stab(5)
NAME
stab − symbol table types
SYNTAX
#include <stab.h>
DESCRIPTION
The stab.h 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, and so on) used by the old debugger sdb and the Berkeley Pascal compiler pc(.). 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 a.out(,), an element of the symbol table consists of the following structure:
/*
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 */
char n_other; /* unused */
short n_desc; /* see struct desc, below */
unsigned n_value; /* address or offset or line */
};
The low bits of the n_type field are used to place a symbol into at most one segment, according to the following masks, defined in <a.out.h>. A symbol can be in none of these segments by having none of these segment bits set.
* Simple values for n_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(1) as an address within the appropriate segment. 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 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 segments. Some of these have already been claimed. The old symbolic debugger, sdb, uses the following n_type values:
#define N_GSYM 0x20 /* global symbol: name,,0,type,0 */
#define N_FNAME 0x22 /* procedure name (f77 kludge): name,,0 */
#define N_FUN 0x24 /* procedure: name,,0,linenumber,address */
#define N_STSYM 0x26 /* static symbol: name,,0,type,address */
#define N_LCSYM 0x28 /* .lcomm symbol: name,,0,type,address */
#define N_RSYM 0x40 /* register sym: name,,0,type,register */
#define N_SLINE 0x44 /* src line: 0,,0,linenumber,address */
#define N_SSYM 0x60 /* struct elt: name,,0,type,struct_offset */
#define N_SO 0x64 /* source file name: name,,0,0,address */
#define N_LSYM 0x80 /* local sym: name,,0,type,offset */
#define N_SOL 0x84 /* #included file name: name,,0,0,address */
#define N_PSYM 0xa0 /* parameter: name,,0,type,offset */
#define N_ENTRY 0xa4 /* alt entry: name,linenumber,address */
#define N_LBRAC 0xc0 /* lft bracket: 0,,0,nesting level,address */
#define N_RBRAC 0xe0 /* rt bracket: 0,,0,nesting level,address */
#define N_BCOMM 0xe2 /* begin common: name,, */
#define N_ECOMM 0xe4 /* end common: name,, */
#define N_ECOML 0xe8 /* end common (local name): ,,address */
#define N_LENG 0xfe /* second stab entry with length information */
where 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 uses the n_desc field to hold a type specifier in the form used by the Portable C Compiler, cc(,), in which a base type is qualified in the following structure:
struct desc {
shortq6:2,
q5:2,
q4:2,
q3:2,
q2:2,
q1:2,
basic:4;
};
There are four qualifications, with q1 the most significant and q6 the least significant:
0none
1pointer
2function
3array
The sixteen basic types are assigned as follows:
0undefined
1function argument
2character
3short
4int
5long
6float
7double
8structure
9union
10enumeration
11member of enumeration
12unsigned character
13unsigned short
14unsigned int
15unsigned long
The Berkeley Pascal compiler, pc(,), uses the following n_type value:
#defineN_PC0x30/* global pascal symbol: name,,0,subtype,line */
and uses the following subtypes to do type checking across separately compiled files:
1source file name
2included file name
3global label
4global constant
5global type
6global variable
7global function
8global procedure
9external function
10external procedure
11library variable
12library routine
RESTRICTIONS
The sdb assumes that a symbol of type N_GSYM with name name is located at address _name.