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Manual page for STAB(5)

stab - symbol table types

SYNOPSIS

#include <stab.h>

DESCRIPTION

Stab.h defines some values of the n_type field of the symbol table of Mach-O (Mach object) files. These are the types for permanent symbols (i.e., not local labels, etc.) used by the symbolic debugger gdb (when the -g option of the compiler cc.1 is used). 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. As described in Mach-O.5 an element of the symbol table consists of the following structure:

/*
* Format of a symbol table entry.
*/
struct nlist {
	union {
		char	*n_name;	/* for use when in-core */
		long	n_strx;	/* index into file string table */
	} n_un;
	unsigned char	n_type;	/* type flag, see below */
	char	n_sect;	/* section number or NO_SECT */
	short	n_desc;	/* see below for specific n_type values */
	unsigned	n_value;	/* value of this symbol (or stab offset) */
};

The value of the n_sect field is used to place a symbol into at most one section. The value of the n_sect field is an ordinal refering to the section the symbol is in. The sections are numbered from 1 and refer to sections in the order their section structures appear in the headers of the file that contains the symbol table entry. This means the same ordinal may very well refer to different sections in different files. A symbol can be in none of these sections by having an n_sect value of NO_SECT.

The n_value field of a symbol is relocated by the linker ld.1 as an address within the appropriate section. N_value fields of symbols not in any section are unchanged by the linker. In addition, the linker will discard certain symbols, according to rules of its own, unless the n_type field has one of the following bits set:

/*
* Other permanent symbol table entries have some of the N_STAB bits set.
* These are given in <stab.h>
*/
#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 sections. Some of these have already been claimed. The symbolic debugger, gdb, uses the following n_type values:

#define	N_GSYM	0x20	/* global symbol: name,,NO_SECT,type,0 */
#define	N_FNAME	0x22	/* procedure name (f77 kludge): name,,NO_SECT,0,0 */
#define	N_FUN	0x24	/* procedure: name,,n_sect,linenumber,address */
#define	N_STSYM	0x26	/* static symbol: name,,n_sect,type,address */
#define	N_LCSYM	0x28	/* .lcomm symbol: name,,n_sect,type,address */
#define	N_RSYM	0x40	/* register sym: name,,NO_SECT,type,register */
#define	N_SLINE	0x44	/* src line: 0,,n_sect,linenumber,address */
#define	N_SSYM	0x60	/* structure elt: name,,NO_SECT,type,struct_offset */
#define	N_SO	0x64	/* source file name: name,,n_sect,0,address */
#define	N_LSYM	0x80	/* local sym: name,,NO_SECT,type,offset */
#define	N_SOL	0x84	/* #included file name: name,,n_sect,0,address */
#define	N_PSYM	0xa0	/* parameter: name,,NO_SECT,type,offset */
#define	N_ENTRY	0xa4	/* alternate entry: name,,n_sect,linenumber,address */
#define	N_LBRAC	0xc0	/* left bracket: 0,,NO_SECT,nesting level,address */
#define	N_RBRAC	0xe0	/* right bracket: 0,,NO_SECT,nesting level,address */
#define	N_BCOMM	0xe2	/* begin common: name,,NO_SECT,0,0 */
#define	N_ECOMM	0xe4	/* end common: name,,n_sect,0,0 */
#define	N_ECOML	0xe8	/* end common (local name): 0,,n_sect,0,address */
#define	N_LENG	0xfe	/* second stab entry with length information */

where the comments give the conventional use for:

	.stabs "n_name", n_type, n_sect, n_desc, n_value

where n_type is the defined constant and not listed in the comment. N_sect is the section ordinal the entry is refering to. Gdb.1 uses the n_desc field to hold a type specifier in the form used by the Portable C Compiler cc.1 see the header file pcc.h for details on the format of these type values.

stab - symbol table types

SYNOPSIS

DESCRIPTION

SEE ALSO