END(3C) END(3C)
end, etext, edata, _end, _etext, _edata, eprol, _ftext, _fdata, _fbss,
_rt_symbol_table, _rt_symbol_table_size, _rt_symbol_string_table,
_procedure_table, _procedure_table_size, _procedure_string_table,
_DYNAMIC, _DYNAMIC_LNK, _DYNAMIC_LINKING, _BASE_ADDRESS, _GOT_OFFSET,
_gp, _gp_disp, __rld_obj_head, _rld_new_interface, __elf_header,
__program_header_table, __dso_displacement, _lib_version,
__lcldta_address, __lcldta_size, - loader defined symbols in a program
extern int end[];
extern int _end[];
extern int etext[];
extern int _etext[];
extern int edata[];
extern int _edata[];
extern int eprol[];
extern int _ftext[];
extern int _fdata[];
extern int _fbss[];
extern int _procedure_table[];
extern int _procedure_table_size[];
extern int _procedure_string_table[];
extern SYMR _rt_symbol_table[];
extern char _rt_symbol_table_size[];
extern char _rt_symbol_string_table[];
extern int _gp[];
extern int _gp_disp[];
extern int _DYNAMIC[];
extern int _DYNAMIC_LINK[];
extern int _DYNAMIC_LINKING[];
extern int _BASE_ADDRESS[];
extern int _GOT_OFFSET[];
extern int __rld_obj_head[];
extern int _rld_new_interface[];
extern int __elf_header[];
extern int __program_header_table[];
extern int __dso_displacement[];
extern int _lib_version[];
extern int __lcldta_address[];
extern int __lcldta_size[];
For the convenience of utilities and other applications which need to
access an executable in special ways, ld(1) may create one or more of the
special symbols in the list above. Of these, only the symbols
_procedure_table _procedure_string_table, _rt_symbol_table and
_rt_symbol_string_table, __rld_obj_head, __elf_header,
__program_header_table, _lib_version, which are described below, actually
have associated data, which is specially created by ld(1). For each of
the remaining special symbols in the list above, no actual data is
associated; only the address of the symbol is meaningful. Ld(1) sets the
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END(3C) END(3C)
symbol's address to either the location of a particular portion of the
executable (e.g., _etext, or _edata) or to the value of some other
interesting parameter (e.g., _procedure_table_size).
NOTE: In order to avoid their interpretation as gp-relative data, each of
these symbols must be declared as an array. Moreover, most of these
arrays are declared to have elements of type int though one must not
conclude that this imposes any restrictions on the alignment of the
addresses.
The symbol versions without a leading underbar may be overridden by
applications. If the application defines the symbol the symbol is no
longer special and the information here is irrelevant for the symbol.
IMPORTANT NOTE: many of the symbols mentioned here are release-specific
and may change meanings or disappear across releases.
The address of etext and _etext are the first address above the text
segment, edata and _edata are the first address above the initialized
data region, and end and _end are the first address above the
uninitialized data region.
The address of _ftext is the first address in the program text, _fdata is
the first address in the initialized data region, and _fbss is the first
address in the uninitialized data region.
Note that the _ftext, _etext, _edata, _ftext, _fdata, and _fbss are
rather questionable things to depend on for programs using dso's or
having multiple text or data regions. There is no standard definition of
the meanings of these symbols when there are multiple text or data
regions present. For example, use of ld options and spec files can
result in the creation of multiple text and/or data regions. Most
programs use dso's and that means there are multiple text, data, and bss
regions in the running application. Instead of presuming that these
symbols mean something specific, applications would be better off using
__elf_header and __program_header_table and __dso_displacement to
determine the memory regions of whatever code refers to the symbol. That
is, from within a dso (or main program), __elf_header refers to the
__elf_header of that dso (or main program). One can use dlopen and dlsym
to access the symbols of a different dso or, from a dso, of the main
program (see the dlopen man page).
eprol is a symbol that was once used by libprof but is no longer used.
User code may freely define its own such symbol for any application
purpose.
