STAB(5)

💖 STAB(5) — symbol table types 💖

okay bestie, welcome to the File Formats Manual page for STAB(5). This is the whole glow-up of the stab symbol table types, and yes we are keeping every single define. iykyk ✨

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🌸 NAME 🌸

stab — symbol table types

✨ SYNOPSIS ✨

#include <mach-o/stab.h>

💅 DESCRIPTION 💅

The file ⟨stab.h⟩ defines some of the symbol table n_type field values for Mach-O a.out files. These are the types for permanent symbols (i.e. not local labels, etc.) used by the old debugger sdb and the Berkeley Pascal compiler pc(1).

Symbol table entries can be produced by the .stabs assembler directive. This lets you 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 comments in <mach-o/nlist.h>, 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 */
        char            n_sect;         /* section number or NO_SECT */
        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 for the symbol type, according to the following masks, defined in ⟨nlist.h⟩. A symbol can be of none these types by having none of these bits set.

/*
* Simple values for n_type.
*/

#define N_UNDF  0x0     /* undefined */
#define N_ABS   0x2     /* absolute */
#define N_SECT  0xe     /* defined in section number n_sect */

#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 section for N_SECT types based on the. (yes, the original sentence just stops at “the” — not me gasping 😭) 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 symbol types. 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,,NO_SECT,type,0 */
#define N_FNAME 0x22    /* procedure name (f77 kludge): name,,NO_SECT */
#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_BNSYM 0x2e    /* begin nsect sym: 0,,n_sect,0,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_ENSYM 0x4e    /* end nsect sym: 0,,n_sect,0,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_BINCL 0x82    /* include file beginning: name,,NO_SECT,0,sum */
#define N_SOL   0x84    /* #included file name: name,,n_sect,0,address */
#define N_PSYM  0xa0    /* parameter: name,,NO_SECT,type,offset */
#define N_EINCL 0xa2    /* include file end: name,,NO_SECT,0,0 */
#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_EXCL  0xc2    /* deleted include file: name,,NO_SECT,0,sum */
#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 sdb conventional use for .stab s and the n_name, n_sect, n_desc, and n_value fields of the given n_type. Sdb 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.

The Berkeley Pascal compiler, pc(1), uses the following n_type value:

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

and uses the following subtypes to do type checking across separately compiled files:

1. source file name
2. included file name
3. global label
4. global constant
5. global type
6. global variable
7. global function
8. global procedure
9. external function
10. external procedure
11. library variable
12. library routine

🦋 SEE ALSO 🦋

Go hang out with these, bestie:

• as(1)
• ld(1)
• dbx(1)
• Mach-O(5)

😭 BUGS 😭

More basic types are needed.

🎀 HISTORY 🎀

The stab file appeared in 4.0BSD.

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— Darwin, November 16, 2001, bestie 💅✨