Symbols In Python (Debugging with GDB)

23.2.2.27 Python representation of Symbols

GDB represents every variable, function and type as an entry in a symbol table. See Examining the Symbol Table. Similarly, Python represents these symbols in GDB with the gdb.Symbol object.

The following symbol-related functions are available in the gdb module:

Function: gdb.lookup_symbol (name [, block [, domain]])

This function searches for a symbol by name. The search scope can be restricted to the parameters defined in the optional domain and block arguments.

name is the name of the symbol. It must be a string. The optional block argument restricts the search to symbols visible in that block. The block argument must be a gdb.Block object. If omitted, the block for the current frame is used. The optional domain argument restricts the search to the domain type. The domain argument must be a domain constant defined in the gdb module and described later in this chapter.

The result is a tuple of two elements. The first element is a gdb.Symbol object or None if the symbol is not found. If the symbol is found, the second element is True if the symbol is a field of a method’s object (e.g., this in C++), otherwise it is False. If the symbol is not found, the second element is False.

Function: gdb.lookup_global_symbol (name [, domain])

This function searches for a global symbol by name. The search scope can be restricted to by the domain argument.

name is the name of the symbol. It must be a string. The optional domain argument restricts the search to the domain type. The domain argument must be a domain constant defined in the gdb module and described later in this chapter.

The result is a gdb.Symbol object or None if the symbol is not found.

Function: gdb.lookup_static_symbol (name [, domain])

This function searches for a global symbol with static linkage by name. The search scope can be restricted to by the domain argument.

The result is a gdb.Symbol object or None if the symbol is not found.

Note that this function will not find function-scoped static variables. To look up such variables, iterate over the variables of the function’s gdb.Block and check that block.addr_class is gdb.SYMBOL_LOC_STATIC.

There can be multiple global symbols with static linkage with the same name. This function will only return the first matching symbol that it finds. Which symbol is found depends on where GDB is currently stopped, as GDB will first search for matching symbols in the current object file, and then search all other object files. If the application is not yet running then GDB will search all object files in the order they appear in the debug information.

Function: gdb.lookup_static_symbols (name [, domain])

Similar to gdb.lookup_static_symbol, this function searches for global symbols with static linkage by name, and optionally restricted by the domain argument. However, this function returns a list of all matching symbols found, not just the first one.

The result is a list of gdb.Symbol objects which could be empty if no matching symbols were found.

A gdb.Symbol object has the following attributes:

Variable: Symbol.type: The type of the symbol or None if no type is recorded. This attribute is represented as a gdb.Type object. See Types In Python. This attribute is not writable.

Variable: Symbol.symtab: The symbol table in which the symbol appears. This attribute is represented as a gdb.Symtab object. See Symbol Tables In Python. This attribute is not writable.

Variable: Symbol.line: The line number in the source code at which the symbol was defined. This is an integer.

Variable: Symbol.name: The name of the symbol as a string. This attribute is not writable.

Variable: Symbol.linkage_name: The name of the symbol, as used by the linker (i.e., may be mangled). This attribute is not writable.

Variable: Symbol.print_name: The name of the symbol in a form suitable for output. This is either name or linkage_name, depending on whether the user asked GDB to display demangled or mangled names.

Variable: Symbol.addr_class: The address class of the symbol. This classifies how to find the value of a symbol. Each address class is a constant defined in the gdb module and described later in this chapter.

Variable: Symbol.needs_frame: This is True if evaluating this symbol’s value requires a frame (see Frames In Python) and False otherwise. Typically, local variables will require a frame, but other symbols will not.

Variable: Symbol.is_argument: True if the symbol is an argument of a function.

Variable: Symbol.is_constant: True if the symbol is a constant.

Variable: Symbol.is_function: True if the symbol is a function or a method.

Variable: Symbol.is_variable: True if the symbol is a variable.

A gdb.Symbol object has the following methods:

Function: Symbol.is_valid (): Returns True if the gdb.Symbol object is valid, False if not. A gdb.Symbol object can become invalid if the symbol it refers to does not exist in GDB any longer. All other gdb.Symbol methods will throw an exception if it is invalid at the time the method is called.

Function: Symbol.value ([frame]): Compute the value of the symbol, as a gdb.Value. For functions, this computes the address of the function, cast to the appropriate type. If the symbol requires a frame in order to compute its value, then frame must be given. If frame is not given, or if frame is invalid, then this method will throw an exception.

The available domain categories in gdb.Symbol are represented as constants in the gdb module:

gdb.SYMBOL_UNDEF_DOMAIN: This is used when a domain has not been discovered or none of the following domains apply. This usually indicates an error either in the symbol information or in GDB’s handling of symbols.
gdb.SYMBOL_VAR_DOMAIN: This domain contains variables, function names, typedef names and enum type values.
gdb.SYMBOL_STRUCT_DOMAIN: This domain holds struct, union and enum type names.
gdb.SYMBOL_LABEL_DOMAIN: This domain contains names of labels (for gotos).
gdb.SYMBOL_MODULE_DOMAIN: This domain contains names of Fortran module types.
gdb.SYMBOL_COMMON_BLOCK_DOMAIN: This domain contains names of Fortran common blocks.

The available address class categories in gdb.Symbol are represented as constants in the gdb module:

gdb.SYMBOL_LOC_UNDEF: If this is returned by address class, it indicates an error either in the symbol information or in GDB’s handling of symbols.
gdb.SYMBOL_LOC_CONST: Value is constant int.
gdb.SYMBOL_LOC_STATIC: Value is at a fixed address.
gdb.SYMBOL_LOC_REGISTER: Value is in a register.
gdb.SYMBOL_LOC_ARG: Value is an argument. This value is at the offset stored within the symbol inside the frame’s argument list.
gdb.SYMBOL_LOC_REF_ARG: Value address is stored in the frame’s argument list. Just like LOC_ARG except that the value’s address is stored at the offset, not the value itself.
gdb.SYMBOL_LOC_REGPARM_ADDR: Value is a specified register. Just like LOC_REGISTER except the register holds the address of the argument instead of the argument itself.
gdb.SYMBOL_LOC_LOCAL: Value is a local variable.
gdb.SYMBOL_LOC_TYPEDEF: Value not used. Symbols in the domain SYMBOL_STRUCT_DOMAIN all have this class.
gdb.SYMBOL_LOC_BLOCK: Value is a block.
gdb.SYMBOL_LOC_CONST_BYTES: Value is a byte-sequence.
gdb.SYMBOL_LOC_UNRESOLVED: Value is at a fixed address, but the address of the variable has to be determined from the minimal symbol table whenever the variable is referenced.
gdb.SYMBOL_LOC_OPTIMIZED_OUT: The value does not actually exist in the program.
gdb.SYMBOL_LOC_COMPUTED: The value’s address is a computed location.
gdb.SYMBOL_LOC_COMPUTED: The value’s address is a symbol. This is only used for Fortran common blocks.