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- <title>Set Breaks (Debugging with GDB)</title>
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- <a name="Set-Breaks"></a>
- <div class="header">
- <p>
- Next: <a href="Set-Watchpoints.html#Set-Watchpoints" accesskey="n" rel="next">Set Watchpoints</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
- </div>
- <hr>
- <a name="Setting-Breakpoints"></a>
- <h4 class="subsection">5.1.1 Setting Breakpoints</h4>
-
-
- <a name="index-break"></a>
- <a name="index-b-_0028break_0029"></a>
- <a name="index-_0024bpnum_002c-convenience-variable"></a>
- <a name="index-latest-breakpoint"></a>
- <p>Breakpoints are set with the <code>break</code> command (abbreviated
- <code>b</code>). The debugger convenience variable ‘<samp>$bpnum</samp>’ 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.
- </p>
- <dl compact="compact">
- <dt><code>break <var>location</var></code></dt>
- <dd><p>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>
- <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>
- <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.
- </p>
- </dd>
- <dt><code>break</code></dt>
- <dd><p>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—except
- that <code>finish</code> does not leave an active breakpoint. If you use
- <code>break</code> without an argument in the innermost frame, <small>GDB</small> stops
- the next time it reaches the current location; this may be useful
- inside loops.
- </p>
- <p><small>GDB</small> 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.
- </p>
- </dd>
- <dt><code>break … if <var>cond</var></code></dt>
- <dd><p>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—that is, if <var>cond</var> evaluates as true.
- ‘<samp>…</samp>’ 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"></a>
- </dd>
- <dt><code>tbreak <var>args</var></code></dt>
- <dd><p>Set a breakpoint enabled only for one stop. The <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"></a>
- <a name="index-hardware-breakpoints"></a>
- </dd>
- <dt><code>hbreak <var>args</var></code></dt>
- <dd><p>Set a hardware-assisted breakpoint. The <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
- <small>GDB</small> 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 <small>GDB</small> will use, see <a href="Remote-Configuration.html#set-remote-hardware_002dbreakpoint_002dlimit">set remote hardware-breakpoint-limit</a>.
- </p>
- <a name="index-thbreak"></a>
- </dd>
- <dt><code>thbreak <var>args</var></code></dt>
- <dd><p>Set a hardware-assisted breakpoint enabled only for one stop. The <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"></a>
- <a name="index-regular-expression"></a>
- <a name="index-breakpoints-at-functions-matching-a-regexp"></a>
- <a name="index-set-breakpoints-in-many-functions"></a>
- </dd>
- <dt><code>rbreak <var>regex</var></code></dt>
- <dd><p>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>
- <p>In programs using different languages, <small>GDB</small> chooses the syntax
- to print the list of all breakpoints it sets according to the
- ‘<samp>set language</samp>’ value: using ‘<samp>set language auto</samp>’
- (see <a href="Automatically.html#Automatically">Set Language Automatically</a>) means to use the
- language of the breakpoint’s function, other values mean to use
- the manually specified language (see <a href="Manually.html#Manually">Set Language Manually</a>).
- </p>
- <p>The syntax of the regular expression is the standard one used with tools
- like <samp>grep</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_002b_002b-functions_002c-set-breakpoint-in"></a>
- <p>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"></a>
- <p>The <code>rbreak</code> command can be used to set breakpoints in
- <strong>all</strong> the functions in a program, like this:
- </p>
- <div class="smallexample">
- <pre class="smallexample">(gdb) rbreak .
- </pre></div>
-
- </dd>
- <dt><code>rbreak <var>file</var>:<var>regex</var></code></dt>
- <dd><p>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:
- </p>
- <div class="smallexample">
- <pre class="smallexample">(gdb) rbreak file.c:.
- </pre></div>
-
- <p>The colon separating the filename qualifier from the regex may
- optionally be surrounded by spaces.
- </p>
- <a name="index-info-breakpoints"></a>
- <a name="index-_0024_005f-and-info-breakpoints"></a>
- </dd>
- <dt><code>info breakpoints <span class="roman">[</span><var>list</var>…<span class="roman">]</span></code></dt>
- <dt><code>info break <span class="roman">[</span><var>list</var>…<span class="roman">]</span></code></dt>
- <dd><p>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(s) (or watchpoint(s) or catchpoint(s)).
