.. _options-phases:
Options related to a particular phase
=====================================
.. _replacing-phases:
Replacing the program for one or more phases
--------------------------------------------
.. index::
single: compilation phases, changing
You may specify that a different program be used for one of the phases
of the compilation system, in place of whatever the ``ghc`` has wired
into it. For example, you might want to try a different assembler. The
following options allow you to change the external program used for a
given compilation phase:
.. ghc-flag:: -pgmL ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the literate pre-processor
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the literate pre-processor.
.. ghc-flag:: -pgmP ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the C pre-processor (with ``-cpp`` only)
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the C pre-processor (with ``-cpp`` only).
.. ghc-flag:: -pgmc ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the C compiler
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the C compiler.
.. ghc-flag:: -pgmlo ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the LLVM optimiser
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the LLVM optimiser.
.. ghc-flag:: -pgmlc ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the LLVM compiler
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the LLVM compiler.
.. ghc-flag:: -pgms ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the splitter
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the splitter.
.. ghc-flag:: -pgma ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the assembler
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the assembler.
.. ghc-flag:: -pgml ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the linker
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the linker.
.. ghc-flag:: -pgmdll ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the DLL generator
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the DLL generator.
.. ghc-flag:: -pgmF ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the pre-processor (with ``-F`` only)
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the pre-processor (with ``-F`` only).
.. ghc-flag:: -pgmwindres ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the program for embedding manifests on Windows.
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the program to use for embedding manifests on Windows.
Normally this is the program ``windres``, which is supplied with a
GHC installation. See ``-fno-embed-manifest`` in
:ref:`options-linker`.
.. ghc-flag:: -pgmlibtool ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the command for libtool (with ``-staticlib`` only).
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the libtool command (when using ``-staticlib`` only).
.. ghc-flag:: -pgmi ⟨cmd⟩
:shortdesc: Use ⟨cmd⟩ as the external interpreter command.
:type: dynamic
:category: phase-programs
Use ⟨cmd⟩ as the external interpreter command (see:
:ref:`external-interpreter`). Default: ``ghc-iserv-prof`` if
``-prof`` is enabled, ``ghc-iserv-dyn`` if ``-dynamic`` is
enabled, or ``ghc-iserv`` otherwise.
.. _forcing-options-through:
Forcing options to a particular phase
-------------------------------------
.. index::
single: forcing GHC-phase options
Options can be forced through to a particular compilation phase, using
the following flags:
.. ghc-flag:: -optL ⟨option⟩
:shortdesc: pass ⟨option⟩ to the literate pre-processor
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the literate pre-processor
.. ghc-flag:: -optP ⟨option⟩
:shortdesc: pass ⟨option⟩ to cpp (with ``-cpp`` only)
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to CPP (makes sense only if ``-cpp`` is also on).
.. ghc-flag:: -optF ⟨option⟩
:shortdesc: pass ⟨option⟩ to the custom pre-processor
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the custom pre-processor (see
:ref:`pre-processor`).
.. ghc-flag:: -optc ⟨option⟩
:shortdesc: pass ⟨option⟩ to the C compiler
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the C compiler.
.. ghc-flag:: -optlo ⟨option⟩
:shortdesc: pass ⟨option⟩ to the LLVM optimiser
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the LLVM optimiser.
.. ghc-flag:: -optlc ⟨option⟩
:shortdesc: pass ⟨option⟩ to the LLVM compiler
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the LLVM compiler.
.. ghc-flag:: -opta ⟨option⟩
:shortdesc: pass ⟨option⟩ to the assembler
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the assembler.
.. ghc-flag:: -optl ⟨option⟩
:shortdesc: pass ⟨option⟩ to the linker
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the linker.
.. ghc-flag:: -optdll ⟨option⟩
:shortdesc: pass ⟨option⟩ to the DLL generator
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the DLL generator.
.. ghc-flag:: -optwindres ⟨option⟩
:shortdesc: pass ⟨option⟩ to ``windres``.
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to ``windres`` when embedding manifests on Windows.
See ``-fno-embed-manifest`` in :ref:`options-linker`.
.. ghc-flag:: -opti ⟨option⟩
:shortdesc: pass ⟨option⟩ to the interpreter sub-process.
:type: dynamic
:category: phase-options
Pass ⟨option⟩ to the interpreter sub-process (see
:ref:`external-interpreter`). A common use for this is to pass
RTS options e.g., ``-opti+RTS -opti-A64m``, or to enable verbosity
with ``-opti-v`` to see what messages are being exchanged by GHC
and the interpreter.
So, for example, to force an ``-Ewurble`` option to the assembler, you
would tell the driver ``-opta-Ewurble`` (the dash before the E is
required).
GHC is itself a Haskell program, so if you need to pass options directly
to GHC's runtime system you can enclose them in ``+RTS ... -RTS`` (see
:ref:`runtime-control`).
