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2007-07-09 Roland McGrath <roland@redhat.com>

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* emultempl/elf32.em (gld${EMULATION_NAME}_add_option): Add --build-id.
	(gld${EMULATION_NAME}_handle_option): Handle --build-id.
	(gld${EMULATION_NAME}_list_options): List --build-id.
	(gld${EMULATION_NAME}_after_open): If --build-id was given,
	synthesize a ".note.gnu.build-id" section and cache it in elf_tdata.
	* ld.texinfo (Options): Describe --build-id.
	* NEWS: Mention --build-id.
This commit is contained in:
Roland McGrath 2007-07-09 21:25:34 +00:00
parent ff59fc360e
commit c0065db732
3 changed files with 209 additions and 128 deletions
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13
ld/NEWS
View file

@ -2,6 +2,9 @@
* Linker sources now released under version 3 of the GNU General Public
License.
* ELF: New --build-id option to generate a unique per-binary identifier
embedded in a note section.
* Added support for National Semicondutor CompactRISC (ie CR16) target.
* -l:foo now searches the library path for a filename called foo,
@ -107,7 +110,7 @@ Changes in 2.15:
* ELF: --as-needed/--no-as-needed options to control if a DT_NEEDED tag should
be added only when a shared library is referenced.
* PE: --large-address-aware option to indicate executables support virtual
addresses greater than 2 gigabytes.
@ -143,7 +146,7 @@ Changes in 2.14:
* Support for Texas Instruments TMS320C4x and TMS320C3x series of
DSP's contributed by Michael Hayes and Svein E. Seldal.
* Added --with-lib-path configure switch to specify default value for
LIB_PATH.
@ -192,7 +195,7 @@ Changes in version 2.11:
* TI C54x support, by Timothy Wall.
* Added command line switch --section-start to set the start address of any
specified section.
specified section.
* Added ability to emit full relocation information in linked executables,
enabled by --emit-relocs. Some post-linkage optimization tools need
@ -208,10 +211,10 @@ Changes in version 2.11:
Changes in version 2.10:
* Added AT> to the linker script language to allow load-time allocation of
* Added AT> to the linker script language to allow load-time allocation of
sections into regions.
* Added garbage collection of unused sections, enabled by --gc-sections.
* Added garbage collection of unused sections, enabled by --gc-sections.
It does require a bit of backend support; currently implemented are
arm-elf, avr-elf, d10v-elf, fr30-elf, i386-elf, m32r-elf, m68k-elf,
mcore-elf, mips-elf, mn10300-elf, ppc-elf, sh-elf, sparc-elf, and v850-elf.

56
ld/emultempl/elf32.em
View file

@ -866,6 +866,45 @@ gld${EMULATION_NAME}_after_open (void)
{
struct bfd_link_needed_list *needed, *l;
if (link_info.emit_note_gnu_build_id)
{
bfd *abfd;
asection *s;
bfd_size_type size;
abfd = link_info.input_bfds;
size = _bfd_id_note_section_size (abfd, &link_info);
if (size == 0)
{
einfo ("%P: warning: unrecognized --build-id style ignored.\n");
free (link_info.emit_note_gnu_build_id);
link_info.emit_note_gnu_build_id = NULL;
}
else
{
s = bfd_make_section_with_flags (abfd, ".note.gnu.build-id",
SEC_ALLOC | SEC_LOAD
| SEC_IN_MEMORY | SEC_LINKER_CREATED
| SEC_READONLY | SEC_DATA);
if (s != NULL && bfd_set_section_alignment (abfd, s, 2))
{
struct elf_obj_tdata *t = elf_tdata (output_bfd);
t->emit_note_gnu_build_id = link_info.emit_note_gnu_build_id;
t->note_gnu_build_id_sec = s;
elf_section_type (s) = SHT_NOTE;
s->size = size;
}
else
{