- For each breakpoint, following columns are printed:
- </p>
- <dl compact="compact">
- <dt><em>Breakpoint Numbers</em></dt>
- <dt><em>Type</em></dt>
- <dd><p>Breakpoint, watchpoint, or catchpoint.
- </p></dd>
- <dt><em>Disposition</em></dt>
- <dd><p>Whether the breakpoint is marked to be disabled or deleted when hit.
- </p></dd>
- <dt><em>Enabled or Disabled</em></dt>
- <dd><p>Enabled breakpoints are marked with ‘<samp>y</samp>’. ‘<samp>n</samp>’ marks breakpoints
- that are not enabled.
- </p></dd>
- <dt><em>Address</em></dt>
- <dd><p>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 ‘<samp><PENDING></samp>’. 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 ‘<samp><MULTIPLE></samp>’ in this field—see below for details.
- </p></dd>
- <dt><em>What</em></dt>
- <dd><p>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.
- </p></dd>
- </dl>
-
- <p>If a breakpoint is conditional, there are two evaluation modes: “host” and
- “target”. If mode is “host”, breakpoint condition evaluation is done by
- <small>GDB</small> on the host’s side. If it is “target”, then the condition
- is evaluated by the target. The <code>info break</code> command shows
- the condition on the line following the affected breakpoint, together with
- its condition evaluation mode in between parentheses.
- </p>
- <p>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>
- <p><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>
- <p><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.
- </p>
- <p>For a breakpoints with an enable count (xref) greater than 1,
- <code>info break</code> also displays that count.
- </p>
- </dd>
- </dl>
-
- <p><small>GDB</small> 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"></a>
- <a name="index-breakpoints_002c-multiple-locations"></a>
- <p>It is possible that a breakpoint corresponds to several locations
- in your program. Examples of this situation are:
- </p>
- <ul>
- <li> Multiple functions in the program may have the same name.
-
- </li><li> For a C<tt>++</tt> constructor, the <small>GCC</small> compiler generates several
- instances of the function body, used in different cases.
-
- </li><li> For a C<tt>++</tt> template function, a given line in the function can
- correspond to any number of instantiations.
-
- </li><li> For an inlined function, a given source line can correspond to
- several places where that function is inlined.
- </li></ul>
-
- <p>In all those cases, <small>GDB</small> will insert a breakpoint at all
- the relevant locations.
- </p>
- <p>A breakpoint with multiple locations is displayed in the breakpoint
- table using several rows—one header row, followed by one row for
- each breakpoint location. The header row has ‘<samp><MULTIPLE></samp>’ 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>
- <p>For example:
- </p>
- <div class="smallexample">
- <pre class="smallexample">Num Type Disp Enb Address What
- 1 breakpoint keep y <MULTIPLE>
- stop only if i==1
- breakpoint already hit 1 time
- 1.1 y 0x080486a2 in void foo<int>() at t.cc:8
- 1.2 y 0x080486ca in void foo<double>() at t.cc:8
- </pre></div>
-
- <p>You cannot delete the individual locations from a breakpoint. However,
- 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. It’s also possible to
- <code>enable</code> and <code>disable</code> a range of <var>location-number</var>
- locations using a <var>breakpoint-number</var> and two <var>location-number</var>s,
- in increasing order, separated by a hyphen, like
- <kbd><var>breakpoint-number</var>.<var>location-number1</var>-<var>location-number2</var></kbd>,
- in which case <small>GDB</small> acts on all the locations in the range (inclusive).
- 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"></a>
- <p>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, <small>GDB</small> 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, <small>GDB</small> will ask you if you want to set
- a so called <em>pending breakpoint</em>—breakpoint whose address
- is not yet resolved.
- </p>
- <p>After the program is run, whenever a new shared library is loaded,
- <small>GDB</small> 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>
- <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>
- <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>
- <p><small>GDB</small> provides some additional commands for controlling what
- happens when the ‘<samp>break</samp>’ command cannot resolve breakpoint
- address specification to an address:
- </p>
- <a name="index-set-breakpoint-pending"></a>
- <a name="index-show-breakpoint-pending"></a>
- <dl compact="compact">
- <dt><code>set breakpoint pending auto</code></dt>
- <dd><p>This is the default behavior. When <small>GDB</small> cannot find the breakpoint
- location, it queries you whether a pending breakpoint should be created.
- </p>
- </dd>
- <dt><code>set breakpoint pending on</code></dt>
- <dd><p>This indicates that an unrecognized breakpoint location should automatically
- result in a pending breakpoint being created.
