<html lang="en">
<head>
<title>Set Breaks - Debugging with GDB</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="Debugging with GDB">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Breakpoints.html#Breakpoints" title="Breakpoints">
<link rel="next" href="Set-Watchpoints.html#Set-Watchpoints" title="Set Watchpoints">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software'' and ``Free Software Needs
Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.

(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
this GNU Manual.  Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom.''-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Set-Breaks"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Set-Watchpoints.html#Set-Watchpoints">Set Watchpoints</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Breakpoints.html#Breakpoints">Breakpoints</a>
<hr>
</div>

<h4 class="subsection">5.1.1 Setting Breakpoints</h4>

<!-- FIXME LMB what does GDB do if no code on line of breakpt? -->
<!-- consider in particular declaration with/without initialization. -->
<!-- FIXME 2 is there stuff on this already? break at fun start, already init? -->
<p><a name="index-break-211"></a><a name="index-b-_0040r_007b_0028_0040code_007bbreak_007d_0029_007d-212"></a><a name="index-g_t_0024bpnum_0040r_007b_002c-convenience-variable_007d-213"></a><a name="index-latest-breakpoint-214"></a>Breakpoints are set with the <code>break</code> command (abbreviated
<code>b</code>).  The debugger convenience variable &lsquo;<samp><span class="samp">$bpnum</span></samp>&rsquo; records the
number of the breakpoint you've set most recently; see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>, for a discussion of what you can do with
convenience variables.

     <dl>
<dt><code>break </code><var>location</var><dd>Set a breakpoint at the given <var>location</var>, which can specify a
function name, a line number, or an address of an instruction. 
(See <a href="Specify-Location.html#Specify-Location">Specify Location</a>, for a list of all the possible ways to
specify a <var>location</var>.)  The breakpoint will stop your program just
before it executes any of the code in the specified <var>location</var>.

     <p>When using source languages that permit overloading of symbols, such as
C<tt>++</tt>, a function name may refer to more than one possible place to break. 
See <a href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous Expressions</a>, for a discussion of
that situation.

     <p>It is also possible to insert a breakpoint that will stop the program
only if a specific thread (see <a href="Thread_002dSpecific-Breakpoints.html#Thread_002dSpecific-Breakpoints">Thread-Specific Breakpoints</a>)
or a specific task (see <a href="Ada-Tasks.html#Ada-Tasks">Ada Tasks</a>) hits that breakpoint.

     <br><dt><code>break</code><dd>When called without any arguments, <code>break</code> sets a breakpoint at
the next instruction to be executed in the selected stack frame
(see <a href="Stack.html#Stack">Examining the Stack</a>).  In any selected frame but the
innermost, this makes your program stop as soon as control
returns to that frame.  This is similar to the effect of a
<code>finish</code> command in the frame inside the selected frame&mdash;except
that <code>finish</code> does not leave an active breakpoint.  If you use
<code>break</code> without an argument in the innermost frame, <span class="sc">gdb</span> stops
the next time it reaches the current location; this may be useful
inside loops.

     <p><span class="sc">gdb</span> normally ignores breakpoints when it resumes execution, until at
least one instruction has been executed.  If it did not do this, you
would be unable to proceed past a breakpoint without first disabling the
breakpoint.  This rule applies whether or not the breakpoint already
existed when your program stopped.

     <br><dt><code>break ... if </code><var>cond</var><dd>Set a breakpoint with condition <var>cond</var>; evaluate the expression
<var>cond</var> each time the breakpoint is reached, and stop only if the
value is nonzero&mdash;that is, if <var>cond</var> evaluates as true. 
&lsquo;<samp><span class="samp">...</span></samp>&rsquo; stands for one of the possible arguments described
above (or no argument) specifying where to break.  See <a href="Conditions.html#Conditions">Break Conditions</a>, for more information on breakpoint conditions.

     <p><a name="index-tbreak-215"></a><br><dt><code>tbreak </code><var>args</var><dd>Set a breakpoint enabled only for one stop.  <var>args</var> are the
same as for the <code>break</code> command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first time your
program stops there.  See <a href="Disabling.html#Disabling">Disabling Breakpoints</a>.