einfo ("%P: warning: Cannot create .note.gnu.build-id section,"
" --build-id ignored.\n");
free (link_info.emit_note_gnu_build_id);
link_info.emit_note_gnu_build_id = NULL;
}
}
}
if (link_info.eh_frame_hdr
&& ! link_info.traditional_format
&& ! link_info.relocatable)
@ -1760,6 +1799,7 @@ cat >>e${EMULATION_NAME}.c <<EOF
#define OPTION_EH_FRAME_HDR (OPTION_GROUP + 1)
#define OPTION_EXCLUDE_LIBS (OPTION_EH_FRAME_HDR + 1)
#define OPTION_HASH_STYLE (OPTION_EXCLUDE_LIBS + 1)
#define OPTION_BUILD_ID (OPTION_HASH_STYLE + 1)
static void
gld${EMULATION_NAME}_add_options
@ -1768,6 +1808,7 @@ gld${EMULATION_NAME}_add_options
{
static const char xtra_short[] = "${PARSE_AND_LIST_SHORTOPTS}z:";
static const struct option xtra_long[] = {
{"build-id", optional_argument, NULL, OPTION_BUILD_ID},
EOF
if test x"$GENERATE_SHLIB_SCRIPT" = xyes; then
@ -1798,6 +1839,8 @@ cat >>e${EMULATION_NAME}.c <<EOF
memcpy (*longopts + nl, &xtra_long, sizeof (xtra_long));
}
#define DEFAULT_BUILD_ID_STYLE "md5"
static bfd_boolean
gld${EMULATION_NAME}_handle_option (int optc)
{
@ -1806,6 +1849,18 @@ gld${EMULATION_NAME}_handle_option (int optc)
default:
return FALSE;
case OPTION_BUILD_ID:
if (link_info.emit_note_gnu_build_id != NULL)
{
free (link_info.emit_note_gnu_build_id);
link_info.emit_note_gnu_build_id = NULL;
}
if (optarg == NULL)
optarg = DEFAULT_BUILD_ID_STYLE;
if (strcmp (optarg, "none"))
link_info.emit_note_gnu_build_id = xstrdup (optarg);
break;
EOF
if test x"$GENERATE_SHLIB_SCRIPT" = xyes; then
@ -1959,6 +2014,7 @@ cat >>e${EMULATION_NAME}.c <<EOF
static void
gld${EMULATION_NAME}_list_options (FILE * file)
{
fprintf (file, _(" --build-id[=STYLE]\tGenerate build ID note\n"));
EOF
if test x"$GENERATE_SHLIB_SCRIPT" = xyes; then

268
ld/ld.texinfo
View file

@ -1143,7 +1143,7 @@ platforms which support shared libraries.
@kindex -Bsymbolic-functions
@item -Bsymbolic-functions
When creating a shared library, bind references to global function
symbols to the definition within the shared library, if any.
symbols to the definition within the shared library, if any.
This option is only meaningful on ELF platforms which support shared
libraries.
@ -1966,6 +1966,28 @@ has been used.
The @option{--reduce-memory-overheads} switch may be also be used to
enable other tradeoffs in future versions of the linker.
@kindex --build-id
@kindex --build-id=@var{style}
@item --build-id
@itemx --build-id=@var{style}
Request creation of @code{.note.gnu.build-id} ELF note section.
The contents of the note are unique bits identifying this linked
file. @var{style} can be @code{uuid} to use 128 random bits,
@code{md5} to use a 128-bit @sc{MD5} hash on the normative parts
of the output contents, or @code{0x@var{hexstring}} to use a
chosen bit string specified as an even number of hexadecimal
digits (@code{-} and @code{:} characters between digit pairs are
ignored). If @var{style} is omitted, @code{md5} is used.
The @code{md5} style produces an identifier that is always the
same in an identical output file, but will be unique among all
nonidentical output files. It is not intended to be compared as
a checksum for the file's contents. A linked file may be
changed later by other tools, but the build ID bit string
identifying the original linked file does not change.
Passing @code{none} for @var{style} disables the setting from any
@code{--build-id} options earlier on the command line.
@end table
@c man end
@ -2200,9 +2222,9 @@ documentation for ld's @code{--enable-auto-import} for details."
