#include <mach-o/nlist.h>
mach-o/nlist.h
$NetBSD: nlist.h,v 1.5 1994/10/26 00:56:11 cgd Exp $
structnlist
Format of a symbol table entry of a Mach-O file for 32-bit architectures.
Modified from the BSD format. The modifications from the original format
were changing n_other (an unused field) to n_sect and the addition of the
N_SECT type. These modifications are required to support symbols in a larger
number of sections not just the three sections (text, data and bss) in a BSD
file.
| unnamed union at mach-o/nlist.h:77:2 | n_un | ||||
| |||||
| uint8_t | n_type | type flag, see below | |||
| uint8_t | n_sect | section number or NO_SECT | |||
| int16_t | n_desc | see <mach-o/stab.h> | |||
| uint32_t | n_value | value of this symbol (or stab offset) | |||
structnlist_64
This is the symbol table entry structure for 64-bit architectures.
| unnamed union at mach-o/nlist.h:93:5 | n_un | ||||
| |||||
| uint8_t | n_type | type flag, see below | |||
| uint8_t | n_sect | section number or NO_SECT | |||
| uint16_t | n_desc | see <mach-o/stab.h> | |||
| uint64_t | n_value | value of this symbol (or stab offset) | |||
macroN_STAB
The n_type field really contains four fields:
unsigned char N_STAB:3,
N_PEXT:1,
N_TYPE:3,
N_EXT:1;
which are used via the following masks.
#define N_STAB 0xe0
if any of these bits set, a symbolic debugging entry
macroN_UNDF
Values for N_TYPE bits of the n_type field.
#define N_UNDF 0x0
undefined, n_sect == NO_SECT
macroNO_SECT
If the type is N_SECT then the n_sect field contains an ordinal of the
section the symbol is defined in. The sections are numbered from 1 and
refer to sections in order they appear in the load commands for the file
they are in. This means the same ordinal may very well refer to different
sections in different files.
The n_value field for all symbol table entries (including N_STAB's) gets
updated by the link editor based on the value of it's n_sect field and where
the section n_sect references gets relocated. If the value of the n_sect
field is NO_SECT then it's n_value field is not changed by the link editor.
#define NO_SECT 0
symbol is not in any section
macroGET_COMM_ALIGN
Common symbols are represented by undefined (N_UNDF) external (N_EXT) types
who's values (n_value) are non-zero. In which case the value of the n_value
field is the size (in bytes) of the common symbol. The n_sect field is set
to NO_SECT. The alignment of a common symbol may be set as a power of 2
between 2^1 and 2^15 as part of the n_desc field using the macros below. If
the alignment is not set (a value of zero) then natural alignment based on
the size is used.
#define GET_COMM_ALIGN(n_desc) (((n_desc) >> 8) & 0x0f)
macroSET_COMM_ALIGN
#define SET_COMM_ALIGN(n_desc, align) (n_desc) = (((n_desc) & 0xf0ff) | (((align) & 0x0f) << 8))
macroREFERENCE_TYPE
To support the lazy binding of undefined symbols in the dynamic link-editor,
the undefined symbols in the symbol table (the nlist structures) are marked
with the indication if the undefined reference is a lazy reference or
non-lazy reference. If both a non-lazy reference and a lazy reference is
made to the same symbol the non-lazy reference takes precedence. A reference
is lazy only when all references to that symbol are made through a symbol
pointer in a lazy symbol pointer section.
The implementation of marking nlist structures in the symbol table for
undefined symbols will be to use some of the bits of the n_desc field as a
reference type. The mask REFERENCE_TYPE will be applied to the n_desc field
of an nlist structure for an undefined symbol to determine the type of
undefined reference (lazy or non-lazy).
The constants for the REFERENCE FLAGS are propagated to the reference table
in a shared library file. In that case the constant for a defined symbol,
REFERENCE_FLAG_DEFINED, is also used.
Reference type bits of the n_desc field of undefined symbols
#define REFERENCE_TYPE 0x7
macroREFERENCE_FLAG_UNDEFINED_NON_LAZY
types of references
#define REFERENCE_FLAG_UNDEFINED_NON_LAZY 0
macroREFERENCE_FLAG_UNDEFINED_LAZY
#define REFERENCE_FLAG_UNDEFINED_LAZY 1
macroREFERENCE_FLAG_DEFINED
#define REFERENCE_FLAG_DEFINED 2
macroREFERENCE_FLAG_PRIVATE_DEFINED
#define REFERENCE_FLAG_PRIVATE_DEFINED 3
macroREFERENCE_FLAG_PRIVATE_UNDEFINED_NON_LAZY
#define REFERENCE_FLAG_PRIVATE_UNDEFINED_NON_LAZY 4
macroREFERENCE_FLAG_PRIVATE_UNDEFINED_LAZY
#define REFERENCE_FLAG_PRIVATE_UNDEFINED_LAZY 5
macroREFERENCED_DYNAMICALLY
To simplify stripping of objects that use are used with the dynamic link
editor, the static link editor marks the symbols defined an object that are
referenced by a dynamically bound object (dynamic shared libraries, bundles).