     <p><a name="index-hbreak-216"></a><a name="index-hardware-breakpoints-217"></a><br><dt><code>hbreak </code><var>args</var><dd>Set a hardware-assisted breakpoint.  <var>args</var> are the same as for the
<code>break</code> command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may not
have this support.  The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction.  This can be used with the new trap-generation
provided by SPARClite DSU and most x86-based targets.  These targets
will generate traps when a program accesses some data or instruction
address that is assigned to the debug registers.  However the hardware
breakpoint registers can take a limited number of breakpoints.  For
example, on the DSU, only two data breakpoints can be set at a time, and
<span class="sc">gdb</span> will reject this command if more than two are used.  Delete
or disable unused hardware breakpoints before setting new ones
(see <a href="Disabling.html#Disabling">Disabling Breakpoints</a>). 
See <a href="Conditions.html#Conditions">Break Conditions</a>. 
For remote targets, you can restrict the number of hardware
breakpoints <span class="sc">gdb</span> will use, see <a href="set-remote-hardware_002dbreakpoint_002dlimit.html#set-remote-hardware_002dbreakpoint_002dlimit">set remote hardware-breakpoint-limit</a>.

     <p><a name="index-thbreak-218"></a><br><dt><code>thbreak </code><var>args</var><dd>Set a hardware-assisted breakpoint enabled only for one stop.  <var>args</var>
are the same as for the <code>hbreak</code> command and the breakpoint is set in
the same way.  However, like the <code>tbreak</code> command,
the breakpoint is automatically deleted after the
first time your program stops there.  Also, like the <code>hbreak</code>
command, the breakpoint requires hardware support and some target hardware
may not have this support.  See <a href="Disabling.html#Disabling">Disabling Breakpoints</a>. 
See also <a href="Conditions.html#Conditions">Break Conditions</a>.

     <p><a name="index-rbreak-219"></a><a name="index-regular-expression-220"></a><a name="index-breakpoints-at-functions-matching-a-regexp-221"></a><a name="index-set-breakpoints-in-many-functions-222"></a><br><dt><code>rbreak </code><var>regex</var><dd>Set breakpoints on all functions matching the regular expression
<var>regex</var>.  This command sets an unconditional breakpoint on all
matches, printing a list of all breakpoints it set.  Once these
breakpoints are set, they are treated just like the breakpoints set with
the <code>break</code> command.  You can delete them, disable them, or make
them conditional the same way as any other breakpoint.

     <p>The syntax of the regular expression is the standard one used with tools
like <samp><span class="file">grep</span></samp>.  Note that this is different from the syntax used by
shells, so for instance <code>foo*</code> matches all functions that include
an <code>fo</code> followed by zero or more <code>o</code>s.  There is an implicit
<code>.*</code> leading and trailing the regular expression you supply, so to
match only functions that begin with <code>foo</code>, use <code>^foo</code>.

     <p><a name="index-non_002dmember-C_0040t_007b_002b_002b_007d-functions_002c-set-breakpoint-in-223"></a>When debugging C<tt>++</tt> programs, <code>rbreak</code> is useful for setting
breakpoints on overloaded functions that are not members of any special
classes.

     <p><a name="index-set-breakpoints-on-all-functions-224"></a>The <code>rbreak</code> command can be used to set breakpoints in
<strong>all</strong> the functions in a program, like this:

     <pre class="smallexample">          (gdb) rbreak .
</pre>
     <br><dt><code>rbreak </code><var>file</var><code>:</code><var>regex</var><dd>If <code>rbreak</code> is called with a filename qualification, it limits
the search for functions matching the given regular expression to the
specified <var>file</var>.  This can be used, for example, to set breakpoints on
every function in a given file:

     <pre class="smallexample">          (gdb) rbreak file.c:.
</pre>
     <p>The colon separating the filename qualifier from the regex may
optionally be surrounded by spaces.

