GDB Internals - Symbol Handling

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# Symbol Handling

Symbols are a key part of GDB's operation. Symbols include variables, functions, and types.

## Symbol Reading

GDB reads symbols from "symbol files". The usual symbol file is the file containing the program which GDB is debugging. GDB can be directed to use a different file for symbols (with the `symbol-file` command), and it can also read more symbols via the "add-file" and "load" commands, or while reading symbols from shared libraries.

Symbol files are initially opened by code in ``symfile.c'` using the BFD library. BFD identifies the type of the file by examining its header. `symfile_init` then uses this identification to locate a set of symbol-reading functions.

Symbol reading modules identify themselves to GDB by calling `add_symtab_fns` during their module initialization. The argument to `add_symtab_fns` is a `struct sym_fns` which contains the name (or name prefix) of the symbol format, the length of the prefix, and pointers to four functions. These functions are called at various times to process symbol-files whose identification matches the specified prefix.

The functions supplied by each module are:

`xyz_symfile_init(struct sym_fns *sf)`
  Called from `symbol_file_add` when we are about to read a new symbol file. This function should clean up any internal state (possibly resulting from half-read previous files, for example) and prepare to read a new symbol file. Note that the symbol file which we are reading might be a new "main" symbol file, or might be a secondary symbol file whose symbols are being added to the existing symbol table. The argument to `xyz_symfile_init` is a newly allocated `struct sym_fns` whose `bfd` field contains the BFD for the new symbol file being read. Its `private` field has been zeroed, and can be modified as desired. Typically, a struct of private information will be `malloc`'d, and a pointer to it will be placed in the `private` field. There is no result from `xyz_symfile_init`, but it can call `error` if it detects an unavoidable problem.
`xyz_new_init()`
  Called from `symbol_file_add` when discarding existing symbols. This function need only handle the symbol-reading module's internal state; the symbol table data structures visible to the rest of GDB will be discarded by `symbol_file_add`. It has no arguments and no result. It may be called after `xyz_symfile_init`, if a new symbol table is being read, or may be called alone if all symbols are simply being discarded.
`xyz_symfile_read(struct sym_fns *sf, CORE_ADDR addr, int mainline)`
  Called from `symbol_file_add` to actually read the symbols from a symbol-file into a set of psymtabs or symtabs. `sf` points to the struct sym_fns originally passed to `xyz_sym_init` for possible initialization. `addr` is the offset between the file's specified start address and its true address in memory. `mainline` is 1 if this is the main symbol table being read, and 0 if a secondary symbol file (e.g. shared library or dynamically loaded file) is being read.

In addition, if a symbol-reading module creates psymtabs when xyz_symfile_read is called, these psymtabs will contain a pointer to a function `xyz_psymtab_to_symtab`, which can be called from any point in the GDB symbol-handling code.

`xyz_psymtab_to_symtab (struct partial_symtab *pst)`
  Called from `psymtab_to_symtab` (or the PSYMTAB_TO_SYMTAB macro) if the psymtab has not already been read in and had its `pst->symtab` pointer set. The argument is the psymtab to be fleshed-out into a symtab. Upon return, pst->readin should have been set to 1, and pst->symtab should contain a pointer to the new corresponding symtab, or zero if there were no symbols in that part of the symbol file.

## Partial Symbol Tables

GDB has three types of symbol tables.

- full symbol tables (symtabs). These contain the main information about symbols and addresses.
- partial symbol tables (psymtabs). These contain enough information to know when to read the corresponding part of the full symbol table.
- minimal symbol tables (msymtabs). These contain information gleaned from non-debugging symbols.

This section describes partial symbol tables.

A psymtab is constructed by doing a very quick pass over an executable file's debugging information. Small amounts of information are extracted -- enough to identify which parts of the symbol table will need to be re-read and fully digested later, when the user needs the information. The speed of this pass causes GDB to start up very quickly. Later, as the detailed rereading occurs, it occurs in small pieces, at various times, and the delay therefrom is mostly invisible to the user.

The symbols that show up in a file's psymtab should be, roughly, those visible to the debugger's user when the program is not running code from that file. These include external symbols and types, static symbols and types, and enum values declared at file scope.

The psymtab also contains the range of instruction addresses that the full symbol table would represent.

The idea is that there are only two ways for the user (or much of the code in the debugger) to reference a symbol:

- by its address (e.g. execution stops at some address which is inside a function in this file). The address will be noticed to be in the range of this psymtab, and the full symtab will be read in. `find_pc_function`, `find_pc_line`, and other `find_pc_...` functions handle this.
- by its name (e.g. the user asks to print a variable, or set a breakpoint on a function). Global names and file-scope names will be found in the psymtab, which will cause the symtab to be pulled in. Local names will have to be qualified by a global name, or a file-scope name, in which case we will have already read in the symtab as we evaluated the qualifier. Or, a local symbol can be referenced when we are "in" a local scope, in which case the first case applies. `lookup_symbol` does most of the work here.

The only reason that psymtabs exist is to cause a symtab to be read in at the right moment. Any symbol that can be elided from a psymtab, while still causing that to happen, should not appear in it. Since psymtabs don't have the idea of scope, you can't put local symbols in them anyway. Psymtabs don't have the idea of the type of a symbol, either, so types need not appear, unless they will be referenced by name.

It is a bug for GDB to behave one way when only a psymtab has been read, and another way if the corresponding symtab has been read in. Such bugs are typically caused by a psymtab that does not contain all the visible symbols, or which has the wrong instruction address ranges.

