Debugging with GDB: Breakpoints In Python

Debugging with GDB: Breakpoints In Python

23.2.2.30 Manipulating breakpoints using Python

Python code can manipulate breakpoints via the gdb.Breakpoint class.

A breakpoint can be created using one of the two forms of the gdb.Breakpoint constructor. The first one accepts a string like one would pass to the break (see Setting Breakpoints) and watch (see Setting Watchpoints) commands, and can be used to create both breakpoints and watchpoints. The second accepts separate Python arguments similar to Explicit Locations, and can only be used to create breakpoints.

Function: Breakpoint.__init__ (spec [, type ][, wp_class ][, internal ][, temporary ][, qualified ])

Create a new breakpoint according to spec, which is a string naming the location of a breakpoint, or an expression that defines a watchpoint. The string should describe a location in a format recognized by the break command (see Setting Breakpoints) or, in the case of a watchpoint, by the watch command (see Setting Watchpoints).

The optional type argument specifies the type of the breakpoint to create, as defined below.

The optional wp_class argument defines the class of watchpoint to create, if type is gdb.BP_WATCHPOINT. If wp_class is omitted, it defaults to gdb.WP_WRITE.

The optional internal argument allows the breakpoint to become invisible to the user. The breakpoint will neither be reported when created, nor will it be listed in the output from info breakpoints (but will be listed with the maint info breakpoints command).

The optional temporary argument makes the breakpoint a temporary breakpoint. Temporary breakpoints are deleted after they have been hit. Any further access to the Python breakpoint after it has been hit will result in a runtime error (as that breakpoint has now been automatically deleted).

The optional qualified argument is a boolean that allows interpreting the function passed in spec as a fully-qualified name. It is equivalent to break’s -qualified flag (see Linespec Locations and Explicit Locations).

Function: Breakpoint.__init__ ([ source ][, function ][, label ][, line ], ][ internal ][, temporary ][, qualified ])

This second form of creating a new breakpoint specifies the explicit location (see Explicit Locations) using keywords. The new breakpoint will be created in the specified source file source, at the specified function, label and line.

internal, temporary and qualified have the same usage as explained previously.

The available types are represented by constants defined in the gdb module:

gdb.BP_BREAKPOINT: Normal code breakpoint.
gdb.BP_WATCHPOINT: Watchpoint breakpoint.
gdb.BP_HARDWARE_WATCHPOINT: Hardware assisted watchpoint.
gdb.BP_READ_WATCHPOINT: Hardware assisted read watchpoint.
gdb.BP_ACCESS_WATCHPOINT: Hardware assisted access watchpoint.

The available watchpoint types represented by constants are defined in the gdb module:

gdb.WP_READ: Read only watchpoint.
gdb.WP_WRITE: Write only watchpoint.
gdb.WP_ACCESS: Read/Write watchpoint.

Function: Breakpoint.stop (self)

The gdb.Breakpoint class can be sub-classed and, in particular, you may choose to implement the stop method. If this method is defined in a sub-class of gdb.Breakpoint, it will be called when the inferior reaches any location of a breakpoint which instantiates that sub-class. If the method returns True, the inferior will be stopped at the location of the breakpoint, otherwise the inferior will continue.

If there are multiple breakpoints at the same location with a stop method, each one will be called regardless of the return status of the previous. This ensures that all stop methods have a chance to execute at that location. In this scenario if one of the methods returns True but the others return False, the inferior will still be stopped.

You should not alter the execution state of the inferior (i.e., step, next, etc.), alter the current frame context (i.e., change the current active frame), or alter, add or delete any breakpoint. As a general rule, you should not alter any data within GDB or the inferior at this time.

Example stop implementation:

class MyBreakpoint (gdb.Breakpoint):
      def stop (self):
        inf_val = gdb.parse_and_eval("foo")
        if inf_val == 3:
          return True
        return False

Function: Breakpoint.is_valid (): Return True if this Breakpoint object is valid, False otherwise. A Breakpoint object can become invalid if the user deletes the breakpoint. In this case, the object still exists, but the underlying breakpoint does not. In the cases of watchpoint scope, the watchpoint remains valid even if execution of the inferior leaves the scope of that watchpoint.

Function: Breakpoint.delete (): Permanently deletes the GDB breakpoint. This also invalidates the Python Breakpoint object. Any further access to this object’s attributes or methods will raise an error.

Variable: Breakpoint.enabled: This attribute is True if the breakpoint is enabled, and False otherwise. This attribute is writable. You can use it to enable or disable the breakpoint.

Variable: Breakpoint.silent

This attribute is True if the breakpoint is silent, and False otherwise. This attribute is writable.

Note that a breakpoint can also be silent if it has commands and the first command is silent. This is not reported by the silent attribute.

Variable: Breakpoint.pending: This attribute is True if the breakpoint is pending, and False otherwise. See Set Breaks. This attribute is read-only.

Variable: Breakpoint.thread: If the breakpoint is thread-specific, this attribute holds the thread’s global id. If the breakpoint is not thread-specific, this attribute is None. This attribute is writable.

Variable: Breakpoint.task: If the breakpoint is Ada task-specific, this attribute holds the Ada task id. If the breakpoint is not task-specific (or the underlying language is not Ada), this attribute is None. This attribute is writable.

Variable: Breakpoint.ignore_count: This attribute holds the ignore count for the breakpoint, an integer. This attribute is writable.

Variable: Breakpoint.number: This attribute holds the breakpoint’s number — the identifier used by the user to manipulate the breakpoint. This attribute is not writable.

Variable: Breakpoint.type: This attribute holds the breakpoint’s type — the identifier used to determine the actual breakpoint type or use-case. This attribute is not writable.

Variable: Breakpoint.visible: This attribute tells whether the breakpoint is visible to the user when set, or when the ‘info breakpoints’ command is run. This attribute is not writable.

Variable: Breakpoint.temporary: This attribute indicates whether the breakpoint was created as a temporary breakpoint. Temporary breakpoints are automatically deleted after that breakpoint has been hit. Access to this attribute, and all other attributes and functions other than the is_valid function, will result in an error after the breakpoint has been hit (as it has been automatically deleted). This attribute is not writable.

Variable: Breakpoint.hit_count: This attribute holds the hit count for the breakpoint, an integer. This attribute is writable, but currently it can only be set to zero.

Variable: Breakpoint.location: This attribute holds the location of the breakpoint, as specified by the user. It is a string. If the breakpoint does not have a location (that is, it is a watchpoint) the attribute’s value is None. This attribute is not writable.

Variable: Breakpoint.expression: This attribute holds a breakpoint expression, as specified by the user. It is a string. If the breakpoint does not have an expression (the breakpoint is not a watchpoint) the attribute’s value is None. This attribute is not writable.

Variable: Breakpoint.condition: This attribute holds the condition of the breakpoint, as specified by the user. It is a string. If there is no condition, this attribute’s value is None. This attribute is writable.

Variable: Breakpoint.commands: This attribute holds the commands attached to the breakpoint. If there are commands, this attribute’s value is a string holding all the commands, separated by newlines. If there are no commands, this attribute is None. This attribute is not writable.