.. _c-pre-processor:
Options affecting the C pre-processor
-------------------------------------
.. extension:: CPP
:shortdesc: Enable the C preprocessor.
:since: 6.8.1
.. index::
single: pre-processing: cpp
single: C pre-processor options
single: cpp, pre-processing with
.. ghc-flag:: -cpp
:shortdesc: Run the C pre-processor on Haskell source files
:type: dynamic
:category: cpp
The C pre-processor :command:`cpp` is run over your Haskell code only if
the ``-cpp`` option -cpp option is given. Unless you are building a
large system with significant doses of conditional compilation, you
really shouldn't need it.
.. ghc-flag:: -D⟨symbol⟩[=⟨value⟩]
:shortdesc: Define a symbol in the C pre-processor
:type: dynamic
:reverse: -U⟨symbol⟩
:category: cpp
Define macro ⟨symbol⟩ in the usual way. When no value is given, the value is
taken to be ``1``. For instance, ``-DUSE_MYLIB`` is equivalent to
``-DUSE_MYLIB=1``.
.. note::
:ghc-flag:`-D⟨symbol⟩[=⟨value⟩]` does *not* affect ``-D``
macros passed to the CÂ compiler when compiling an unregisterised build! In
this case use the ``-optc-Dfoo`` hack… (see :ref:`forcing-options-through`).
.. ghc-flag:: -U⟨symbol⟩
:shortdesc: Undefine a symbol in the C pre-processor
:type: dynamic
:category: cpp
Undefine macro ⟨symbol⟩ in the usual way.
.. ghc-flag:: -I⟨dir⟩
:shortdesc: Add ⟨dir⟩ to the directory search list for ``#include`` files
:type: dynamic
:category: cpp
Specify a directory in which to look for ``#include`` files, in the
usual C way.
The GHC driver pre-defines several macros when processing Haskell source
code (``.hs`` or ``.lhs`` files).
.. _standard-cpp-macros:
Standard CPP macros
~~~~~~~~~~~~~~~~~~~
The symbols defined by GHC are listed below. To check which symbols are
defined by your local GHC installation, the following trick is useful:
.. code-block:: sh
$ ghc -E -optP-dM -cpp foo.hs
$ cat foo.hspp
(you need a file ``foo.hs``, but it isn't actually used).
``__GLASGOW_HASKELL__``
.. index::
single: __GLASGOW_HASKELL__
For version ``x.y.z`` of GHC, the value of ``__GLASGOW_HASKELL__``
is the integer ⟨xyy⟩ (if ⟨y⟩ is a single digit, then a leading zero
is added, so for example in version 6.2 of GHC,
``__GLASGOW_HASKELL__==602``). More information in
:ref:`version-numbering`.
With any luck, ``__GLASGOW_HASKELL__`` will be undefined in all
other implementations that support C-style pre-processing.
.. note::
The comparable symbols for other systems are:
``__HUGS__`` for Hugs, ``__NHC__`` for nhc98, and ``__HBC__`` for
hbc).
NB. This macro is set when pre-processing both Haskell source and C
source, including the C source generated from a Haskell module (i.e.
``.hs``, ``.lhs``, ``.c`` and ``.hc`` files).
``__GLASGOW_HASKELL_PATCHLEVEL1__``; \ ``__GLASGOW_HASKELL_PATCHLEVEL2__``
.. index::
single: __GLASGOW_HASKELL_PATCHLEVEL2__
.. index::
single: __GLASGOW_HASKELL_PATCHLEVEL1__
These macros are available starting with GHC 7.10.1.
For three-part GHC version numbers ``x.y.z``, the value of
``__GLASGOW_HASKELL_PATCHLEVEL1__`` is the integer ⟨z⟩.
For four-part GHC version numbers ``x.y.z.z'``, the value of
``__GLASGOW_HASKELL_PATCHLEVEL1__`` is the integer ⟨z⟩ while the
value of ``__GLASGOW_HASKELL_PATCHLEVEL2__`` is set to the integer
⟨z'⟩.
These macros are provided for allowing finer granularity than is
provided by ``__GLASGOW_HASKELL__``. Usually, this should not be
necessary as it's expected for most APIs to remain stable between
patchlevel releases, but occasionally internal API changes are
necessary to fix bugs. Also conditional compilation on the
patchlevel can be useful for working around bugs in older releases.
.. tip::
These macros are set when pre-processing both Haskell source and
C source, including the C source generated from a Haskell module
(i.e. ``.hs``, ``.lhs``, ``.c`` and ``.hc`` files).
``MIN_VERSION_GLASGOW_HASKELL(x,y,z,z')``
.. index::
single: MIN_VERSION_GLASGOW_HASKELL
This macro is available starting with GHC 7.10.1.
This macro is provided for convenience to write CPP conditionals
testing whether the GHC version used is version ``x.y.z.z'`` or
later.
