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 field are used to place a symbol into at most
one segment, according to the following masks, defined in 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 field of a symbol is relocated by the linker, as an address within the appropriate segment. 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 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, 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 /* structure 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 /* alternate entry: name,linenumber,address */ #define N_LBRAC 0xc0 /* left bracket: 0,,0,nesting level,address */ #define N_RBRAC 0xe0 /* right 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 conventional use for and the and fields of the given uses the field to hold a type specifier in the form used by the Portable C Compiler, see the header file for details on the format of these type values. The Berkeley Pascal compiler, uses the following value: #define N_PC 0x30 /* global pascal symbol: name,,0,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