/*
* 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.