If compatibility with Haskell compilers (including GHC prior to
version 7.10.1) which do not define ``MIN_VERSION_GLASGOW_HASKELL``
is required, the presence of the ``MIN_VERSION_GLASGOW_HASKELL``
macro needs to be ensured before it is called, e.g.:
.. code-block:: c
#ifdef MIN_VERSION_GLASGOW_HASKELL
#if MIN_VERSION_GLASGOW_HASKELL(7,10,2,0)
/* code that applies only to GHC 7.10.2 or later */
#endif
#endif
.. tip::
This macro is set when pre-processing both Haskell source and C
source, including the C source generated from a Haskell module (i.e.
``.hs``, ``.lhs``, ``.c`` and ``.hc`` files).
``__GLASGOW_HASKELL_TH__``
.. index::
single: __GLASGOW_HASKELL_TH__
This is set to ``1`` when the compiler supports Template Haskell,
and to ``0`` when not. The latter is the case for a stage-1
compiler during bootstrapping, or on architectures where the
interpreter is not available.
``__GLASGOW_HASKELL_LLVM__``
.. index::
single: __GLASGOW_HASKELL_LLVM__
Only defined when ``-fllvm`` is specified. When GHC is using version
``x.y.z`` of LLVM, the value of ``__GLASGOW_HASKELL_LLVM__`` is the
integer ⟨xyy⟩ (if ⟨y⟩ is a single digit, then a leading zero
is added, so for example when using version 3.7 of LLVM,
``__GLASGOW_HASKELL_LLVM__==307``).
``__PARALLEL_HASKELL__``
.. index::
single: __PARALLEL_HASKELL__
Only defined when ``-parallel`` is in use! This symbol is defined
when pre-processing Haskell (input) and pre-processing C (GHC
output).
``os_HOST_OS=1``
This define allows conditional compilation based on the Operating
System, where⟨os⟩ is the name of the current Operating System (eg.
``linux``, ``mingw32`` for Windows, ``solaris``, etc.).
``arch_HOST_ARCH=1``
This define allows conditional compilation based on the host
architecture, where⟨arch⟩ is the name of the current architecture
(eg. ``i386``, ``x86_64``, ``powerpc``, ``sparc``, etc.).
``VERSION_pkgname``
This macro is available starting GHC 8.0. It is defined for every
exposed package. This macro expands to a string recording the
version of ``pkgname`` that is exposed for module import.
It is identical in behavior to the ``VERSION_pkgname`` macros
that Cabal defines.
``MIN_VERSION_pkgname(x,y,z)``
This macro is available starting GHC 8.0. It is defined for every
exposed package. This macro is provided for convenience to write CPP
conditionals testing if a package version is ``x.y.z`` or
later. It is identical in behavior to the ``MIN_VERSION_pkgname``
macros that Cabal defines.
.. _cpp-string-gaps:
CPP and string gaps
~~~~~~~~~~~~~~~~~~~
.. index::
single: -cpp vs string gaps
single: string gaps vs -cpp.
A small word of warning: :ghc-flag:`-cpp` is not friendly to "string gaps".
In other words, strings such as the following: ::
strmod = "\
\ p \
\ "
don't work with :ghc-flag:`-cpp`; :command:`/usr/bin/cpp` elides the backslash-newline
pairs.
However, it appears that if you add a space at the end of the line, then
``cpp`` (at least GNU ``cpp`` and possibly other ``cpp``\ s) leaves the
backslash-space pairs alone and the string gap works as expected.
.. _pre-processor:
Options affecting a Haskell pre-processor
-----------------------------------------
.. index::
single: pre-processing: custom
single: pre-processor options
.. ghc-flag:: -F
:shortdesc: Enable the use of a :ref:`pre-processor <pre-processor>`
(set with :ghc-flag:`-pgmF ⟨cmd⟩`)
:type: dynamic
:category: phases
A custom pre-processor is run over your Haskell source file only if
the ``-F`` option is given.
Running a custom pre-processor at compile-time is in some settings
appropriate and useful. The ``-F`` option lets you run a
pre-processor as part of the overall GHC compilation pipeline, which
has the advantage over running a Haskell pre-processor separately in
that it works in interpreted mode and you can continue to take reap
the benefits of GHC's recompilation checker.
The pre-processor is run just before the Haskell compiler proper
processes the Haskell input, but after the literate markup has been
stripped away and (possibly) the C pre-processor has washed the
Haskell input.
Use :ghc-flag:`-pgmF ⟨cmd⟩` to select the program to use as the
preprocessor. When invoked, the ⟨cmd⟩ pre-processor is given at least
three arguments on its command-line: the first argument is the name of the
original source file, the second is the name of the file holding the input,
and the third is the name of the file where ⟨cmd⟩ should write its output
to.
Additional arguments to the pre-processor can be passed in using the
:ghc-flag:`-optF ⟨option⟩` option. These are fed to ⟨cmd⟩ on the command
line after the three standard input and output arguments.