This message occurs when some (sub)expression accesses an address
ultimately given by the sum of two constants (Win32 import tables only
allow one). Instances where this may occur include accesses to member
fields of struct variables imported from a DLL, as well as using a
constant index into an array variable imported from a DLL. Any
allow one). Instances where this may occur include accesses to member
fields of struct variables imported from a DLL, as well as using a
constant index into an array variable imported from a DLL. Any
multiword variable (arrays, structs, long long, etc) may trigger
this error condition. However, regardless of the exact data type
of the offending exported variable, ld will always detect it, issue
@ -2215,14 +2237,14 @@ One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
of adjusting references in your client code for runtime environment, so
this method works only when runtime environment supports this feature.
A second solution is to force one of the 'constants' to be a variable --
that is, unknown and un-optimizable at compile time. For arrays,
there are two possibilities: a) make the indexee (the array's address)
A second solution is to force one of the 'constants' to be a variable --
that is, unknown and un-optimizable at compile time. For arrays,
there are two possibilities: a) make the indexee (the array's address)
a variable, or b) make the 'constant' index a variable. Thus:
@example
extern type extern_array[];
extern_array[1] -->
extern_array[1] -->
@{ volatile type *t=extern_array; t[1] @}
@end example
@ -2230,16 +2252,16 @@ or
@example
extern type extern_array[];
extern_array[1] -->
extern_array[1] -->
@{ volatile int t=1; extern_array[t] @}
@end example
For structs (and most other multiword data types) the only option
For structs (and most other multiword data types) the only option
is to make the struct itself (or the long long, or the ...) variable:
@example
extern struct s extern_struct;
extern_struct.field -->
extern_struct.field -->
@{ volatile struct s *t=&extern_struct; t->field @}
@end example
@ -2252,12 +2274,12 @@ extern_ll -->
@end example
A third method of dealing with this difficulty is to abandon
'auto-import' for the offending symbol and mark it with
'auto-import' for the offending symbol and mark it with
@code{__declspec(dllimport)}. However, in practise that
requires using compile-time #defines to indicate whether you are
building a DLL, building client code that will link to the DLL, or
merely building/linking to a static library. In making the choice
between the various methods of resolving the 'direct address with
building a DLL, building client code that will link to the DLL, or
merely building/linking to a static library. In making the choice
between the various methods of resolving the 'direct address with
constant offset' problem, you should consider typical real-world usage:
Original:
@ -2302,7 +2324,7 @@ void main(int argc, char **argv)@{
@}
@end example
A fourth way to avoid this problem is to re-code your
A fourth way to avoid this problem is to re-code your
library to use a functional interface rather than a data interface
for the offending variables (e.g. set_foo() and get_foo() accessor
functions).
@ -2310,7 +2332,7 @@ functions).
@kindex --disable-auto-import
@item --disable-auto-import
Do not attempt to do sophisticated linking of @code{_symbol} to
Do not attempt to do sophisticated linking of @code{_symbol} to
@code{__imp__symbol} for DATA imports from DLLs.
[This option is specific to the i386 PE targeted port of the linker]
@ -2319,7 +2341,7 @@ Do not attempt to do sophisticated linking of @code{_symbol} to
If your code contains expressions described in --enable-auto-import section,
that is, DATA imports from DLL with non-zero offset, this switch will create
a vector of 'runtime pseudo relocations' which can be used by runtime
environment to adjust references to such data in your client code.
environment to adjust references to such data in your client code.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --disable-runtime-pseudo-reloc
@ -3096,7 +3118,7 @@ Then the C source code to perform the copy would be:
@smallexample
@group
extern char start_of_ROM, end_of_ROM, start_of_FLASH;
memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
@end group
@end smallexample
@ -3416,7 +3438,7 @@ sections have the same name.
It will sort the input sections by alignment first, then by name if 2
sections have the same alignment.
@item
@code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
@code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
@item
@code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
@ -4191,12 +4213,12 @@ output sections directed to a memory region are too large for the
region, the linker will issue an error message.