     <p><a name="index-info-breakpoints-225"></a><a name="index-g_t_0040code_007b_0024_005f_007d-and-_0040code_007binfo-breakpoints_007d-226"></a><br><dt><code>info breakpoints </code><span class="roman">[</span><var>n</var><span class="roman">]</span><dt><code>info break </code><span class="roman">[</span><var>n</var><span class="roman">]</span><dd>Print a table of all breakpoints, watchpoints, and catchpoints set and
not deleted.  Optional argument <var>n</var> means print information only
about the specified breakpoint (or watchpoint or catchpoint).  For
each breakpoint, following columns are printed:

          <dl>
<dt><em>Breakpoint Numbers</em><br><dt><em>Type</em><dd>Breakpoint, watchpoint, or catchpoint. 
<br><dt><em>Disposition</em><dd>Whether the breakpoint is marked to be disabled or deleted when hit. 
<br><dt><em>Enabled or Disabled</em><dd>Enabled breakpoints are marked with &lsquo;<samp><span class="samp">y</span></samp>&rsquo;.  &lsquo;<samp><span class="samp">n</span></samp>&rsquo; marks breakpoints
that are not enabled. 
<br><dt><em>Address</em><dd>Where the breakpoint is in your program, as a memory address.  For a
pending breakpoint whose address is not yet known, this field will
contain &lsquo;<samp><span class="samp">&lt;PENDING&gt;</span></samp>&rsquo;.  Such breakpoint won't fire until a shared
library that has the symbol or line referred by breakpoint is loaded. 
See below for details.  A breakpoint with several locations will
have &lsquo;<samp><span class="samp">&lt;MULTIPLE&gt;</span></samp>&rsquo; in this field&mdash;see below for details. 
<br><dt><em>What</em><dd>Where the breakpoint is in the source for your program, as a file and
line number.  For a pending breakpoint, the original string passed to
the breakpoint command will be listed as it cannot be resolved until
the appropriate shared library is loaded in the future. 
</dl>

     <p class="noindent">If a breakpoint is conditional, <code>info break</code> shows the condition on
the line following the affected breakpoint; breakpoint commands, if any,
are listed after that.  A pending breakpoint is allowed to have a condition
specified for it.  The condition is not parsed for validity until a shared
library is loaded that allows the pending breakpoint to resolve to a
valid location.

     <p class="noindent"><code>info break</code> with a breakpoint
number <var>n</var> as argument lists only that breakpoint.  The
convenience variable <code>$_</code> and the default examining-address for
the <code>x</code> command are set to the address of the last breakpoint
listed (see <a href="Memory.html#Memory">Examining Memory</a>).

     <p class="noindent"><code>info break</code> displays a count of the number of times the breakpoint
has been hit.  This is especially useful in conjunction with the
<code>ignore</code> command.  You can ignore a large number of breakpoint
hits, look at the breakpoint info to see how many times the breakpoint
was hit, and then run again, ignoring one less than that number.  This
will get you quickly to the last hit of that breakpoint. 
</dl>

   <p><span class="sc">gdb</span> allows you to set any number of breakpoints at the same place in
your program.  There is nothing silly or meaningless about this.  When
the breakpoints are conditional, this is even useful
(see <a href="Conditions.html#Conditions">Break Conditions</a>).

   <p><a name="index-multiple-locations_002c-breakpoints-227"></a><a name="index-breakpoints_002c-multiple-locations-228"></a>It is possible that a breakpoint corresponds to several locations
in your program.  Examples of this situation are:

     <ul>
<li>For a C<tt>++</tt> constructor, the <span class="sc">gcc</span> compiler generates several
instances of the function body, used in different cases.

     <li>For a C<tt>++</tt> template function, a given line in the function can
correspond to any number of instantiations.

     <li>For an inlined function, a given source line can correspond to
several places where that function is inlined. 
</ul>

   <p>In all those cases, <span class="sc">gdb</span> will insert a breakpoint at all
the relevant locations<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.