An example of a pre-processor is to convert your source files to the
input encoding that GHC expects, i.e. create a script ``convert.sh``
containing the lines:
.. code-block:: sh
#!/bin/sh
( echo "{-# LINE 1 \"$2\" #-}" ; iconv -f l1 -t utf-8 $2 ) > $3
and pass ``-F -pgmF convert.sh`` to GHC. The ``-f l1`` option tells
iconv to convert your Latin-1 file, supplied in argument ``$2``,
while the "-t utf-8" options tell iconv to return a UTF-8 encoded
file. The result is redirected into argument ``$3``. The
``echo "{-# LINE 1 \"$2\" #-}"`` just makes sure that your error
positions are reported as in the original source file.
.. _options-codegen:
Options affecting code generation
---------------------------------
.. ghc-flag:: -fasm
:shortdesc: Use the :ref:`native code generator <native-code-gen>`
:type: dynamic
:reverse: -fllvm
:category: codegen
Use GHC's :ref:`native code generator <native-code-gen>` rather than
compiling via LLVM. ``-fasm`` is the default.
.. ghc-flag:: -fllvm
:shortdesc: Compile using the :ref:`LLVM code generator <llvm-code-gen>`
:type: dynamic
:reverse: -fasm
:category: codegen
Compile via :ref:`LLVM <llvm-code-gen>` instead of using the native
code generator. This will generally take slightly longer than the
native code generator to compile. Produced code is generally the
same speed or faster than the other two code generators. Compiling
via LLVM requires LLVM's :command:`opt` and :command:`llc` executables to be
in :envvar:`PATH`.
.. note::
Note that this GHC release expects an LLVM version in the |llvm-version|
release series.
.. ghc-flag:: -fno-code
:shortdesc: Omit code generation
:type: dynamic
:category: codegen
Omit code generation (and all later phases) altogether. This is
useful if you're only interested in type checking code.
.. ghc-flag:: -fwrite-interface
:shortdesc: Always write interface files
:type: dynamic
:category: codegen
Always write interface files. GHC will normally write interface
files automatically, but this flag is useful with :ghc-flag:`-fno-code`,
which normally suppresses generation of interface files. This is
useful if you want to type check over multiple runs of GHC without
compiling dependencies.
.. ghc-flag:: -fobject-code
:shortdesc: Generate object code
:type: dynamic
:category: codegen
Generate object code. This is the default outside of GHCi, and can
be used with GHCi to cause object code to be generated in preference
to bytecode.
.. ghc-flag:: -fbyte-code
:shortdesc: Generate byte-code
:type: dynamic
:category: codegen
Generate byte-code instead of object-code. This is the default in
GHCi. Byte-code can currently only be used in the interactive
interpreter, not saved to disk. This option is only useful for
reversing the effect of :ghc-flag:`-fobject-code`.
.. ghc-flag:: -fPIC
:shortdesc: Generate position-independent code (where available)
:type: dynamic
:category: codegen
Generate position-independent code (code that can be put into shared
libraries). This currently works on Linux x86 and x86-64. On
Windows, position-independent code is never used so the flag is a
no-op on that platform.
.. ghc-flag:: -fPIE
:shortdesc: Generate code for a position-independent executable (where available)
:type: dynamic
:category: codegen
Generate code in such a way to be linkable into a position-independent
executable This currently works on Linux x86 and x86-64. On Windows,
position-independent code is never used so the flag is a no-op on that
platform. To link the final executable use :ghc-flag:`-pie`.
.. ghc-flag:: -dynamic
:shortdesc: Build dynamically-linked object files and executables
:type: dynamic
:category: codegen
:noindex:
When generating code, assume that entities imported from a different
package will be dynamically linked. This can reduce code size
tremendously, but may slow-down cross-package calls of non-inlined
functions. There can be some complications combining :ghc-flag:`-shared`
with this flag relating to linking in the RTS under Linux. See
:ghc-ticket:`10352`.
Note that using this option when linking causes GHC to link against
shared libraries.
.. ghc-flag:: -dynamic-too
:shortdesc: Build dynamic object files *as well as* static object files
during compilation
:type: dynamic
:category: codegen
Generates both dynamic and static object files in a single run of
GHC. This option is functionally equivalent to running GHC twice,
the second time adding ``-dynamic -osuf dyn_o -hisuf dyn_hi``.
Although it is equivalent to running GHC twice, using
``-dynamic-too`` is more efficient, because the earlier phases of
the compiler up to code generation are performed just once.
When using ``-dynamic-too``, the options ``-dyno``, ``-dynosuf``,
and ``-dynhisuf`` are the counterparts of ``-o``, ``-osuf``, and
``-hisuf`` respectively, but applying to the dynamic compilation.
.. _options-linker:
Options affecting linking
-------------------------
.. index::
single: linker options
single: ld options
GHC has to link your code with various libraries, possibly including:
user-supplied, GHC-supplied, and system-supplied (``-lm`` math library,
for example).
