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stab - Symbol table types  


#include <stab.h>  


The stab.h header file defines some values of the n_type field of the symbol table of a.out files. These are the types for permanent symbols (that is, not local labels, etc.) used by the old debugger sdb. Symbol table entries can be produced by the .stabs assembler directive. This allows one to specify a double-quote delimited name, a symbol type, one char and one short of information about the symbol, and an unsigned long (usually an address). To avoid having to produce an explicit label for the address field, the .stabd directive can be used to implicitly address the current location. If no name is needed, symbol table entries can be generated using the .stabn directive. The loader promises to preserve the order of symbol table entries produced by .stab directives. An element of the symbol table consists of the following structure:

/* SYMBOL INFORMATION ENTRY * This is used for defined symbols, imports and stabs. The type (kind) * of the associated symbols load command determines which. typedef struct symbol_info_t {        union   {mo_offset_t    symbol_name;
                mo_ptr_t       symbol_nameP;

      } si_name;
       mo_short_t              si_package_index;
       mo_short_t              si_type;
       mo_short_t              si_flags;
       mo_byte_t               si_reserved byte;
       mo_byte_t               si_sc_type;
       union   {mo_addr_t      def_val;        /* defined section, offset */
                mo_long_t      imp_val;        /* index in import list */
                mo_long_t      lit_val;        /* literal value */
                mo_vm_addr_t   abs_val;        /* absolute value */

      } si_value; } symbol_info_t;

The low bits of the si_sc_type field are used to place a symbol into at most one segment, according to the following masks. A symbol can be in none of these segments by having none of these segment bits set.

* Simple values for si_sc_type. */ #define N_UNDF  0x0     /* undefined */
#define        N_ABS   0x2     /* absolute */
#define        N_TEXT  0x4     /* text */
#define        N_DATA  0x6     /* data */
#define        N_BSS   0x8     /* bss */
#define       N_EXT   01      /* external bit, or'ed in */

The n_value field of a symbol is relocated by the linker, ld, as an address within the appropriate segment. The n_value fields of symbols not in any segment are unchanged by the linker. In addition, the linker will discard certain symbols, according to rules of its own, unless the si_sc_type field has one of the following bits set:

#define N_STAB        0xe0    /* if any of these bits set, don't discard */

This allows up to 112 (7 * 16) symbol types, split between the various segments. Some of these have already been claimed. The old symbolic debugger, sdb, uses the following n_type values: 0x20  /* global symbol: name,,0,type,0 */
0x22   /* procedurename(f77 kludge): name,,0 */
0x24   /* procedure: name,,0,linenumber,address */
0x26   /* static symbol: name,,0,type,address */
0x28   /* .lcomm symbol: name,,0,type,address */
0x40   /* register sym: name,,0,type,register */
0x44   /* src line: 0,,0,linenumber,address */
0x60   /* structure elt: name,,0,type,struct_offset */
0x64   /* sourcefilename: name,,0,0,address */
0x80   /* local sym: name,,0,type,offset */
0x84   /* #includedfilename: name,,0,0,address */
0xa0   /* parameter: name,,0,type,offset */
0xa4   /* alternate entry: name,,linenumber,address */
0xc0   /* left bracket: 0,,0,nestinglevel,address */
0xe0   /* right bracket: 0,,0,nestinglevel,address */
0xe2   /* begincommon: name,, */
0xe4   /* endcommon: name,, */
0xe8   /* endcommon(local name): ,,address */
0xfe   /* secondstabentrywithlengthinformation */

The comments give sdb conventional use for .stabs and the n_name, n_other, n_desc, and n_value fields of the given n_type. The sdb debugger uses the n_desc field to hold a type specifier in the form used by the Portable C Compiler, cc.

The Berkeley Pascal compiler, pc, uses the following si_sc_type value:

#define N_PC 0x30 /* global pascal symbol: name,,0,subtype,line */

and uses the following subtypes to do type checking across separately compiled files: Source filename Included filename Global label Global constant Global type Global variable Global function Global procedure External function External procedure Library variable "12" Library routine  


Commands: as(1), ld(1) delim off




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Time: 02:40:05 GMT, October 02, 2010