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Frame recognizers allow for retrieving information about special frames based on - ABI, arguments or other special properties of that frame, even without source - code or debug info. Currently, one use case is to extract function arguments - that would otherwise be unaccesible, or augment existing arguments.

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Adding a custom frame recognizer is done by implementing a Python class - and using the 'frame recognizer add' command. The Python class should have a - 'get_recognized_arguments' method and it will receive an argument of type - lldb.SBFrame representing the current frame that we are trying to recognize. - The method should return a (possibly empty) list of lldb.SBValue objects that - represent the recognized arguments.

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An example of a recognizer that retrieves the file descriptor values from libc - functions 'read', 'write' and 'close' follows:

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  class LibcFdRecognizer(object):
-    def get_recognized_arguments(self, frame):
-      if frame.name in ["read", "write", "close"]:
-        fd = frame.EvaluateExpression("$arg1").unsigned
-        value = lldb.target.CreateValueFromExpression("fd", "(int)%d" % fd)
-        return [value]
-      return []
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The file containing this implementation can be imported via 'command script - import' and then we can register this recognizer with 'frame recognizer add'. - It's important to restrict the recognizer to the libc library (which is - libsystem_kernel.dylib on macOS) to avoid matching functions with the same name in other modules:

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(lldb) command script import .../fd_recognizer.py
-(lldb) frame recognizer add -l fd_recognizer.LibcFdRecognizer -n read -s libsystem_kernel.dylib
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