.. ghc-flag:: -l ⟨lib⟩
:shortdesc: Link in library ⟨lib⟩
:type: dynamic
:category: linking
Link in the ⟨lib⟩ library. On Unix systems, this will be in a file
called :file:`lib{lib}.a` or :file:`lib{lib}.so` which resides somewhere on the
library directories path.
Because of the sad state of most UNIX linkers, the order of such
options does matter. If library ⟨foo⟩ requires library ⟨bar⟩, then
in general ``-l ⟨foo⟩`` should come *before* ``-l ⟨bar⟩`` on the
command line.
There's one other gotcha to bear in mind when using external
libraries: if the library contains a ``main()`` function, then this
will be a link conflict with GHC's own ``main()`` function (eg.
``libf2c`` and ``libl`` have their own ``main()``\ s).
You can use an external main function if you initialize the RTS manually
and pass ``-no-hs-main``. See also :ref:`using-own-main`.
.. ghc-flag:: -c
:shortdesc: Stop after generating object (``.o``) file
:type: mode
:category: linking
Omits the link step. This option can be used with :ghc-flag:`--make` to
avoid the automatic linking that takes place if the program contains
a ``Main`` module.
.. ghc-flag:: -package ⟨name⟩
:shortdesc: Expose package ⟨pkg⟩
:type: dynamic/ ``:set``
:category: linking
If you are using a Haskell "package" (see :ref:`packages`), don't
forget to add the relevant ``-package`` option when linking the
program too: it will cause the appropriate libraries to be linked in
with the program. Forgetting the ``-package`` option will likely
result in several pages of link errors.
.. ghc-flag:: -framework ⟨name⟩
:shortdesc: On Darwin/OS X/iOS only, link in the framework ⟨name⟩. This
option corresponds to the ``-framework`` option for Apple's Linker.
:type: dynamic
:category: linking
On Darwin/OS X/iOS only, link in the framework ⟨name⟩. This option
corresponds to the ``-framework`` option for Apple's Linker. Please
note that frameworks and packages are two different things -
frameworks don't contain any Haskell code. Rather, they are Apple's
way of packaging shared libraries. To link to Apple's "Carbon" API,
for example, you'd use ``-framework Carbon``.
.. ghc-flag:: -staticlib
:shortdesc: Generate a standalone static library (as opposed to an
executable). This is useful when cross compiling. The
library together with all its dependencies ends up in in a
single static library that can be linked against.
:type: dynamic
:category: linking
Link all passed files into a static library suitable for linking.
To control the name, use the :ghc-flag:`-o ⟨file⟩` option
as usual. The default name is ``liba.a``.
.. ghc-flag:: -L ⟨dir⟩
:shortdesc: Add ⟨dir⟩ to the list of directories searched for libraries
:type: dynamic
:category: linking
Where to find user-supplied libraries… Prepend the directory ⟨dir⟩
to the library directories path.
.. ghc-flag:: -framework-path ⟨dir⟩
:shortdesc: On Darwin/OS X/iOS only, add ⟨dir⟩ to the list of directories
searched for frameworks. This option corresponds to the ``-F``
option for Apple's Linker.
:type: dynamic
:category: linking
On Darwin/OS X/iOS only, prepend the directory ⟨dir⟩ to the
framework directories path. This option corresponds to the ``-F``
option for Apple's Linker (``-F`` already means something else for
GHC).
.. ghc-flag:: -split-objs
:shortdesc: Split objects (for libraries)
:type: dynamic
:category: linking
Tell the linker to split the single object file that would normally
be generated into multiple object files, one per top-level Haskell
function or type in the module. This only makes sense for libraries,
where it means that executables linked against the library are
smaller as they only link against the object files that they need.
However, assembling all the sections separately is expensive, so
this is slower than compiling normally. Additionally, the size of
the library itself (the ``.a`` file) can be a factor of 2 to 2.5
larger. We use this feature for building GHC's libraries.
.. ghc-flag:: -split-sections
:shortdesc: Split sections for link-time dead-code stripping
:type: dynamic
:category: linking
Place each generated function or data item into its own section in the
output file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name in the
output file.
When linking, the linker can automatically remove all unreferenced sections
and thus produce smaller executables. The effect is similar to
:ghc-flag:`-split-objs`, but somewhat more efficient - the generated library
files are about 30% smaller than with :ghc-flag:`-split-objs`.
.. ghc-flag:: -static
:shortdesc: Use static Haskell libraries
:type: dynamic
:category: linking
Tell the linker to avoid shared Haskell libraries, if possible. This
is the default.
.. ghc-flag:: -dynamic
:shortdesc: Build dynamically-linked object files and executables
:type: dynamic
:category: linking
This flag tells GHC to link against shared Haskell libraries. This
flag only affects the selection of dependent libraries, not the form
of the current target (see -shared). See :ref:`using-shared-libs` on
how to create them.
Note that this option also has an effect on code generation (see
above).
