  Just as you can run a compiler from Emacs and then visit the lines
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with compilation errors, you can also run @command{grep} and then
visit the lines on which matches were found.  This works by treating
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the matches reported by @command{grep} as if they were errors.
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The output buffer uses Grep mode, which is a variant of Compilation
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mode (@pxref{Compilation Mode}).

@table @kbd
@item M-x grep
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@itemx M-x lgrep
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Run @command{grep} asynchronously under Emacs, listing matching lines in
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the buffer named @file{*grep*}.
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@item M-x grep-find
@itemx M-x find-grep
@itemx M-x rgrep
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Run @command{grep} via @code{find}, and collect output in the
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@file{*grep*} buffer.
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@item M-x zrgrep
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Run @code{zgrep} and collect output in the @file{*grep*} buffer.
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@item M-x kill-grep
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Kill the running @command{grep} subprocess.
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@end table

@findex grep
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  To run @command{grep}, type @kbd{M-x grep}, then enter a command line
that specifies how to run @command{grep}.  Use the same arguments you
would give @command{grep} when running it normally: a @command{grep}-style
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regexp (usually in single-quotes to quote the shell's special
characters) followed by file names, which may use wildcards.  If you
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specify a prefix argument for @kbd{M-x grep}, it finds the identifier
(@pxref{Xref}) in the buffer around point, and puts that into the
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default @command{grep} command.
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  Your command need not simply run @command{grep}; you can use any shell
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command that produces output in the same format.  For instance, you
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can chain @command{grep} commands, like this:
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@example
grep -nH -e foo *.el | grep bar | grep toto
@end example

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  The output from @command{grep} goes in the @file{*grep*} buffer.  You
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can find the corresponding lines in the original files using @w{@kbd{C-x
`}}, @key{RET}, and so forth, just like compilation errors.

  Some grep programs accept a @samp{--color} option to output special
markers around matches for the purpose of highlighting.  You can make
use of this feature by setting @code{grep-highlight-matches} to
@code{t}.  When displaying a match in the source buffer, the exact
match will be highlighted, instead of the entire source line.

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  As with compilation commands (@pxref{Compilation}), while the grep
command runs, the mode line shows the running number of matches found
and highlighted so far.

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  The @command{grep} commands will offer to save buffers before
running.  This is controlled by the @code{grep-save-buffers} variable.
The possible values are either @code{nil} (don't save), @code{ask}
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(ask before saving), or a function which will be used as a predicate
(and is called with the file name as the parameter and should return
non-@code{nil} if the buffer is to be saved).  Any other
non-@code{nil} value means that all buffers should be saved without
asking.
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@findex grep-find
@findex find-grep
  The command @kbd{M-x grep-find} (also available as @kbd{M-x
find-grep}) is similar to @kbd{M-x grep}, but it supplies a different
initial default for the command---one that runs both @code{find} and
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@command{grep}, so as to search every file in a directory tree.  See also
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the @code{find-grep-dired} command, in @ref{Dired and Find}.

@findex lgrep
@findex rgrep
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@findex zrgrep
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  The commands @kbd{M-x lgrep} (local grep) and @kbd{M-x rgrep}
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(recursive grep) are more user-friendly versions of @command{grep} and
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@code{grep-find}, which prompt separately for the regular expression
to match, the files to search, and the base directory for the search.
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Case sensitivity of the search is controlled by the current value of
@code{case-fold-search}.  The command @kbd{M-x zrgrep} is similar to
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@kbd{M-x rgrep}, but it calls @command{zgrep} instead of
@command{grep} to search the contents of gzipped files.
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  These commands build the shell commands based on the variables
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@code{grep-template} (for @code{lgrep}) and @code{grep-find-template}
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(for @code{rgrep}).  The files to search can use aliases defined in
the variable @code{grep-files-aliases}.
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@vindex grep-find-ignored-directories
  Directories listed in the variable
@code{grep-find-ignored-directories} are automatically skipped by
@kbd{M-x rgrep}.  The default value includes the data directories used
by various version control systems.
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@vindex grep-find-hide
  The boolean option @code{grep-find-hide} controls shortening of the
displayed command line by hiding the part containing ignored
directories and files.  The hidden part can be revealed by clicking on
the button with ellipsis.

