Set Breaks (Debugging with GDB)
5.1.1 Setting Breakpoints
Breakpoints are set with the
break command (abbreviated
b). The debugger convenience variable ‘
$bpnum’ records the number of the breakpoint you’ve set most recently; see Convenience Variables, for a discussion of what you can do with convenience variables.
Set a breakpoint at the given
location, which can specify a function name, a line number, or an address of an instruction. (See Specify Location, for a list of all the possible ways to specify a
location.) The breakpoint will stop your program just before it executes any of the code in the specified
When using source languages that permit overloading of symbols, such as C++, a function name may refer to more than one possible place to break. See Ambiguous Expressions, for a discussion of that situation.
When called without any arguments,
breaksets a breakpoint at the next instruction to be executed in the selected stack frame (see Examining the Stack). 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
finishcommand in the frame inside the selected frame—except that
finishdoes not leave an active breakpoint. If you use
breakwithout an argument in the innermost frame, GDB stops the next time it reaches the current location; this may be useful inside loops.
GDB 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.
break … if cond
Set a breakpoint with condition
cond; evaluate the expression
condeach time the breakpoint is reached, and stop only if the value is nonzero—that is, if
condevaluates as true. ‘
…’ stands for one of the possible arguments described above (or no argument) specifying where to break. See Break Conditions, for more information on breakpoint conditions.
The breakpoint may be mapped to multiple locations. If the breakpoint condition
condis invalid at some but not all of the locations, the locations for which the condition is invalid are disabled. For example, GDB reports below that two of the three locations are disabled.
(gdb) break func if a == 10 warning: failed to validate condition at location 0x11ce, disabling: No symbol "a" in current context. warning: failed to validate condition at location 0x11b6, disabling: No symbol "a" in current context. Breakpoint 1 at 0x11b6: func. (3 locations)
Locations that are disabled because of the condition are denoted by an uppercase
Nin the output of the
(gdb) info breakpoints Num Type Disp Enb Address What 1 breakpoint keep y <MULTIPLE> stop only if a == 10 1.1 N* 0x00000000000011b6 in ... 1.2 y 0x00000000000011c2 in ... 1.3 N* 0x00000000000011ce in ... (*): Breakpoint condition is invalid at this location.
If the breakpoint condition
condis invalid in the context of all the locations of the breakpoint, GDB refuses to define the breakpoint. For example, if variable
foois an undefined variable:
(gdb) break func if foo No symbol "foo" in current context.
break … -force-condition if cond
There may be cases where the condition
condis invalid at all the current locations, but the user knows that it will be valid at a future location; for example, because of a library load. In such cases, by using the
-force-conditionkeyword before ‘
if’, GDB can be forced to define the breakpoint with the given condition expression instead of refusing it.
(gdb) break func -force-condition if foo warning: failed to validate condition at location 1, disabling: No symbol "foo" in current context. warning: failed to validate condition at location 2, disabling: No symbol "foo" in current context. warning: failed to validate condition at location 3, disabling: No symbol "foo" in current context. Breakpoint 1 at 0x1158: test.c:18. (3 locations)
This causes all the present locations where the breakpoint would otherwise be inserted, to be disabled, as seen in the example above. However, if there exist locations at which the condition is valid, the
-force-conditionkeyword has no effect.
Set a breakpoint enabled only for one stop. The
argsare the same as for the
breakcommand, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there. See Disabling Breakpoints.
Set a hardware-assisted breakpoint. The
argsare the same as for the
breakcommand 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 GDB will reject this command if more than two are used. Delete or disable unused hardware breakpoints before setting new ones (see Disabling Breakpoints). See Break Conditions. For remote targets, you can restrict the number of hardware breakpoints GDB will use, see set remote hardware-breakpoint-limit.
Set a hardware-assisted breakpoint enabled only for one stop. The
argsare the same as for the
hbreakcommand and the breakpoint is set in the same way. However, like the
tbreakcommand, the breakpoint is automatically deleted after the first time your program stops there. Also, like the
hbreakcommand, the breakpoint requires hardware support and some target hardware may not have this support. See Disabling Breakpoints. See also Break Conditions.
Set breakpoints on all functions matching the regular expression
regex. 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
breakcommand. You can delete them, disable them, or make them conditional the same way as any other breakpoint.
In programs using different languages, GDB chooses the syntax to print the list of all breakpoints it sets according to the ‘
set language’ value: using ‘
set language auto’ (see Set Language Automatically) means to use the language of the breakpoint’s function, other values mean to use the manually specified language (see Set Language Manually).
The syntax of the regular expression is the standard one used with tools like
grep. Note that this is different from the syntax used by shells, so for instance
foo*matches all functions that include an
fofollowed by zero or more
os. There is an implicit
.*leading and trailing the regular expression you supply, so to match only functions that begin with
When debugging C++ programs,
rbreakis useful for setting breakpoints on overloaded functions that are not members of any special classes.
rbreakcommand can be used to set breakpoints in all the functions in a program, like this:
(gdb) rbreak .
rbreakis called with a filename qualification, it limits the search for functions matching the given regular expression to the specified
file. This can be used, for example, to set breakpoints on every function in a given file:
(gdb) rbreak file.c:.