.. ghc-flag:: -shared
:shortdesc: Generate a shared library (as opposed to an executable)
:type: dynamic
:category: linking
Instead of creating an executable, GHC produces a shared object with
this linker flag. Depending on the operating system target, this
might be an ELF DSO, a Windows DLL, or a Mac OS dylib. GHC hides the
operating system details beneath this uniform flag.
The flags :ghc-flag:`-dynamic` and :ghc-flag:`-static` control whether the
resulting shared object links statically or dynamically to Haskell package
libraries given as :ghc-flag:`-package ⟨pkg⟩` option. Non-Haskell libraries
are linked as gcc would regularly link it on your system, e.g. on most ELF
system the linker uses the dynamic libraries when found.
Object files linked into shared objects must be compiled with
:ghc-flag:`-fPIC`, see :ref:`options-codegen`
When creating shared objects for Haskell packages, the shared object
must be named properly, so that GHC recognizes the shared object
when linked against this package. See shared object name mangling.
.. ghc-flag:: -dynload
:shortdesc: Selects one of a number of modes for finding shared libraries at runtime.
:type: dynamic
:category: linking
This flag selects one of a number of modes for finding shared
libraries at runtime. See :ref:`finding-shared-libs` for a
description of each mode.
.. ghc-flag:: -main-is ⟨thing⟩
:shortdesc: Set main module and function
:type: dynamic
:category: linking
.. index::
single: specifying your own main function
The normal rule in Haskell is that your program must supply a
``main`` function in module ``Main``. When testing, it is often
convenient to change which function is the "main" one, and the
``-main-is`` flag allows you to do so. The ⟨thing⟩ can be one of:
- A lower-case identifier ``foo``. GHC assumes that the main
function is ``Main.foo``.
- A module name ``A``. GHC assumes that the main function is
``A.main``.
- A qualified name ``A.foo``. GHC assumes that the main function is
``A.foo``.
Strictly speaking, ``-main-is`` is not a link-phase flag at all; it
has no effect on the link step. The flag must be specified when
compiling the module containing the specified main function (e.g.
module ``A`` in the latter two items above). It has no effect for
other modules, and hence can safely be given to ``ghc --make``.
However, if all the modules are otherwise up to date, you may need
to force recompilation both of the module where the new "main" is,
and of the module where the "main" function used to be; ``ghc`` is
not clever enough to figure out that they both need recompiling. You
can force recompilation by removing the object file, or by using the
:ghc-flag:`-fforce-recomp` flag.
.. ghc-flag:: -no-hs-main
:shortdesc: Don't assume this program contains ``main``
:type: dynamic
:category: linking
.. index::
single: linking Haskell libraries with foreign code
In the event you want to include ghc-compiled code as part of
another (non-Haskell) program, the RTS will not be supplying its
definition of ``main()`` at link-time, you will have to. To signal
that to the compiler when linking, use ``-no-hs-main``. See also
:ref:`using-own-main`.
Notice that since the command-line passed to the linker is rather
involved, you probably want to use ``ghc`` to do the final link of
your \`mixed-language' application. This is not a requirement
though, just try linking once with :ghc-flag:`-v` on to see what options the
driver passes through to the linker.
The ``-no-hs-main`` flag can also be used to persuade the compiler
to do the link step in :ghc-flag:`--make` mode when there is no Haskell
``Main`` module present (normally the compiler will not attempt
linking when there is no ``Main``).
The flags :ghc-flag:`-rtsopts[=⟨none|some|all⟩]` and
:ghc-flag:`-with-rtsopts=⟨opts⟩` have no effect when used with
:ghc-flag:`-no-hs-main`, because they are implemented by changing the
definition of ``main`` that GHC generates. See :ref:`using-own-main` for
how to get the effect of :ghc-flag:`-rtsopts[=⟨none|some|all⟩]` and
:ghc-flag:`-with-rtsopts=⟨opts⟩` when using your own ``main``.
.. ghc-flag:: -debug
:shortdesc: Use the debugging runtime
:type: dynamic
:category: linking
Link the program with a debugging version of the runtime system. The
debugging runtime turns on numerous assertions and sanity checks,
and provides extra options for producing debugging output at runtime
(run the program with ``+RTS -?`` to see a list).
.. ghc-flag:: -threaded
:shortdesc: Use the threaded runtime
:type: dynamic
:category: linking
Link the program with the "threaded" version of the runtime system.
The threaded runtime system is so-called because it manages multiple
OS threads, as opposed to the default runtime system which is purely
single-threaded.
Note that you do *not* need ``-threaded`` in order to use
concurrency; the single-threaded runtime supports concurrency
between Haskell threads just fine.
The threaded runtime system provides the following benefits:
- It enables the :rts-flag:`-N ⟨x⟩` RTS option to be used,
which allows threads to run in parallelparallelism on a
multiprocessormultiprocessorSMP or multicoremulticore machine.
See :ref:`using-smp`.