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@node Flymake
@section Finding Syntax Errors On The Fly
@cindex checking syntax

  Flymake mode is a minor mode that performs on-the-fly syntax
checking for many programming and markup languages, including C, C++,
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Perl, HTML, and @TeX{}/@LaTeX{}.  It is somewhat analogous to Flyspell
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mode, which performs spell checking for ordinary human languages in a
similar fashion (@pxref{Spelling}).  As you edit a file, Flymake mode
runs an appropriate syntax checking tool in the background, using a
temporary copy of the buffer.  It then parses the error and warning
messages, and highlights the erroneous lines in the buffer.  The
syntax checking tool used depends on the language; for example, for
C/C++ files this is usually the C compiler.  Flymake can also use
build tools such as @code{make} for checking complicated projects.

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  To enable Flymake mode, type @kbd{M-x flymake-mode}.  You can jump
to the errors that it finds by using @kbd{M-x flymake-goto-next-error}
and @kbd{M-x flymake-goto-prev-error}.  To display any error messages
associated with the current line, type @kbd{M-x
flymake-display-err-menu-for-current-line}.
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  For more details about using Flymake,
@ifnottex
see @ref{Top, Flymake, Flymake, flymake, The Flymake Manual}.
@end ifnottex
@iftex
see the Flymake Info manual, which is distributed with Emacs.
@end iftex
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@node Debuggers
@section Running Debuggers Under Emacs
@cindex debuggers
@cindex GUD library
@cindex GDB
@cindex DBX
@cindex SDB
@cindex XDB
@cindex Perldb
@cindex JDB
@cindex PDB

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The GUD (Grand Unified Debugger) library provides an Emacs interface
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to a wide variety of symbolic debuggers.  It can run the GNU Debugger
(GDB), as well as DBX, SDB, XDB, Perl's debugging mode, the Python
debugger PDB, and the Java Debugger JDB.
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  Emacs provides a special interface to GDB, which uses extra Emacs
windows to display the state of the debugged program.  @xref{GDB
Graphical Interface}.

  Emacs also has a built-in debugger for Emacs Lisp programs.
@xref{Debugging,, The Lisp Debugger, elisp, the Emacs Lisp Reference
Manual}.
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@menu
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* Starting GUD::        How to start a debugger subprocess.
* Debugger Operation::  Connection between the debugger and source buffers.
* Commands of GUD::     Key bindings for common commands.
* GUD Customization::   Defining your own commands for GUD.
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* GDB Graphical Interface::  An enhanced mode that uses GDB features to
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                        implement a graphical debugging environment.
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@end menu

@node Starting GUD
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@subsection Starting GUD
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  There are several commands for starting a debugger subprocess, each
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corresponding to a particular debugger program.
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@table @kbd
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@item M-x gdb
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@findex gdb
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Run GDB as a subprocess, and interact with it via an IDE-like Emacs
interface.  @xref{GDB Graphical Interface}, for more information about
this command.
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@item M-x gud-gdb
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@findex gud-gdb
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Run GDB, using a GUD interaction buffer for input and output to the
GDB subprocess (@pxref{Debugger Operation}).  If such a buffer already
exists, switch to it; otherwise, create the buffer and switch to it.
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The other commands in this list do the same, for other debugger
programs.
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@item M-x perldb
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@findex perldb
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Run the Perl interpreter in debug mode.
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@item M-x jdb
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@findex jdb
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Run the Java debugger.
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@item M-x pdb
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@findex pdb
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Run the Python debugger.

@item M-x dbx
@findex dbx
Run the DBX debugger.

@item M-x xdb
@findex xdb
@vindex gud-xdb-directories
Run the XDB debugger.