In ISO/ANSI C the symbols end, edata, and etext are elements of the space
of names reserved for the user. Thus, by default these symbols are not
defined by the loader (ld(1)). If, however, a reference to end, edata, or
etext is unsatisfied during the link, it will be defined by the loader.
The address of the resultant symbol will be identical to the address of
the symbol of the same name prefixed by an underbar. (Thus, if an
Page 2
END(3C) END(3C)
unsatisfied reference to end remains at the end of the link, ld(1) will
satisfy the reference by giving it the same address as _end, which is
always defined.)
Some symbols have a dso-specific specific meaning. That is, a query of
the value within a dso can get a dso-specific value: the values are not
global, but specific to the main program or dso taking the address of the
symbol. The dso-specific symbols are: _gp, _gp_disp, _procedure_table,
_procedure_table_size, _procedure_string_table, _rt_symbol_table,
_rt_symbol_table_size, _rt_symbol_string_table, _data_init_table,
_DYNAMIC_LINK, _DYNAMIC_LINKING, and _BASE_ADDRESS. All the other
symbols are global and therefore a single value (that of the main
program) will be seen from all dsos and the main program because of the
global symbol resolution rules.
All of the symbols beginning with _ (underbar) are reserved to the
implementation by ISO/ANSI C rules and it is unwise to attempt to define
these as global symbols.
_gp is the address of the region of global data accessed by offsets of
the global-pointer register (ie, its address is the run-time value of the
global-pointer register.)
_gp_disp has the same value as _gp.
When execution begins, the program break coincides with _end, but it is
reset by the routines brk(2), malloc(3), standard input/output
(stdio(3)), the profile (-p) option of cc(1), etc. The current value of
the program break is reliably returned by `sbrk(0)', see brk(2).
_DYNAMIC_LINKING and _DYNAMIC_LINK are identical. If referred to, they
have special value generated by the linker: 0 means this code is nonshared,
1 means the code is in a KPIC executable, and 2 means the code is
in a dso.
_DYNAMIC is a symbol which is no longer used.
_BASE_ADDRESS is the virtual address of the first loadable segment in the
dso/executable.
_GOT_OFFSET is no longer used or set.
__rld_obj_head is set to the virtual address of a pointer to the list of
dso's in a 32-bit executable. See also <obj.h> and <obj_list.h> and
rld(1).
_rld_new_interface is set to the address of a function within rld that
implements various run-time linking facilities. See rld for further
information.
Page 3
END(3C) END(3C)
__elf_header is set to point to the elf header of the executing object or
dso.
__program_header_table is set to point to the program header table of the
executing object or dso.
__dso_displacement is set to point to a 32-bit word (in a 32-bit program)
which is the amount the executable or dso has been moved by rld at runtime.
_lib_version is set to point to the address of a 32-bit integer. The
value of the integer is 0 normally. It can be non-zero with applications
linked -abi and -cckr which can affect slightly the behavior of abi
applications calling scanf, ldexp, or atof.
__lcldta_address is set to the address of a special section used to keep
thread-local data for MP programming. See the MP documentation for
further information. This symbol is not applicable to applications
compiled as 64-bit programs.
__lcldta_size is set to the size (in bytes) of the special section
referred to above. This symbol is not applicable to applications
compiled as 64-bit programs.
The loader defined symbols _procedure_table, _procedure_table_size, and
_procedure_string_table refer to data structures of the runtime procedure
table. The procedure table data structures are built by ld(1) only if
they are referenced. See the include file <sym.h> for the definition of
the runtime procedure table and see the include file <exception.h> for
its uses. These symbols are not applicable to applications compiled as
64-bit programs.
The loader defined symbols _rt_symbol_table, _rt_symbol_table_size, and
_rt_symbol_string_table refer to data structures of the runtime symbol
table. The runtime symbol table structures are built by ld(1) only if
they are referenced. The runtime symbol table includes all stProc and
stGlobal symbols and is used by the kernel for dynamically loadable
kernel modules. See the include file <sym.h> for the definition of the
runtime symbol table. These symbols are not applicable to applications
compiled as 64-bit programs.
brk(2), malloc(3c), ld(1), cc(1), f77(1), pc(1), mload(4), rld(1)
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