- If a thread makes a foreign call (and the call is not marked
``unsafe``), then other Haskell threads in the program will
continue to run while the foreign call is in progress.
Additionally, ``foreign export``\ ed Haskell functions may be
called from multiple OS threads simultaneously. See
:ref:`ffi-threads`.
.. ghc-flag:: -eventlog
:shortdesc: Enable runtime event tracing
:type: dynamic
:category: linking
Link the program with the "eventlog" version of the runtime system.
A program linked in this way can generate a runtime trace of events
(such as thread start/stop) to a binary file :file:`{program}.eventlog`,
which can then be interpreted later by various tools. See
:ref:`rts-eventlog` for more information.
:ghc-flag:`-eventlog` can be used with :ghc-flag:`-threaded`. It is implied by
:ghc-flag:`-debug`.
.. ghc-flag:: -rtsopts[=⟨none|some|all⟩]
:shortdesc: Control whether the RTS behaviour can be tweaked via command-line
flags and the ``GHCRTS`` environment variable. Using ``none``
means no RTS flags can be given; ``some`` means only a minimum
of safe options can be given (the default), and ``all`` (or no
argument at all) means that all RTS flags are permitted.
:type: dynamic
:category: linking
:default: all
This option affects the processing of RTS control options given
either on the command line or via the :envvar:`GHCRTS` environment
variable. There are three possibilities:
``-rtsopts=none``
Disable all processing of RTS options. If ``+RTS`` appears
anywhere on the command line, then the program will abort with
an error message. If the ``GHCRTS`` environment variable is set,
then the program will emit a warning message, ``GHCRTS`` will be
ignored, and the program will run as normal.
``-rtsopts=ignore``
Disables all processing of RTS options. Unlike ``none`` this treats
all RTS flags appearing on the command line the same way as regular
arguments. (Passing them on to your program as arguments).
``GHCRTS`` options will be processed normally.
``-rtsopts=ignoreAll``
Same as ``ignore`` but also ignores ``GHCRTS``.
``-rtsopts=some``
[this is the default setting] Enable only the "safe" RTS
options: (Currently only ``-?`` and ``--info``.) Any other RTS
options on the command line or in the ``GHCRTS`` environment
variable causes the program with to abort with an error message.
``-rtsopts=all`` or just ``-rtsopts``
Enable *all* RTS option processing, both on the command line and
through the ``GHCRTS`` environment variable.
In GHC 6.12.3 and earlier, the default was to process all RTS
options. However, since RTS options can be used to write logging
data to arbitrary files under the security context of the running
program, there is a potential security problem. For this reason, GHC
7.0.1 and later default to ``-rtsopts=some``.
Note that ``-rtsopts`` has no effect when used with :ghc-flag:`-no-hs-main`;
see :ref:`using-own-main` for details.
.. ghc-flag:: -with-rtsopts=⟨opts⟩
:shortdesc: Set the default RTS options to ⟨opts⟩.
:type: dynamic
:category: linking
This option allows you to set the default RTS options at link-time.
For example, ``-with-rtsopts="-H128m"`` sets the default heap size
to 128MB. This will always be the default heap size for this
program, unless the user overrides it. (Depending on the setting of
the ``-rtsopts`` option, the user might not have the ability to
change RTS options at run-time, in which case ``-with-rtsopts``
would be the *only* way to set them.)
Note that ``-with-rtsopts`` has no effect when used with
``-no-hs-main``; see :ref:`using-own-main` for details.
.. ghc-flag:: -no-rtsopts-suggestions
:shortdesc: Don't print RTS suggestions about linking with
:ghc-flag:`-rtsopts[=⟨none|some|all⟩]`.
:type: dynamic
:category: linking
This option disables RTS suggestions about linking with
:ghc-flag:`-rtsopts[=⟨none|some|all⟩]` when they are not available. These
suggestions would be unhelpful if the users have installed Haskell programs
through their package managers. With this option enabled, these suggestions
will not appear. It is recommended for people distributing binaries to
build with either ``-rtsopts`` or ``-no-rtsopts-suggestions``.
.. ghc-flag:: -fno-gen-manifest
:shortdesc: Do not generate a manifest file (Windows only)
:type: dynamic
:category: linking
On Windows, GHC normally generates a manifestmanifest file when
linking a binary. The manifest is placed in the file
:file:`{prog}.exe.manifest`` where ⟨prog.exe⟩ is the name of the
executable. The manifest file currently serves just one purpose: it
disables the "installer detection" in Windows
Vista that attempts to elevate privileges for executables with
certain names (e.g. names containing "install", "setup" or "patch").
Without the manifest file to turn off installer detection,
attempting to run an executable that Windows deems to be an
installer will return a permission error code to the invoker.
Depending on the invoker, the result might be a dialog box asking
the user for elevated permissions, or it might simply be a
permission denied error.
Installer detection can be also turned off globally for the system
using the security control panel, but GHC by default generates
binaries that don't depend on the user having disabled installer
detection.