@item M-x sdb
@findex sdb
Run the SDB debugger.
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@end table

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  Each of these commands reads a command line to invoke the debugger,
using the minibuffer.  The minibuffer's initial contents contain the
standard executable name and options for the debugger, and sometimes
also a guess for the name of the executable file you want to debug.
Shell wildcards and variables are not allowed in this command line.
Emacs assumes that the first command argument which does not start
with a @samp{-} is the executable file name.
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@cindex remote host, debugging on
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  Tramp provides a facility for remote debugging, whereby both the
debugger and the program being debugged are on the same remote host.
@xref{Running a debugger on a remote host,,, tramp, The Tramp Manual},
for details.  This is separate from GDB's remote debugging feature,
where the program and the debugger run on different machines
(@pxref{Remote Debugging,, Debugging Remote Programs, gdb, The GNU
debugger}).
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@node Debugger Operation
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@subsection Debugger Operation
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@cindex GUD interaction buffer
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  The @dfn{GUD interaction buffer} is an Emacs buffer which is used to
send text commands to a debugger subprocess, and record its output.
This is the basic interface for interacting with a debugger, used by
@kbd{M-x gud-gdb} and other commands listed in
@iftex
the preceding section.
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@end iftex
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@ifnottex
@ref{Starting GUD}.
@end ifnottex
The @kbd{M-x gdb} command extends this interface with additional
specialized buffers for controlling breakpoints, stack frames, and
other aspects of the debugger state (@pxref{GDB Graphical Interface}).

  The GUD interaction buffer uses a variant of Shell mode, so the
Emacs commands defined by Shell mode are available (@pxref{Shell
Mode}).  Completion is available for most debugger commands
(@pxref{Completion}), and you can use the usual Shell mode history
commands to repeat them.
@iftex
See the next section
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@end iftex
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@ifnottex
@xref{Commands of GUD},
@end ifnottex
for special commands that can be used in the GUD interaction buffer.

  As you debug a program, Emacs displays the relevant source files by
visiting them in Emacs buffers, with an arrow in the left fringe
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indicating the current execution line.  (On a text terminal, the arrow
appears as @samp{=>}, overlaid on the first two text columns.)  Moving
point in such a buffer does not move the arrow.  You are free to edit
these source files, but note that inserting or deleting lines will
throw off the arrow's positioning, as Emacs has no way to figure out
which edited source line corresponds to the line reported by the
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debugger subprocess.  To update this information, you typically have
to recompile and restart the program.

@cindex GUD Tooltip mode
@cindex mode, GUD Tooltip
@findex gud-tooltip-mode
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@vindex gud-tooltip-echo-area
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  GUD Tooltip mode is a global minor mode that adds tooltip support to
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GUD@.  To toggle this mode, type @kbd{M-x gud-tooltip-mode}.  It is
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disabled by default.  If enabled, you can move the mouse pointer over a
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variable, a function, or a macro (collectively called
@dfn{identifiers}) to show their values in tooltips
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(@pxref{Tooltips}).  If just placing the mouse pointer over an
expression doesn't show the value of the expression you had in mind,
you can tell Emacs more explicitly what expression to evaluate by
dragging the mouse over the expression, then leaving the mouse inside
the marked area.  The GUD Tooltip mode takes effect in the GUD
interaction buffer, and in all source buffers with major modes listed
in the variable @code{gud-tooltip-modes}.  If the variable
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@code{gud-tooltip-echo-area} is non-@code{nil}, or if you turned off
the tooltip mode, values are shown in the echo area instead of a
tooltip.

  When using GUD Tooltip mode with @kbd{M-x gud-gdb}, displaying an
expression's value in GDB can sometimes expand a macro, potentially
causing side effects in the debugged program.  For that reason, using
tooltips in @code{gud-gdb} is disabled.  If you use the @kbd{M-x gdb}
interface, this problem does not occur, as there is special code to
avoid side-effects; furthermore, you can display macro definitions
associated with an identifier when the program is not executing.
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@node Commands of GUD
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@subsection Commands of GUD
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  GUD provides commands for setting and clearing breakpoints,
selecting stack frames, and stepping through the program.
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@table @kbd
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@item C-x C-a C-b
@kindex C-x C-a C-b
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Set a breakpoint on the source line that point is on.
@end table