With this marking strip knows not to strip these symbols.
#define REFERENCED_DYNAMICALLY 0x0010
macroGET_LIBRARY_ORDINAL
For images created by the static link editor with the -twolevel_namespace
option in effect the flags field of the mach header is marked with
MH_TWOLEVEL. And the binding of the undefined references of the image are
determined by the static link editor. Which library an undefined symbol is
bound to is recorded by the static linker in the high 8 bits of the n_desc
field using the SET_LIBRARY_ORDINAL macro below. The ordinal recorded
references the libraries listed in the Mach-O's LC_LOAD_DYLIB,
LC_LOAD_WEAK_DYLIB, LC_REEXPORT_DYLIB, LC_LOAD_UPWARD_DYLIB, and
LC_LAZY_LOAD_DYLIB, etc. load commands in the order they appear in the
headers. The library ordinals start from 1.
For a dynamic library that is built as a two-level namespace image the
undefined references from module defined in another use the same nlist struct
an in that case SELF_LIBRARY_ORDINAL is used as the library ordinal. For
defined symbols in all images they also must have the library ordinal set to
SELF_LIBRARY_ORDINAL. The EXECUTABLE_ORDINAL refers to the executable
image for references from plugins that refer to the executable that loads
them.
The DYNAMIC_LOOKUP_ORDINAL is for undefined symbols in a two-level namespace
image that are looked up by the dynamic linker with flat namespace semantics.
This ordinal was added as a feature in Mac OS X 10.3 by reducing the
value of MAX_LIBRARY_ORDINAL by one. So it is legal for existing binaries
or binaries built with older tools to have 0xfe (254) dynamic libraries. In
this case the ordinal value 0xfe (254) must be treated as a library ordinal
for compatibility.
#define GET_LIBRARY_ORDINAL(n_desc) (((n_desc) >> 8) & 0xff)
macroSET_LIBRARY_ORDINAL
#define SET_LIBRARY_ORDINAL(n_desc, ordinal) (n_desc) = (((n_desc) & 0x00ff) | (((ordinal) & 0xff) << 8))
macroSELF_LIBRARY_ORDINAL
#define SELF_LIBRARY_ORDINAL 0x0
macroMAX_LIBRARY_ORDINAL
#define MAX_LIBRARY_ORDINAL 0xfd
macroDYNAMIC_LOOKUP_ORDINAL
#define DYNAMIC_LOOKUP_ORDINAL 0xfe
macroEXECUTABLE_ORDINAL
#define EXECUTABLE_ORDINAL 0xff
macroN_NO_DEAD_STRIP
The N_NO_DEAD_STRIP bit of the n_desc field only ever appears in a
relocatable .o file (MH_OBJECT filetype). And is used to indicate to the
static link editor it is never to dead strip the symbol.
#define N_NO_DEAD_STRIP 0x0020
symbol is not to be dead stripped
macroN_DESC_DISCARDED
The N_DESC_DISCARDED bit of the n_desc field never appears in linked image.
But is used in very rare cases by the dynamic link editor to mark an in
memory symbol as discared and longer used for linking.
#define N_DESC_DISCARDED 0x0020
symbol is discarded
macroN_WEAK_REF
The N_WEAK_REF bit of the n_desc field indicates to the dynamic linker that
the undefined symbol is allowed to be missing and is to have the address of
zero when missing.
#define N_WEAK_REF 0x0040
symbol is weak referenced
macroN_WEAK_DEF
The N_WEAK_DEF bit of the n_desc field indicates to the static and dynamic
linkers that the symbol definition is weak, allowing a non-weak symbol to
also be used which causes the weak definition to be discared. Currently this
is only supported for symbols in coalesed sections.
#define N_WEAK_DEF 0x0080
coalesed symbol is a weak definition
macroN_REF_TO_WEAK
The N_REF_TO_WEAK bit of the n_desc field indicates to the dynamic linker
that the undefined symbol should be resolved using flat namespace searching.
#define N_REF_TO_WEAK 0x0080
reference to a weak symbol
macroN_ARM_THUMB_DEF
The N_ARM_THUMB_DEF bit of the n_desc field indicates that the symbol is
a defintion of a Thumb function.
#define N_ARM_THUMB_DEF 0x0008
symbol is a Thumb function (ARM)
macroN_SYMBOL_RESOLVER
The N_SYMBOL_RESOLVER bit of the n_desc field indicates that the
that the function is actually a resolver function and should
be called to get the address of the real function to use.
This bit is only available in .o files (MH_OBJECT filetype)
#define N_SYMBOL_RESOLVER 0x0100
macroN_ALT_ENTRY
The N_ALT_ENTRY bit of the n_desc field indicates that the
symbol is pinned to the previous content.
#define N_ALT_ENTRY 0x0200
macroN_COLD_FUNC
The N_COLD_FUNC bit of the n_desc field indicates that the symbol is used
infrequently and the linker should order it towards the end of the section.
#define N_COLD_FUNC 0x0400
functionnlist
extern int nlist(const char *filename, struct nlist *list)
The function nlist(3) from the C library.