- </p>
- </dd>
- <dt><code>set breakpoint pending off</code></dt>
- <dd><p>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.
- </p>
- </dd>
- <dt><code>show breakpoint pending</code></dt>
- <dd><p>Show the current behavior setting for creating pending breakpoints.
- </p></dd>
- </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"></a>
- <p>For some targets, <small>GDB</small> 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>, <small>GDB</small> will always use hardware
- breakpoints.
- </p>
- <p>You can control this automatic behaviour with the following commands:
- </p>
- <a name="index-set-breakpoint-auto_002dhw"></a>
- <a name="index-show-breakpoint-auto_002dhw"></a>
- <dl compact="compact">
- <dt><code>set breakpoint auto-hw on</code></dt>
- <dd><p>This is the default behavior. When <small>GDB</small> sets a breakpoint, it
- will try to use the target memory map to decide if software or hardware
- breakpoint must be used.
- </p>
- </dd>
- <dt><code>set breakpoint auto-hw off</code></dt>
- <dd><p>This indicates <small>GDB</small> should not automatically select breakpoint
- type. If the target provides a memory map, <small>GDB</small> will warn when
- trying to set software breakpoint at a read-only address.
- </p></dd>
- </dl>
-
- <p><small>GDB</small> 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, <small>GDB</small> 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"></a>
- <a name="index-show-breakpoint-always_002dinserted"></a>
- <dl compact="compact">
- <dt><code>set breakpoint always-inserted off</code></dt>
- <dd><p>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. This is the default mode.
- </p>
- </dd>
- <dt><code>set breakpoint always-inserted on</code></dt>
- <dd><p>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 deleted.
- </p></dd>
- </dl>
-
- <p><small>GDB</small> handles conditional breakpoints by evaluating these conditions
- when a breakpoint breaks. If the condition is true, then the process being
- debugged stops, otherwise the process is resumed.
- </p>
- <p>If the target supports evaluating conditions on its end, <small>GDB</small> may
- download the breakpoint, together with its conditions, to it.
- </p>
- <p>This feature can be controlled via the following commands:
- </p>
- <a name="index-set-breakpoint-condition_002devaluation"></a>
- <a name="index-show-breakpoint-condition_002devaluation"></a>
- <dl compact="compact">
- <dt><code>set breakpoint condition-evaluation host</code></dt>
- <dd><p>This option commands <small>GDB</small> to evaluate the breakpoint
- conditions on the host’s side. Unconditional breakpoints are sent to
- the target which in turn receives the triggers and reports them back to GDB
- for condition evaluation. This is the standard evaluation mode.
- </p>
- </dd>
- <dt><code>set breakpoint condition-evaluation target</code></dt>
- <dd><p>This option commands <small>GDB</small> to download breakpoint conditions
- to the target at the moment of their insertion. The target
- is responsible for evaluating the conditional expression and reporting
- breakpoint stop events back to <small>GDB</small> whenever the condition
- is true. Due to limitations of target-side evaluation, some conditions
- cannot be evaluated there, e.g., conditions that depend on local data
- that is only known to the host. Examples include
- conditional expressions involving convenience variables, complex types
- that cannot be handled by the agent expression parser and expressions
- that are too long to be sent over to the target, specially when the
- target is a remote system. In these cases, the conditions will be
- evaluated by <small>GDB</small>.
- </p>
- </dd>
- <dt><code>set breakpoint condition-evaluation auto</code></dt>
- <dd><p>This is the default mode. If the target supports evaluating breakpoint
- conditions on its end, <small>GDB</small> will download breakpoint conditions to
- the target (limitations mentioned previously apply). If the target does
- not support breakpoint condition evaluation, then <small>GDB</small> will fallback
- to evaluating all these conditions on the host’s side.
- </p></dd>
- </dl>
-
-
- <a name="index-negative-breakpoint-numbers"></a>
- <a name="index-internal-GDB-breakpoints"></a>
- <p><small>GDB</small> 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>; ‘<samp>info breakpoints</samp>’ does not display them.
- You can see these breakpoints with the <small>GDB</small> maintenance command
- ‘<samp>maint info breakpoints</samp>’ (see <a href="Maintenance-Commands.html#maint-info-breakpoints">maint info breakpoints</a>).
- </p>
-
- <hr>
- <div class="header">
- <p>
- Next: <a href="Set-Watchpoints.html#Set-Watchpoints" accesskey="n" rel="next">Set Watchpoints</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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