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  @kbd{C-x C-a C-b} (@code{gud-break}), when called in a source
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buffer, sets a debugger breakpoint on the current source line.  This
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command is available only after starting GUD@.  If you call it in a
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buffer that is not associated with any debugger subprocess, it signals
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an error.
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@kindex C-x C-a @r{(GUD)}
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  The following commands are available both in the GUD interaction
buffer and globally, but with different key bindings.  The keys
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starting with @kbd{C-c} are available only in the GUD interaction
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buffer, while those starting with @kbd{C-x C-a} are available
globally.  Some of these commands are also available via the tool bar;
some are not supported by certain debuggers.
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@table @kbd
@item C-c C-l
@kindex C-c C-l @r{(GUD)}
@itemx C-x C-a C-l
@findex gud-refresh
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Display, in another window, the last source line referred to in the
GUD interaction buffer (@code{gud-refresh}).
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@item C-c C-s
@kindex C-c C-s @r{(GUD)}
@itemx C-x C-a C-s
@findex gud-step
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Execute the next single line of code (@code{gud-step}).  If the line
contains a function call, execution stops after entering the called
function.
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@item C-c C-n
@kindex C-c C-n @r{(GUD)}
@itemx C-x C-a C-n
@findex gud-next
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Execute the next single line of code, stepping across function calls
without stopping inside the functions (@code{gud-next}).
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@item C-c C-i
@kindex C-c C-i @r{(GUD)}
@itemx C-x C-a C-i
@findex gud-stepi
Execute a single machine instruction (@code{gud-stepi}).

@item C-c C-p
@kindex C-c C-p @r{(GUD)}
@itemx C-x C-a C-p
@findex gud-print
Evaluate the expression at point (@code{gud-print}).  If Emacs
does not print the exact expression that you want, mark it as a region
first.

@need 3000
@item C-c C-r
@kindex C-c C-r @r{(GUD)}
@itemx C-x C-a C-r
@findex gud-cont
Continue execution without specifying any stopping point.  The program
will run until it hits a breakpoint, terminates, or gets a signal that
the debugger is checking for (@code{gud-cont}).

@need 1000
@item C-c C-d
@kindex C-c C-d @r{(GUD)}
@itemx C-x C-a C-d
@findex gud-remove
Delete the breakpoint(s) on the current source line, if any
(@code{gud-remove}).  If you use this command in the GUD interaction
buffer, it applies to the line where the program last stopped.

@item C-c C-t
@kindex C-c C-t @r{(GUD)}
@itemx C-x C-a C-t
@findex gud-tbreak
Set a temporary breakpoint on the current source line, if any
(@code{gud-tbreak}).  If you use this command in the GUD interaction
buffer, it applies to the line where the program last stopped.

@item C-c <
@kindex C-c < @r{(GUD)}
@itemx C-x C-a <
@findex gud-up
Select the next enclosing stack frame (@code{gud-up}).  This is
equivalent to the GDB command @samp{up}.

@item C-c >
@kindex C-c > @r{(GUD)}
@itemx C-x C-a >
@findex gud-down
Select the next inner stack frame (@code{gud-down}).  This is
equivalent to the GDB command @samp{down}.

@item C-c C-u
@kindex C-c C-u @r{(GUD)}
@itemx C-x C-a C-u
@findex gud-until
Continue execution to the current line (@code{gud-until}).  The
program will run until it hits a breakpoint, terminates, gets a signal
that the debugger is checking for, or reaches the line on which the
cursor currently sits.

@item C-c C-f
@kindex C-c C-f @r{(GUD)}
@itemx C-x C-a C-f
@findex gud-finish
Run the program until the selected stack frame returns or
stops for some other reason (@code{gud-finish}).
@end table

  If you are using GDB, these additional key bindings are available:

@table @kbd
@item C-x C-a C-j
@kindex C-x C-a C-j @r{(GUD)}
@findex gud-jump
Only useful in a source buffer, @code{gud-jump} transfers the
program's execution point to the current line.  In other words, the
next line that the program executes will be the one where you gave the
command.  If the new execution line is in a different function from
the previously one, GDB prompts for confirmation since the results may
be bizarre.  See the GDB manual entry regarding @code{jump} for
details.