   <p>A breakpoint with multiple locations is displayed in the breakpoint
table using several rows&mdash;one header row, followed by one row for
each breakpoint location.  The header row has &lsquo;<samp><span class="samp">&lt;MULTIPLE&gt;</span></samp>&rsquo; in the
address column.  The rows for individual locations contain the actual
addresses for locations, and show the functions to which those
locations belong.  The number column for a location is of the form
<var>breakpoint-number</var>.<var>location-number</var>.

   <p>For example:

<pre class="smallexample">     Num     Type           Disp Enb  Address    What
     1       breakpoint     keep y    &lt;MULTIPLE&gt;
             stop only if i==1
             breakpoint already hit 1 time
     1.1                         y    0x080486a2 in void foo&lt;int&gt;() at t.cc:8
     1.2                         y    0x080486ca in void foo&lt;double&gt;() at t.cc:8
</pre>
   <p>Each location can be individually enabled or disabled by passing
<var>breakpoint-number</var>.<var>location-number</var> as argument to the
<code>enable</code> and <code>disable</code> commands.  Note that you cannot
delete the individual locations from the list, you can only delete the
entire list of locations that belong to their parent breakpoint (with
the <kbd>delete </kbd><var>num</var> command, where <var>num</var> is the number of
the parent breakpoint, 1 in the above example).  Disabling or enabling
the parent breakpoint (see <a href="Disabling.html#Disabling">Disabling</a>) affects all of the locations
that belong to that breakpoint.

   <p><a name="index-pending-breakpoints-229"></a>It's quite common to have a breakpoint inside a shared library. 
Shared libraries can be loaded and unloaded explicitly,
and possibly repeatedly, as the program is executed.  To support
this use case, <span class="sc">gdb</span> updates breakpoint locations whenever
any shared library is loaded or unloaded.  Typically, you would
set a breakpoint in a shared library at the beginning of your
debugging session, when the library is not loaded, and when the
symbols from the library are not available.  When you try to set
breakpoint, <span class="sc">gdb</span> will ask you if you want to set
a so called <dfn>pending breakpoint</dfn>&mdash;breakpoint whose address
is not yet resolved.

   <p>After the program is run, whenever a new shared library is loaded,
<span class="sc">gdb</span> reevaluates all the breakpoints.  When a newly loaded
shared library contains the symbol or line referred to by some
pending breakpoint, that breakpoint is resolved and becomes an
ordinary breakpoint.  When a library is unloaded, all breakpoints
that refer to its symbols or source lines become pending again.

   <p>This logic works for breakpoints with multiple locations, too.  For
example, if you have a breakpoint in a C<tt>++</tt> template function, and
a newly loaded shared library has an instantiation of that template,
a new location is added to the list of locations for the breakpoint.

   <p>Except for having unresolved address, pending breakpoints do not
differ from regular breakpoints.  You can set conditions or commands,
enable and disable them and perform other breakpoint operations.

   <p><span class="sc">gdb</span> provides some additional commands for controlling what
happens when the &lsquo;<samp><span class="samp">break</span></samp>&rsquo; command cannot resolve breakpoint
address specification to an address:

   <p><a name="index-set-breakpoint-pending-230"></a><a name="index-show-breakpoint-pending-231"></a>
     <dl>
<dt><code>set breakpoint pending auto</code><dd>This is the default behavior.  When <span class="sc">gdb</span> cannot find the breakpoint
location, it queries you whether a pending breakpoint should be created.

     <br><dt><code>set breakpoint pending on</code><dd>This indicates that an unrecognized breakpoint location should automatically
result in a pending breakpoint being created.

     <br><dt><code>set breakpoint pending off</code><dd>This indicates that pending breakpoints are not to be created.  Any
unrecognized breakpoint location results in an error.  This setting does
not affect any pending breakpoints previously created.

     <br><dt><code>show breakpoint pending</code><dd>Show the current behavior setting for creating pending breakpoints. 
</dl>

   <p>The settings above only affect the <code>break</code> command and its
variants.  Once breakpoint is set, it will be automatically updated
as shared libraries are loaded and unloaded.