It is possible to access the origin and length of a memory in an
expression via the @code{ORIGIN(@var{memory})} and
expression via the @code{ORIGIN(@var{memory})} and
@code{LENGTH(@var{memory})} functions:
@smallexample
@group
_fstack = ORIGIN(ram) + LENGTH(ram) - 4;
_fstack = ORIGIN(ram) + LENGTH(ram) - 4;
@end group
@end smallexample
@ -4397,10 +4419,10 @@ VERS_1.2 @{
VERS_2.0 @{
bar1; bar2;
extern "C++" @{
extern "C++" @{
ns::*;
"int f(int, double)";
@}
@}
@} VERS_1.2;
@end smallexample
@ -4518,7 +4540,7 @@ You can also specify the language in the version script:
VERSION extern "lang" @{ version-script-commands @}
@end smallexample
The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
The linker will iterate over the list of symbols at the link time and
demangle them according to @samp{lang} before matching them to the
patterns specified in @samp{version-script-commands}.
@ -5284,20 +5306,20 @@ page of memory, and changes them to use the eight-bit address form.
top page of memory).
@item bit manipulation instructions
@command{ld} finds all bit manipulation instructions like @code{band, bclr,
@command{ld} finds all bit manipulation instructions like @code{band, bclr,
biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
which use 32 bit and 16 bit absolute address form, but refer to the top
which use 32 bit and 16 bit absolute address form, but refer to the top
page of memory, and changes them to use the 8 bit address form.
(That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
@samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
@samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
the top page of memory).
@item system control instructions
@command{ld} finds all @code{ldc.w, stc.w} instructions which use the
32 bit absolute address form, but refer to the top page of memory, and
@command{ld} finds all @code{ldc.w, stc.w} instructions which use the
32 bit absolute address form, but refer to the top page of memory, and
changes them to use 16 bit address form.
(That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
@samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
@samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
the top page of memory).
@end table
@ -5422,7 +5444,7 @@ addressing mode. These instructions consists of @code{bclr} or
@cindex trampoline generation on M68HC12
For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
call a far function using a normal @code{jsr} instruction. The linker
will also change the relocation to some far function to use the
will also change the relocation to some far function to use the
trampoline address instead of the function address. This is typically the
case when a pointer to a function is taken. The pointer will in fact
point to the function trampoline.
@ -5663,14 +5685,14 @@ in this section will be uploaded to the MPU.
Defines an information memory section (if applicable). Any code in
this section will be uploaded to the MPU.
@item @samp{.infomemnobits}
@item @samp{.infomemnobits}
This is the same as the @samp{.infomem} section except that any code
in this section will not be uploaded to the MPU.
@item @samp{.noinit}
Denotes a portion of RAM located above @samp{.bss} section.
The last two sections are used by gcc.
The last two sections are used by gcc.
@end table
@ifclear GENERIC
@ -5907,7 +5929,7 @@ on calls to non-overlay regions.
the address range 0 to 256k. This option may be used to change the
range. Disable the check entirely with @option{--local-store=0:0}.
@cindex SPU
@cindex SPU
@kindex --stack-analysis
@item --stack-analysis
SPU local store space is limited. Over-allocation of stack space
@ -5926,7 +5948,7 @@ dynamic allocation, e.g. alloca, will not be detected. If a link map
is requested, detailed information about each function's stack usage
and calls will be given.
@cindex SPU
@cindex SPU
@kindex --emit-stack-syms
@item --emit-stack-syms
This option, if given along with @option{--stack-analysis} will result
@ -5936,7 +5958,7 @@ functions, and @code{__stack_<number>_<function_name>} for static
functions. @code{<number>} is the section id in hex. The value of
such symbols is the stack requirement for the corresponding function.
The symbol size will be zero, type @code{STT_NOTYPE}, binding
@code{STB_LOCAL}, and section @code{SHN_ABS}.
@code{STB_LOCAL}, and section @code{SHN_ABS}.
@end table
@ifclear GENERIC
@ -5972,13 +5994,13 @@ header format depends on the default specified by the specific target.
@node WIN32
@section @command{ld} and WIN32 (cygwin/mingw)
This section describes some of the win32 specific @command{ld} issues.
This section describes some of the win32 specific @command{ld} issues.
See @ref{Options,,Command Line Options} for detailed description of the
command line options mentioned here.
@table @emph
@cindex import libraries
@item import libraries
@cindex import libraries
@item import libraries
The standard Windows linker creates and uses so-called import
libraries, which contains information for linking to dll's. They are
regular static archives and are handled as any other static
@ -5986,8 +6008,8 @@ archive. The cygwin and mingw ports of @command{ld} have specific
support for creating such libraries provided with the
@samp{--out-implib} command line option.