@item @key{TAB}
@kindex TAB @r{(GUD)}
@findex gud-gdb-complete-command
With GDB, complete a symbol name (@code{gud-gdb-complete-command}).
This key is available only in the GUD interaction buffer.
@end table

  These commands interpret a numeric argument as a repeat count, when
that makes sense.

  Because @key{TAB} serves as a completion command, you can't use it to
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enter a tab as input to the program you are debugging with GDB@.
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Instead, type @kbd{C-q @key{TAB}} to enter a tab.

@node GUD Customization
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@subsection GUD Customization
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@vindex gdb-mode-hook
@vindex dbx-mode-hook
@vindex sdb-mode-hook
@vindex xdb-mode-hook
@vindex perldb-mode-hook
@vindex pdb-mode-hook
@vindex jdb-mode-hook
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  On startup, GUD runs one of the following hooks:
@code{gdb-mode-hook}, if you are using GDB; @code{dbx-mode-hook}, if
you are using DBX; @code{sdb-mode-hook}, if you are using SDB;
@code{xdb-mode-hook}, if you are using XDB; @code{perldb-mode-hook},
for Perl debugging mode; @code{pdb-mode-hook}, for PDB;
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@code{jdb-mode-hook}, for JDB@.  @xref{Hooks}.
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  The @code{gud-def} Lisp macro (@pxref{Defining Macros,,, elisp, the
Emacs Lisp Reference Manual}) provides a convenient way to define an
Emacs command that sends a particular command string to the debugger,
and set up a key binding for in the GUD interaction buffer:
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@findex gud-def
@example
(gud-def @var{function} @var{cmdstring} @var{binding} @var{docstring})
@end example

  This defines a command named @var{function} which sends
@var{cmdstring} to the debugger process, and gives it the documentation
string @var{docstring}.  You can then use the command @var{function} in any
buffer.  If @var{binding} is non-@code{nil}, @code{gud-def} also binds
the command to @kbd{C-c @var{binding}} in the GUD buffer's mode and to
@kbd{C-x C-a @var{binding}} generally.

  The command string @var{cmdstring} may contain certain
@samp{%}-sequences that stand for data to be filled in at the time
@var{function} is called:

@table @samp
@item %f
The name of the current source file.  If the current buffer is the GUD
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buffer, then the current source file is the file that the program
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stopped in.

@item %l
The number of the current source line.  If the current buffer is the GUD
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buffer, then the current source line is the line that the program
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stopped in.

@item %e
In transient-mark-mode the text in the region, if it is active.
Otherwise the text of the C lvalue or function-call expression at or
adjacent to point.

@item %a
The text of the hexadecimal address at or adjacent to point.

@item %p
The numeric argument of the called function, as a decimal number.  If
the command is used without a numeric argument, @samp{%p} stands for the
empty string.

If you don't use @samp{%p} in the command string, the command you define
ignores any numeric argument.

@item %d
The name of the directory of the current source file.

@item %c
Fully qualified class name derived from the expression surrounding point
(jdb only).
@end table

@node GDB Graphical Interface
@subsection GDB Graphical Interface

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  The command @kbd{M-x gdb} starts GDB in an IDE-like interface, with
specialized buffers for controlling breakpoints, stack frames, and
other aspects of the debugger state.  It also provides additional ways
to control the debugging session with the mouse, such as clicking in
the fringe of a source buffer to set a breakpoint there.
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@vindex gud-gdb-command-name
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  To run GDB using just the GUD interaction buffer interface, without
these additional features, use @kbd{M-x gud-gdb} (@pxref{Starting
GUD}).  You must use this if you want to debug multiple programs
within one Emacs session, as that is currently unsupported by @kbd{M-x
gdb}.