The ``-fno-gen-manifest`` disables generation of the manifest file.
One reason to do this would be if you had a manifest file of your
own, for example.
In the future, GHC might use the manifest file for more things, such
as supplying the location of dependent DLLs.
:ghc-flag:`-fno-gen-manifest` also implies :ghc-flag:`-fno-embed-manifest`, see
below.
.. ghc-flag:: -fno-embed-manifest
:shortdesc: Do not embed the manifest in the executable (Windows only)
:type: dynamic
:category: linking
.. index::
single: windres
The manifest file that GHC generates when linking a binary on Windows is
also embedded in the executable itself, by default. This means that the
binary can be distributed without having to supply the manifest file too.
The embedding is done by running :command:`windres`; to see exactly what
GHC does to embed the manifest, use the :ghc-flag:`-v` flag. A GHC
installation comes with its own copy of ``windres`` for this reason.
See also :ghc-flag:`-pgmwindres ⟨cmd⟩` (:ref:`replacing-phases`) and
:ghc-flag:`-optwindres ⟨option⟩` (:ref:`forcing-options-through`).
.. ghc-flag:: -fno-shared-implib
:shortdesc: Don't generate an import library for a DLL (Windows only)
:type: dynamic
:category: linking
DLLs on Windows are typically linked to by linking to a
corresponding ``.lib`` or ``.dll.a`` — the so-called import library.
GHC will typically generate such a file for every DLL you create by
compiling in :ghc-flag:`-shared` mode. However, sometimes you don't want to
pay the disk-space cost of creating this import library, which can
be substantial — it might require as much space as the code itself,
as Haskell DLLs tend to export lots of symbols.
As long as you are happy to only be able to link to the DLL using
``GetProcAddress`` and friends, you can supply the
:ghc-flag:`-fno-shared-implib` flag to disable the creation of the import
library entirely.
.. ghc-flag:: -dylib-install-name ⟨path⟩
:shortdesc: Set the install name (via ``-install_name`` passed to Apple's
linker), specifying the full install path of the library file.
Any libraries or executables that link with it later will pick
up that path as their runtime search location for it.
(Darwin/OS X only)
:type: dynamic
:category: linking
On Darwin/OS X, dynamic libraries are stamped at build time with an
"install name", which is the ultimate install path of the library
file. Any libraries or executables that subsequently link against it
will pick up that path as their runtime search location for it. By
default, ghc sets the install name to the location where the library
is built. This option allows you to override it with the specified
file path. (It passes ``-install_name`` to Apple's linker.) Ignored
on other platforms.
.. ghc-flag:: -rdynamic
:shortdesc: This instructs the linker to add all symbols, not only used
ones, to the dynamic symbol table. Currently Linux and
Windows/MinGW32 only. This is equivalent to using
``-optl -rdynamic`` on Linux, and ``-optl -export-all-symbols``
on Windows.
:type: dynamic
:category: linking
This instructs the linker to add all symbols, not only used ones, to
the dynamic symbol table. Currently Linux and Windows/MinGW32 only.
This is equivalent to using ``-optl -rdynamic`` on Linux, and
``-optl -export-all-symbols`` on Windows.
.. ghc-flag:: -fwhole-archive-hs-libs
:shortdesc: When linking a binary executable, this inserts the flag
``-Wl,--whole-archive`` before any ``-l`` flags for Haskell
libraries, and ``-Wl,--no-whole-archive`` afterwards
:type: dynamic
:category: linking
When linking a binary executable, this inserts the flag
``-Wl,--whole-archive`` before any ``-l`` flags for Haskell
libraries, and ``-Wl,--no-whole-archive`` afterwards (on OS X, the
flag is ``-Wl,-all_load``, there is no equivalent for
``-Wl,--no-whole-archive``). This flag also disables the use of
``-Wl,--gc-sections`` (``-Wl,-dead_strip`` on OS X).
This is for specialist applications that may require symbols
defined in these Haskell libraries at runtime even though they
aren't referenced by any other code linked into the executable.
If you're using ``-fwhole-archive-hs-libs``, you probably also
want ``-rdynamic``.
.. ghc-flag:: -pie
:shortdesc: Instruct the linker to produce a position-independent executable.
:type: dynamic
:category: linking
:since: 8.2.2
This instructs the linker to produce a position-independent executable.
This flag is only valid while producing executables and all object code
being linked must have been produced with :ghc-flag:`-fPIE`.
Position independent executables are required by some platforms as they
enable address-space layout randomization (ASLR), a common security measure.
They can also be useful as they can be dynamically loaded and used as shared
libraries by other executables.
Position independent executables should be dynamically-linked (e.g. built
with :ghc-flag:`-dynamic` and only loaded into other dynamically-linked
executables to ensure that only one ``libHSrts`` is present if
loaded into the address space of another Haskell process.
Also, you may need to use the :ghc-flag:`-rdynamic` flag to ensure that
that symbols are not dropped from your PIE objects.