@item exporting DLL symbols
@cindex exporting DLL symbols
@item exporting DLL symbols
@cindex exporting DLL symbols
The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
@table @emph
@ -6002,7 +6024,7 @@ which is controlled by the following command line options:
@item --exclude-libs
@end itemize
If, however, @samp{--export-all-symbols} is not given explicitly on the
If, however, @samp{--export-all-symbols} is not given explicitly on the
command line, then the default auto-export behavior will be @emph{disabled}
if either of the following are true:
@ -6011,8 +6033,8 @@ if either of the following are true:
@item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
@end itemize
@item using a DEF file
@cindex using a DEF file
@item using a DEF file
@cindex using a DEF file
Another way of exporting symbols is using a DEF file. A DEF file is
an ASCII file containing definitions of symbols which should be
exported when a dll is created. Usually it is named @samp{<dll
@ -6037,7 +6059,7 @@ bar
_bar = bar
another_foo = abc.dll.afoo
var1 DATA
@end example
@end example
This example defines a DLL with a non-default base address and five
symbols in the export table. The third exported symbol @code{_bar} is an
@ -6053,11 +6075,11 @@ the default library suffix, @samp{.DLL} is appended.
When the .DEF file is used to build an application, rather than a
library, the @code{NAME <name>} command should be used instead of
@code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
executable suffix, @samp{.EXE} is appended.
executable suffix, @samp{.EXE} is appended.
With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
specification @code{BASE = <number>} may be used to specify a
non-default base address for the image.
non-default base address for the image.
If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
or they specify an empty string, the internal name is the same as the
@ -6070,7 +6092,7 @@ EXPORTS
( ( ( <name1> [ = <name2> ] )
| ( <name1> = <module-name> . <external-name>))
[ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] ) *
@end example
@end example
Declares @samp{<name1>} as an exported symbol from the DLL, or declares
@samp{<name1>} as an exported alias for @samp{<name2>}; or declares
@ -6105,7 +6127,7 @@ it into the static import library used to resolve imports at link time. The
symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
API at runtime or by by using the GNU ld extension of linking directly to
the DLL without an import library.
See ld/deffilep.y in the binutils sources for the full specification of
other DEF file statements
@ -6130,7 +6152,7 @@ this way, then the normal auto-export behavior is disabled, unless
the @samp{--export-all-symbols} option is also used.
Note that object files that wish to access these symbols must @emph{not}
decorate them with dllexport. Instead, they should use dllimport,
decorate them with dllexport. Instead, they should use dllimport,
instead:
@example
@ -6138,63 +6160,63 @@ __declspec(dllimport) int a_variable
__declspec(dllimport) void a_function(int with_args)
@end example
This complicates the structure of library header files, because
when included by the library itself the header must declare the
This complicates the structure of library header files, because
when included by the library itself the header must declare the
variables and functions as dllexport, but when included by client
code the header must declare them as dllimport. There are a number
of idioms that are typically used to do this; often client code can
of idioms that are typically used to do this; often client code can
omit the __declspec() declaration completely. See
@samp{--enable-auto-import} and @samp{automatic data imports} for more
information.
@end table
@end table
@cindex automatic data imports
@item automatic data imports
The standard Windows dll format supports data imports from dlls only
by adding special decorations (dllimport/dllexport), which let the
compiler produce specific assembler instructions to deal with this
issue. This increases the effort necessary to port existing Un*x
issue. This increases the effort necessary to port existing Un*x
code to these platforms, especially for large
c++ libraries and applications. The auto-import feature, which was
initially provided by Paul Sokolovsky, allows one to omit the
initially provided by Paul Sokolovsky, allows one to omit the
decorations to achieve a behavior that conforms to that on POSIX/Un*x
platforms. This feature is enabled with the @samp{--enable-auto-import}
platforms. This feature is enabled with the @samp{--enable-auto-import}
command-line option, although it is enabled by default on cygwin/mingw.