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  Internally, @kbd{M-x gdb} informs GDB that its screen size is
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unlimited; for correct operation, you must not change GDB's screen
height and width values during the debugging session.
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@menu
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* GDB User Interface Layout::   Control the number of displayed buffers.
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* Source Buffers::              Use the mouse in the fringe/margin to
                                control your program.
* Breakpoints Buffer::          A breakpoint control panel.
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* Threads Buffer::              Displays your threads.
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* Stack Buffer::                Select a frame from the call stack.
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* Other GDB Buffers::           Other buffers for controlling the GDB state.
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* Watch Expressions::           Monitor variable values in the speedbar.
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* Multithreaded Debugging::     Debugging programs with several threads.
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@end menu

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@node GDB User Interface Layout
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@subsubsection GDB User Interface Layout
@cindex GDB User Interface layout

@vindex gdb-many-windows
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  If the variable @code{gdb-many-windows} is @code{nil} (the default),
@kbd{M-x gdb} normally displays only the GUD interaction buffer.
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However, if the variable @code{gdb-show-main} is also non-@code{nil},
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it starts with two windows: one displaying the GUD interaction buffer,
and the other showing the source for the @code{main} function of the
program you are debugging.
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  If @code{gdb-many-windows} is non-@code{nil}, then @kbd{M-x gdb}
displays the following frame layout:

@smallexample
@group
+--------------------------------+--------------------------------+
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|   GUD interaction buffer       |   Locals/Registers buffer      |
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|--------------------------------+--------------------------------+
|   Primary Source buffer        |   I/O buffer for debugged pgm  |
|--------------------------------+--------------------------------+
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|   Stack buffer                 |   Breakpoints/Threads buffer   |
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+--------------------------------+--------------------------------+
@end group
@end smallexample

@findex gdb-restore-windows
@findex gdb-many-windows
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  If you ever change the window layout, you can restore the many-windows
layout by typing @kbd{M-x gdb-restore-windows}.  To toggle
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between the many windows layout and a simple layout with just the GUD
interaction buffer and a source file, type @kbd{M-x gdb-many-windows}.
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  If you have an elaborate window setup, and don't want
@code{gdb-many-windows} to disrupt that, it is better to invoke
@kbd{M-x gdb} in a separate frame to begin with, then the arrangement
of windows on your original frame will not be affected.  A separate
frame for GDB sessions can come in especially handy if you work on a
text-mode terminal, where the screen estate for windows could be at a
premium.

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  You may also specify additional GDB-related buffers to display,
either in the same frame or a different one.  Select the buffers you
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want by typing @kbd{M-x gdb-display-@var{buffertype}-buffer} or
@kbd{M-x gdb-frame-@var{buffertype}-buffer}, where @var{buffertype}
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is the relevant buffer type, such as @samp{breakpoints}.  You can do
the same with the menu bar, with the @samp{GDB-Windows} and
@samp{GDB-Frames} sub-menus of the @samp{GUD} menu.

  When you finish debugging, kill the GUD interaction buffer with
@kbd{C-x k}, which will also kill all the buffers associated with the
session.  However you need not do this if, after editing and
re-compiling your source code within Emacs, you wish to continue
debugging.  When you restart execution, GDB automatically finds the
new executable.  Keeping the GUD interaction buffer has the advantage
of keeping the shell history as well as GDB's breakpoints.  You do
need to check that the breakpoints in recently edited source files are
still in the right places.
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@node Source Buffers
@subsubsection Source Buffers
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@cindex fringes, for debugging
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@table @asis
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@item @kbd{mouse-1} (in fringe)
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Set or clear a breakpoint on that line.
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@item @kbd{C-mouse-1} (in fringe)
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Enable or disable a breakpoint on that line.
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@item @kbd{mouse-3} (in fringe)
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Continue execution to that line.
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@item @kbd{C-mouse-3} (in fringe)
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Jump to that line.
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@end table

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  On a graphical display, you can click @kbd{mouse-1} in the fringe of
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a source buffer, to set a breakpoint on that line (@pxref{Fringes}).
A red dot appears in the fringe, where you clicked.  If a breakpoint
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already exists there, the click removes it.  A @kbd{C-mouse-1} click
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enables or disables an existing breakpoint; a breakpoint that is
disabled, but not unset, is indicated by a gray dot.

