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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. In addition, it may bring in more symbols while loading shared libraries.
Symbol files are initially opened by code in symfile.c using the BFD library (see Support Libraries). BFD identifies the type of the file by examining its header. find_sym_fns 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.
Called from symbol_file_add when discarding existing symbols. This function needs 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.
For a more in depth look at what happens during symbol file loading, see How gdb loads symbol files