The psymtab for a particular section of a symbol-file (objfile) could be thrown away after the symtab has been read in. The symtab should always be searched before the psymtab, so the psymtab will never be used (in a bug-free environment). Currently, psymtabs are allocated on an obstack, and all the psymbols themselves are allocated in a pair of large arrays on an obstack, so there is little to be gained by trying to free them unless you want to do a lot more work.

## Types

Fundamental Types (e.g., FT_VOID, FT_BOOLEAN).

These are the fundamental types that GDB uses internally. Fundamental types from the various debugging formats (stabs, ELF, etc) are mapped into one of these. They are basically a union of all fundamental types that gdb knows about for all the languages that GDB knows about.

Type Codes (e.g., TYPE_CODE_PTR, TYPE_CODE_ARRAY).

Each time GDB builds an internal type, it marks it with one of these types. The type may be a fundamental type, such as TYPE_CODE_INT, or a derived type, such as TYPE_CODE_PTR which is a pointer to another type. Typically, several FT_* types map to one TYPE_CODE_* type, and are distinguished by other members of the type struct, such as whether the type is signed or unsigned, and how many bits it uses.

Builtin Types (e.g., builtin_type_void, builtin_type_char).

These are instances of type structs that roughly correspond to fundamental types and are created as global types for GDB to use for various ugly historical reasons. We eventually want to eliminate these. Note for example that builtin_type_int initialized in gdbtypes.c is basically the same as a TYPE_CODE_INT type that is initialized in c-lang.c for an FT_INTEGER fundamental type. The difference is that the builtin_type is not associated with any particular objfile, and only one instance exists, while c-lang.c builds as many TYPE_CODE_INT types as needed, with each one associated with some particular objfile.

## Object File Formats

### a.out

The ``a.out'` format is the original file format for Unix. It consists of three sections: text, data, and bss, which are for program code, initialized data, and uninitialized data, respectively.

The ``a.out'` format is so simple that it doesn't have any reserved place for debugging information. (Hey, the original Unix hackers used ``adb'`, which is a machine-language debugger.) The only debugging format for ``a.out'` is stabs, which is encoded as a set of normal symbols with distinctive attributes.

The basic ``a.out'` reader is in ``dbxread.c'`.

### COFF

The COFF format was introduced with System V Release 3 (SVR3) Unix. COFF files may have multiple sections, each prefixed by a header. The number of sections is limited.

The COFF specification includes support for debugging. Although this was a step forward, the debugging information was woefully limited. For instance, it was not possible to represent code that came from an included file.

The COFF reader is in ``coffread.c'`.

### ECOFF

ECOFF is an extended COFF originally introduced for Mips and Alpha workstations.

The basic ECOFF reader is in ``mipsread.c'`.

### XCOFF

The IBM RS/6000 running AIX uses an object file format called XCOFF. The COFF sections, symbols, and line numbers are used, but debugging symbols are dbx-style stabs whose strings are located in the ``.debug'` section (rather than the string table). For more information, see See section `Top' in The Stabs Debugging Format.

The shared library scheme has a clean interface for figuring out what shared libraries are in use, but the catch is that everything which refers to addresses (symbol tables and breakpoints at least) needs to be relocated for both shared libraries and the main executable. At least using the standard mechanism this can only be done once the program has been run (or the core file has been read).

### PE

Windows 95 and NT use the PE (Portable Executable) format for their executables. PE is basically COFF with additional headers.

While BFD includes special PE support, GDB needs only the basic COFF reader.

### ELF

The ELF format came with System V Release 4 (SVR4) Unix. ELF is similar to COFF in being organized into a number of sections, but it removes many of COFF's limitations.

The basic ELF reader is in ``elfread.c'`.

### SOM

SOM is HP's object file and debug format (not to be confused with IBM's SOM, which is a cross-language ABI).

The SOM reader is in ``hpread.c'`.

### Other File Formats

Other file formats that have been supported by GDB include Netware Loadable Modules (``nlmread.c'`.

## Debugging File Formats

This section describes characteristics of debugging information that are independent of the object file format.

### stabs

`stabs` started out as special symbols within the `a.out` format. Since then, it has been encapsulated into other file formats, such as COFF and ELF.

While ``dbxread.c'` does some of the basic stab processing, including for encapsulated versions, ``stabsread.c'` does the real work.

### COFF

The basic COFF definition includes debugging information. The level of support is minimal and non-extensible, and is not often used.

### Mips debug (Third Eye)

ECOFF includes a definition of a special debug format.

The file ``mdebugread.c'` implements reading for this format.

### DWARF 1

DWARF 1 is a debugging format that was originally designed to be used with ELF in SVR4 systems.

The DWARF 1 reader is in ``dwarfread.c'`.

### DWARF 2

DWARF 2 is an improved but incompatible version of DWARF 1.

The DWARF 2 reader is in ``dwarf2read.c'`.

### SOM

Like COFF, the SOM definition includes debugging information.

## Adding a New Symbol Reader to GDB

If you are using an existing object file format (a.out, COFF, ELF, etc), there is probably little to be done.

If you need to add a new object file format, you must first add it to BFD. This is beyond the scope of this document.

You must then arrange for the BFD code to provide access to the debugging symbols. Generally GDB will have to call swapping routines from BFD and a few other BFD internal routines to locate the debugging information. As much as possible, GDB should not depend on the BFD internal data structures.

For some targets (e.g., COFF), there is a special transfer vector used to call swapping routines, since the external data structures on various platforms have different sizes and layouts. Specialized routines that will only ever be implemented by one object file format may be called directly. This interface should be described in a file ``bfd/libxyz.h'`, which is included by GDB.

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