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  On a text terminal, or when fringes are disabled, enabled
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breakpoints are indicated with a @samp{B} character in the left margin
of the window.  Disabled breakpoints are indicated with @samp{b}.
(The margin is only displayed if a breakpoint is present.)

  A solid arrow in the left fringe of a source buffer indicates the
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line of the innermost frame where the debugged program has stopped.  A
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hollow arrow indicates the current execution line of a higher-level
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frame.  If you drag the arrow in the fringe with @kbd{mouse-1}, that
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causes execution to advance to the line where you release the button.
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Alternatively, you can click @kbd{mouse-3} in the fringe to advance to
that line.  You can click @kbd{C-mouse-3} in the fringe to jump to
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that line without executing the intermediate lines.  This command
allows you to go backwards, which can be useful for running through
code that has already executed, in order to examine its execution in
more detail.
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@node Breakpoints Buffer
@subsubsection Breakpoints Buffer

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  The GDB Breakpoints buffer shows the breakpoints, watchpoints and
catchpoints in the debugger session.  @xref{Breakpoints,,, gdb, The
GNU debugger}.  It provides the following commands, which mostly apply
to the @dfn{current breakpoint} (the breakpoint which point is on):
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@table @kbd
@item @key{SPC}
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@kindex SPC @r{(GDB Breakpoints buffer)}
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@findex gdb-toggle-breakpoint
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Enable/disable current breakpoint (@code{gdb-toggle-breakpoint}).  On
a graphical display, this changes the color of the dot in the fringe
of the source buffer at that line.  The dot is red when the breakpoint
is enabled, and gray when it is disabled.
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@item D
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@kindex D @r{(GDB Breakpoints buffer)}
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@findex gdb-delete-breakpoint
Delete the current breakpoint (@code{gdb-delete-breakpoint}).

@item @key{RET}
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@kindex RET @r{(GDB Breakpoints buffer)}
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@findex gdb-goto-breakpoint
Visit the source line for the current breakpoint
(@code{gdb-goto-breakpoint}).

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@item mouse-2
@kindex mouse-2 @r{(GDB Breakpoints buffer)}
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Visit the source line for the breakpoint you click on.
@end table

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@vindex gdb-show-threads-by-default
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  When @code{gdb-many-windows} is non-@code{nil}, the GDB Breakpoints
buffer shares its window with the GDB Threads buffer.  To switch from
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one to the other click with @kbd{mouse-1} on the relevant button in
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the header line.  If @code{gdb-show-threads-by-default} is
non-@code{nil}, the GDB Threads buffer is the one shown by default.
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@node Threads Buffer
@subsubsection Threads Buffer

@findex gdb-select-thread
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  The GDB Threads buffer displays a summary of the threads in the
debugged program.  @xref{Threads, Threads, Debugging programs with
multiple threads, gdb, The GNU debugger}.  To select a thread, move
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point there and press @key{RET} (@code{gdb-select-thread}), or click on
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it with @kbd{mouse-2}.  This also displays the associated source
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buffer, and updates the contents of the other GDB buffers.
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  You can customize variables under @code{gdb-buffers} group to select
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fields included in GDB Threads buffer.
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@table @code
@item gdb-thread-buffer-verbose-names
@vindex gdb-thread-buffer-verbose-names
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Show long thread names like @samp{Thread 0x4e2ab70 (LWP 1983)}.
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@item gdb-thread-buffer-arguments
@vindex gdb-thread-buffer-arguments
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Show arguments of thread top frames.
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@item gdb-thread-buffer-locations
@vindex gdb-thread-buffer-locations
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Show file information or library names.
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@item gdb-thread-buffer-addresses
@vindex gdb-thread-buffer-addresses
Show addresses for thread frames in threads buffer.
@end table

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  To view information for several threads simultaneously, use the
following commands from the GDB Threads buffer.
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@table @kbd
@item d
@kindex d @r{(GDB threads buffer)}
@findex gdb-display-disassembly-for-thread
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Display disassembly buffer for the thread at current line
(@code{gdb-display-disassembly-for-thread}).
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@item f
@kindex f @r{(GDB threads buffer)}
@findex gdb-display-stack-for-thread
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Display the GDB Stack buffer for the thread at current line
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(@code{gdb-display-stack-for-thread}).