The @samp{--enable-auto-import} option itself now serves mainly to
suppress any warnings that are ordinarily emitted when linked objects
trigger the feature's use.
auto-import of variables does not always work flawlessly without
auto-import of variables does not always work flawlessly without
additional assistance. Sometimes, you will see this message
"variable '<var>' can't be auto-imported. Please read the
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's @code{--enable-auto-import} for details."
The @samp{--enable-auto-import} documentation explains why this error
occurs, and several methods that can be used to overcome this difficulty.
One of these methods is the @emph{runtime pseudo-relocs} feature, described
The @samp{--enable-auto-import} documentation explains why this error
occurs, and several methods that can be used to overcome this difficulty.
One of these methods is the @emph{runtime pseudo-relocs} feature, described
below.
@cindex runtime pseudo-relocation
For complex variables imported from DLLs (such as structs or classes),
object files typically contain a base address for the variable and an
offset (@emph{addend}) within the variable--to specify a particular
field or public member, for instance. Unfortunately, the runtime loader used
in win32 environments is incapable of fixing these references at runtime
For complex variables imported from DLLs (such as structs or classes),
object files typically contain a base address for the variable and an
offset (@emph{addend}) within the variable--to specify a particular
field or public member, for instance. Unfortunately, the runtime loader used
in win32 environments is incapable of fixing these references at runtime
without the additional information supplied by dllimport/dllexport decorations.
The standard auto-import feature described above is unable to resolve these
The standard auto-import feature described above is unable to resolve these
references.
The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
be resolved without error, while leaving the task of adjusting the references
themselves (with their non-zero addends) to specialized code provided by the
runtime environment. Recent versions of the cygwin and mingw environments and
compilers provide this runtime support; older versions do not. However, the
support is only necessary on the developer's platform; the compiled result will
The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
be resolved without error, while leaving the task of adjusting the references
themselves (with their non-zero addends) to specialized code provided by the
runtime environment. Recent versions of the cygwin and mingw environments and
compilers provide this runtime support; older versions do not. However, the
support is only necessary on the developer's platform; the compiled result will
run without error on an older system.
@samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
enabled as needed.
@samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
enabled as needed.
@cindex direct linking to a dll
@item direct linking to a dll
@ -6202,16 +6224,16 @@ The cygwin/mingw ports of @command{ld} support the direct linking,
including data symbols, to a dll without the usage of any import
libraries. This is much faster and uses much less memory than does the
traditional import library method, especially when linking large
libraries or applications. When @command{ld} creates an import lib, each
function or variable exported from the dll is stored in its own bfd, even
though a single bfd could contain many exports. The overhead involved in
libraries or applications. When @command{ld} creates an import lib, each
function or variable exported from the dll is stored in its own bfd, even
though a single bfd could contain many exports. The overhead involved in
storing, loading, and processing so many bfd's is quite large, and explains the
tremendous time, memory, and storage needed to link against particularly
tremendous time, memory, and storage needed to link against particularly
large or complex libraries when using import libs.
Linking directly to a dll uses no extra command-line switches other than
Linking directly to a dll uses no extra command-line switches other than
@samp{-L} and @samp{-l}, because @command{ld} already searches for a number
of names to match each library. All that is needed from the developer's
of names to match each library. All that is needed from the developer's
perspective is an understanding of this search, in order to force ld to
select the dll instead of an import library.
@ -6231,14 +6253,14 @@ xxx.dll
before moving on to the next directory in the search path.
(*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
where @samp{<prefix>} is set by the @command{ld} option
@samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
(*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
where @samp{<prefix>} is set by the @command{ld} option
@samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
@samp{cygxxx.dll}.
Other win32-based unix environments, such as mingw or pw32, may use other
@samp{<prefix>}es, although at present only cygwin makes use of this feature. It
Other win32-based unix environments, such as mingw or pw32, may use other
@samp{<prefix>}es, although at present only cygwin makes use of this feature. It
was originally intended to help avoid name conflicts among dll's built for the
various win32/un*x environments, so that (for example) two versions of a zlib dll
could coexist on the same machine.
