NAME

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 ∗/
longn_strx;/∗ index into file string table ∗/
} n_un;
unsigned charn_type; /∗ type flag, see below ∗/
charn_sect;/∗ section number or NO_SECT ∗/
shortn_desc;/∗ see below for specific n_type values ∗/
unsignedn_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>
∗/

#defineN_STAB0xe0 /∗ 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:

#defineN_GSYM0x20/∗ global symbol: name,,NO_SECT,type,0 ∗/
#defineN_FNAME0x22/∗ procedure name (f77 kludge): name,,NO_SECT,0,0 ∗/
#defineN_FUN0x24/∗ procedure: name,,n_sect,linenumber,address ∗/
#defineN_STSYM0x26/∗ static symbol: name,,n_sect,type,address ∗/
#defineN_LCSYM0x28/∗ .lcomm symbol: name,,n_sect,type,address ∗/
#defineN_RSYM0x40/∗ register sym: name,,NO_SECT,type,register ∗/
#defineN_SLINE0x44/∗ src line: 0,,n_sect,linenumber,address ∗/
#defineN_SSYM0x60/∗ structure elt: name,,NO_SECT,type,struct_offset ∗/
#defineN_SO0x64/∗ source file name: name,,n_sect,0,address ∗/
#defineN_LSYM0x80/∗ local sym: name,,NO_SECT,type,offset ∗/
#defineN_SOL0x84/∗ #included file name: name,,n_sect,0,address ∗/
#defineN_PSYM0xa0/∗ parameter: name,,NO_SECT,type,offset ∗/
#defineN_ENTRY0xa4/∗ alternate entry: name,,n_sect,linenumber,address ∗/
#defineN_LBRAC0xc0/∗ left bracket: 0,,NO_SECT,nesting level,address ∗/
#defineN_RBRAC0xe0/∗ right bracket: 0,,NO_SECT,nesting level,address ∗/
#defineN_BCOMM0xe2/∗ begin common: name,,NO_SECT,0,0 ∗/
#defineN_ECOMM0xe4/∗ end common: name,,n_sect,0,0 ∗/
#defineN_ECOML0xe8/∗ end common (local name): 0,,n_sect,0,address ∗/
#defineN_LENG0xfe/∗ 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. 

SEE ALSO

as(1), ld(1), gdb(1), Mach-O(5)

4th Berkeley Distribution  —  March 1, 1989