@item l
@kindex l @r{(GDB threads buffer)}
@findex gdb-display-locals-for-thread
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Display the GDB Locals buffer for the thread at current line
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(@code{gdb-display-locals-for-thread}).

@item r
@kindex r @r{(GDB threads buffer)}
@findex gdb-display-registers-for-thread
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Display the GDB Registers buffer for the thread at current line
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(@code{gdb-display-registers-for-thread}).
@end table

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@noindent
Their upper-case counterparts, @kbd{D}, @kbd{F} ,@kbd{L} and @kbd{R},
display the corresponding buffer in a new frame.
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  When you create a buffer showing information about some specific
thread, it becomes bound to that thread and keeps showing actual
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information while you debug your program.  The mode indicator for each
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GDB buffer shows the number of the thread whose information that
buffer displays.  The thread number is also included in the name of
each bound buffer.
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  Further commands are available in the GDB Threads buffer which
depend on the mode of GDB that is used for controlling execution of
your program.  @xref{Multithreaded Debugging}.
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@node Stack Buffer
@subsubsection Stack Buffer

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  The GDB Stack buffer displays a @dfn{call stack}, with one line for
each of the nested subroutine calls (@dfn{stack frames}) in the
debugger session.  @xref{Backtrace,, Backtraces, gdb, The GNU
debugger}.
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@findex gdb-frames-select
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  On graphical displays, the selected stack frame is indicated by an
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arrow in the fringe.  On text terminals, or when fringes are disabled,
the selected stack frame is displayed in reverse contrast.  To select
a stack frame, move point in its line and type @key{RET}
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(@code{gdb-frames-select}), or click @kbd{mouse-2} on it.  Doing so
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also updates the Locals buffer
@ifnottex
(@pxref{Other GDB Buffers}).
@end ifnottex
@iftex
(described in the next section).
@end iftex
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@node Other GDB Buffers
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@subsubsection Other GDB Buffers
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@table @asis
@item Locals Buffer
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This buffer displays the values of local variables of the current
frame for simple data types (@pxref{Frame Info, Frame Info,
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Information on a frame, gdb, The GNU debugger}).  Press @key{RET} or
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click @kbd{mouse-2} on the value if you want to edit it.
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Arrays and structures display their type only.  With GDB 6.4 or later,
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you can examine the value of the local variable at point by typing
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@key{RET}, or with a @kbd{mouse-2} click.  With earlier versions of
GDB, use @key{RET} or @kbd{mouse-2} on the type description
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(@samp{[struct/union]} or @samp{[array]}).  @xref{Watch Expressions}.

@item Registers Buffer
@findex toggle-gdb-all-registers
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This buffer displays the values held by the registers
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(@pxref{Registers,,, gdb, The GNU debugger}).  Press @key{RET} or
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click @kbd{mouse-2} on a register if you want to edit its value.  With
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GDB 6.4 or later, recently changed register values display with
@code{font-lock-warning-face}.
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@item Assembler Buffer
The assembler buffer displays the current frame as machine code.  An
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arrow points to the current instruction, and you can set and remove
breakpoints as in a source buffer.  Breakpoint icons also appear in
the fringe or margin.

@item Memory Buffer
The memory buffer lets you examine sections of program memory
(@pxref{Memory, Memory, Examining memory, gdb, The GNU debugger}).
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Click @kbd{mouse-1} on the appropriate part of the header line to
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change the starting address or number of data items that the buffer
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displays.  Alternatively, use @kbd{S} or @kbd{N} respectively.  Click
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@kbd{mouse-3} on the header line to select the display format or unit
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size for these data items.
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@end table

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When @code{gdb-many-windows} is non-@code{nil}, the locals buffer
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shares its window with the registers buffer, just like breakpoints and
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threads buffers.  To switch from one to the other, click with