The colon separating the filename qualifier from the regex may optionally be surrounded by spaces.
info breakpoints [list…]
info break [list…]
Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted. Optional argument
nmeans print information only about the specified breakpoint(s) (or watchpoint(s) or catchpoint(s)). For each breakpoint, following columns are printed:
- Breakpoint Numbers
Breakpoint, watchpoint, or catchpoint.
Whether the breakpoint is marked to be disabled or deleted when hit.
- Enabled or Disabled
Enabled breakpoints are marked with ‘
n’ marks breakpoints that are not enabled.
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 ‘
<PENDING>’. 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 ‘
<MULTIPLE>’ in this field—see below for details.
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.
If a breakpoint is conditional, there are two evaluation modes: “host” and “target”. If mode is “host”, breakpoint condition evaluation is done by GDB on the host’s side. If it is “target”, then the condition is evaluated by the target. The
info breakcommand shows the condition on the line following the affected breakpoint, together with its condition evaluation mode in between parentheses.
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.
info breakwith a breakpoint number
nas argument lists only that breakpoint. The convenience variable
$_and the default examining-address for the
xcommand are set to the address of the last breakpoint listed (see Examining Memory).
info breakdisplays a count of the number of times the breakpoint has been hit. This is especially useful in conjunction with the
ignorecommand. 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.
For a breakpoints with an enable count (xref) greater than 1,
info breakalso displays that count.
- Breakpoint Numbers
GDB 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 Break Conditions).
It is possible that a breakpoint corresponds to several locations in your program. Examples of this situation are:
- Multiple functions in the program may have the same name.
- For a C++ constructor, the GCC compiler generates several instances of the function body, used in different cases.
- For a C++ template function, a given line in the function can correspond to any number of instantiations.
- For an inlined function, a given source line can correspond to several places where that function is inlined.
In all those cases, GDB will insert a breakpoint at all the relevant locations.
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 ‘
<MULTIPLE>’ 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
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
You cannot delete the individual locations from a breakpoint. However, each location can be individually enabled or disabled by passing
location-number as argument to the
disable commands. It’s also possible to
disable a range of
location-number locations using a
breakpoint-number and two
location-numbers, in increasing order, separated by a hyphen, like breakpoint-number.location-number1-location-number2, in which case GDB acts on all the locations in the range (inclusive). Disabling or enabling the parent breakpoint (see Disabling) affects all of the locations that belong to that breakpoint.
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, GDB 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, GDB will ask you if you want to set a so called pending breakpoint—breakpoint whose address is not yet resolved.
After the program is run, whenever a new shared library is loaded, GDB 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.
This logic works for breakpoints with multiple locations, too. For example, if you have a breakpoint in a C++ 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.
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.
GDB provides some additional commands for controlling what happens when the ‘
break’ command cannot resolve breakpoint address specification to an address:
set breakpoint pending auto
- This is the default behavior. When GDB cannot find the breakpoint location, it queries you whether a pending breakpoint should be created.
set breakpoint pending on
- This indicates that an unrecognized breakpoint location should automatically result in a pending breakpoint being created.
set breakpoint pending off
- 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.
show breakpoint pending
- Show the current behavior setting for creating pending breakpoints.
The settings above only affect the
break command and its variants. Once breakpoint is set, it will be automatically updated as shared libraries are loaded and unloaded.
For some targets, GDB 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
break command as well as to internal breakpoints set by commands like
finish. For breakpoints set with
hbreak, GDB will always use hardware breakpoints.
You can control this automatic behaviour with the following commands:
set breakpoint auto-hw on
- This is the default behavior. When GDB sets a breakpoint, it will try to use the target memory map to decide if software or hardware breakpoint must be used.
set breakpoint auto-hw off
- This indicates GDB should not automatically select breakpoint type. If the target provides a memory map, GDB will warn when trying to set software breakpoint at a read-only address.
GDB 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, GDB 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::
set breakpoint always-inserted off
- 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.
set breakpoint always-inserted on
- 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.
GDB 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.
If the target supports evaluating conditions on its end, GDB may download the breakpoint, together with its conditions, to it.
This feature can be controlled via the following commands:
set breakpoint condition-evaluation host
- This option commands GDB 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.
set breakpoint condition-evaluation target
- This option commands GDB 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 GDB 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 GDB.
set breakpoint condition-evaluation auto
- This is the default mode. If the target supports evaluating breakpoint conditions on its end, GDB will download breakpoint conditions to the target (limitations mentioned previously apply). If the target does not support breakpoint condition evaluation, then GDB will fallback to evaluating all these conditions on the host’s side.
GDB itself sometimes sets breakpoints in your program for special purposes, such as proper handling of
longjmp (in C programs). These internal breakpoints are assigned negative numbers, starting with
info breakpoints’ does not display them. You can see these breakpoints with the GDB maintenance command ‘
maint info breakpoints’ (see maint info breakpoints).