@ -6252,16 +6274,16 @@ bin/
cygxxx.dll
lib/
libxxx.dll.a (in case of dll's)
libxxx.a (in case of static archive)
libxxx.a (in case of static archive)
@end example
Linking directly to a dll without using the import library can be
done two ways:
Linking directly to a dll without using the import library can be
done two ways:
1. Use the dll directly by adding the @samp{bin} path to the link line
@example
gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
@end example
@end example
However, as the dll's often have version numbers appended to their names
(@samp{cygncurses-5.dll}) this will often fail, unless one specifies
@ -6275,13 +6297,13 @@ making the app/dll.
@example
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
@end example
@end example
Then you can link without any make environment changes.
@example
gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
@end example
@end example
This technique also avoids the version number problems, because the following is
perfectly legal
@ -6290,7 +6312,7 @@ perfectly legal
bin/
cygxxx-5.dll
lib/
libxxx.dll.a -> ../bin/cygxxx-5.dll
libxxx.dll.a -> ../bin/cygxxx-5.dll
@end example
Linking directly to a dll without using an import lib will work
@ -6315,71 +6337,71 @@ in which symbols are usually exported as undecorated aliases of their
stdcall-decorated assembly names.
So, import libs are not going away. But the ability to replace
true import libs with a simple symbolic link to (or a copy of)
a dll, in many cases, is a useful addition to the suite of tools
binutils makes available to the win32 developer. Given the
true import libs with a simple symbolic link to (or a copy of)
a dll, in many cases, is a useful addition to the suite of tools
binutils makes available to the win32 developer. Given the
massive improvements in memory requirements during linking, storage
requirements, and linking speed, we expect that many developers
will soon begin to use this feature whenever possible.
@item symbol aliasing
@item symbol aliasing
@table @emph
@item adding additional names
Sometimes, it is useful to export symbols with additional names.
@item adding additional names
Sometimes, it is useful to export symbols with additional names.
A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
exported as @samp{_foo} by using special directives in the DEF file
when creating the dll. This will affect also the optional created
import library. Consider the following DEF file:
import library. Consider the following DEF file:
@example
@example
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
foo
foo
_foo = foo
@end example
@end example
The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
Another method for creating a symbol alias is to create it in the
source code using the "weak" attribute:
@example
void foo () @{ /* Do something. */; @}
@example
void foo () @{ /* Do something. */; @}
void _foo () __attribute__ ((weak, alias ("foo")));
@end example
@end example
See the gcc manual for more information about attributes and weak
symbols.
@item renaming symbols
Sometimes it is useful to rename exports. For instance, the cygwin
kernel does this regularly. A symbol @samp{_foo} can be exported as
kernel does this regularly. A symbol @samp{_foo} can be exported as
@samp{foo} but not as @samp{_foo} by using special directives in the
DEF file. (This will also affect the import library, if it is
created). In the following example:
created). In the following example:
@example
@example
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
_foo = foo
@end example
@end example
The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
@samp{_foo}.
@end table
@end table
Note: using a DEF file disables the default auto-export behavior,
unless the @samp{--export-all-symbols} command line option is used.
unless the @samp{--export-all-symbols} command line option is used.
If, however, you are trying to rename symbols, then you should list
@emph{all} desired exports in the DEF file, including the symbols
that are not being renamed, and do @emph{not} use the
@samp{--export-all-symbols} option. If you list only the
renamed symbols in the DEF file, and use @samp{--export-all-symbols}
to handle the other symbols, then the both the new names @emph{and}
the original names for the renamed symbols will be exported.
In effect, you'd be aliasing those symbols, not renaming them,
@emph{all} desired exports in the DEF file, including the symbols
that are not being renamed, and do @emph{not} use the
@samp{--export-all-symbols} option. If you list only the
renamed symbols in the DEF file, and use @samp{--export-all-symbols}
to handle the other symbols, then the both the new names @emph{and}
the original names for the renamed symbols will be exported.
In effect, you'd be aliasing those symbols, not renaming them,
which is probably not what you wanted.
@cindex weak externals
@ -6615,7 +6637,7 @@ location where that name is stored in memory; perhaps, if the name
were different, the contents of that location would fool the linker
into doing the right thing despite the bug. Play it safe and give a
specific, complete example. That is the easiest thing for you to do,
and the most helpful.
and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix
the bug if it is new to us. Therefore, always write your bug reports