   <p><a name="index-automatic-hardware-breakpoints-232"></a>For some targets, <span class="sc">gdb</span> can automatically decide if hardware or
software breakpoints should be used, depending on whether the
breakpoint address is read-only or read-write.  This applies to
breakpoints set with the <code>break</code> command as well as to internal
breakpoints set by commands like <code>next</code> and <code>finish</code>.  For
breakpoints set with <code>hbreak</code>, <span class="sc">gdb</span> will always use hardware
breakpoints.

   <p>You can control this automatic behaviour with the following commands::

   <p><a name="index-set-breakpoint-auto_002dhw-233"></a><a name="index-show-breakpoint-auto_002dhw-234"></a>
     <dl>
<dt><code>set breakpoint auto-hw on</code><dd>This is the default behavior.  When <span class="sc">gdb</span> sets a breakpoint, it
will try to use the target memory map to decide if software or hardware
breakpoint must be used.

     <br><dt><code>set breakpoint auto-hw off</code><dd>This indicates <span class="sc">gdb</span> should not automatically select breakpoint
type.  If the target provides a memory map, <span class="sc">gdb</span> will warn when
trying to set software breakpoint at a read-only address. 
</dl>

   <p><span class="sc">gdb</span> normally implements breakpoints by replacing the program code
at the breakpoint address with a special instruction, which, when
executed, given control to the debugger.  By default, the program
code is so modified only when the program is resumed.  As soon as
the program stops, <span class="sc">gdb</span> restores the original instructions.  This
behaviour guards against leaving breakpoints inserted in the
target should gdb abrubptly disconnect.  However, with slow remote
targets, inserting and removing breakpoint can reduce the performance. 
This behavior can be controlled with the following commands::

   <p><a name="index-set-breakpoint-always_002dinserted-235"></a><a name="index-show-breakpoint-always_002dinserted-236"></a>
     <dl>
<dt><code>set breakpoint always-inserted off</code><dd>All breakpoints, including newly added by the user, are inserted in
the target only when the target is resumed.  All breakpoints are
removed from the target when it stops.

     <br><dt><code>set breakpoint always-inserted on</code><dd>Causes all breakpoints to be inserted in the target at all times.  If
the user adds a new breakpoint, or changes an existing breakpoint, the
breakpoints in the target are updated immediately.  A breakpoint is
removed from the target only when breakpoint itself is removed.

     <p><a name="index-non_002dstop-mode_002c-and-_0040code_007bbreakpoint-always_002dinserted_007d-237"></a><br><dt><code>set breakpoint always-inserted auto</code><dd>This is the default mode.  If <span class="sc">gdb</span> is controlling the inferior
in non-stop mode (see <a href="Non_002dStop-Mode.html#Non_002dStop-Mode">Non-Stop Mode</a>), gdb behaves as if
<code>breakpoint always-inserted</code> mode is on.  If <span class="sc">gdb</span> is
controlling the inferior in all-stop mode, <span class="sc">gdb</span> behaves as if
<code>breakpoint always-inserted</code> mode is off. 
</dl>

   <p><a name="index-negative-breakpoint-numbers-238"></a><a name="index-internal-_0040value_007bGDBN_007d-breakpoints-239"></a><span class="sc">gdb</span> itself sometimes sets breakpoints in your program for
special purposes, such as proper handling of <code>longjmp</code> (in C
programs).  These internal breakpoints are assigned negative numbers,
starting with <code>-1</code>; &lsquo;<samp><span class="samp">info breakpoints</span></samp>&rsquo; does not display them. 
You can see these breakpoints with the <span class="sc">gdb</span> maintenance command
&lsquo;<samp><span class="samp">maint info breakpoints</span></samp>&rsquo; (see <a href="maint-info-breakpoints.html#maint-info-breakpoints">maint info breakpoints</a>).

   <div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> 
As of this writing, multiple-location breakpoints work only if there's
line number information for all the locations.  This means that they
will generally not work in system libraries, unless you have debug
info with line numbers for them.</p>

   <hr></div>

   </body></html>