Chapter 13. GHC FAQ
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## Chapter 13. GHC FAQ

This section has the answers to questions that get asked regularly on the GHC mailing lists, in no particular order. Please let us know if you think there's a question/answer that should be added here.

How do I port GHC to platform X?

  There are two distinct possibilities: either

  - The hardware architecture for your system is already supported by GHC, but you're running an OS that isn't supported (or perhaps has been supported in the past, but currently isn't). This is the easiest type of porting job, but it still requires some careful bootstrapping.

  - Your system's hardware architecture isn't supported by GHC. This will be a more difficult port (though by comparison perhaps not as difficult as porting gcc).

  Both ways require you to bootstrap from intermediate `HC` files: these are the stylised C files generated by GHC when it compiles Haskell source. Basically the idea is to take the HC files for GHC itself to the target machine and compile them with `gcc` to get a working GHC, and go from there.

  The Building Guide has all the details on how to bootstrap GHC on a new platform.

Do I have to recompile all my code if I upgrade GHC?

  Yes. There are two reasons for this:

  - GHC does a lot of cross-module optimisation, so compiled code will include parts of the libraries it was compiled against (including the Prelude), so will be deeply tied to the actual version of those libraries it was compiled against. When you upgrade GHC, the libraries may change; even if the external interface of the libraries doesn't change, sometimes internal details may change because GHC optimised the code in the library differently.

  - We sometimes change the ABI (application binary interface) between versions of GHC. Code compiled with one version of GHC is not necessarily compatible with code compiled by a different version, even if you arrange to keep the same libraries.

Why doesn't GHC use shared libraries?

  The subject of shared libraries has come up several times in the past — take a look through the mailing-list archives for some of the previous discussions. The upshot is that shared libraries wouldn't really buy much unless you really need to save the disk space: in all other considerations, static linking comes out better.

  Unfortunately GHC-compiled libraries are very tightly coupled, which means it's unlikely you'd be able to swap out a shared library for a newer version unless it was compiled with exactly the same compiler and set of libraries as the old version.

I can't get string gaps to work

  If you're also using CPP, beware of the known pitfall with string gaps mentioned in Section 4.10.3.1, “CPP and string gaps”.

GHCi complains about missing symbols like `CC_LIST` when loading a previously compiled .o file.

  This probably means the .o files in question were compiled for profiling (with `-prof`). Workaround: recompile them without profiling. We really ought to detect this situation and give a proper error message.

Linking a program causes the following error on Linux: `/usr/bin/ld: cannot open -lgmp: No such file or directory`

  The problem is that your system doesn't have the GMP library installed. If this is a RedHat distribution, install the RedHat-supplied `gmp-devel` package, and the `gmp` package if you don't already have it. There have been reports that installing the RedHat packages also works for SuSE (SuSE don't supply a shared gmp library).

I Can't run GHCi on Linux, because it complains about a missing `libreadline.so.3`.

  The "correct" fix for this problem is to install the correct RPM for the particular flavour of Linux on your machine. If this isn't an option, however, there is a hack that might work: make a symbolic link from `libreadline.so.4` to `libreadline.so.3` in `/usr/lib`. We tried this on a SuSE 7.1 box and it seemed to work, but YMMV.

Solaris users may sometimes get link errors due to libraries needed by GNU Readline.

  We suggest you try linking in some combination of the `termcap`, `curses` and `ncurses` libraries, by giving `-ltermcap`, `-lcurses` and `-lncurses` respectively. If you encounter this problem, we would appreciate feedback on it, since we don't fully understand what's going on here.

The build fails in readline.

  It has been reported that if you have multiple versions of the readline library installed on Linux, then this may cause the build to fail. If you have multiple versions of readline, try uninstalling all except the most recent version.

When I try to start ghci (probably one I compiled myself) it says `ghc-5.02: not built for interactive use`

  To build a working ghci, you need to build GHC 5.02 with itself; the above message appears if you build it with 4.08.X, for example. It'll still work fine for batch-mode compilation, though. Note that you really must build with exactly the same version of the compiler. Building 5.02 with 5.00.2, for example, may or may not give a working interactive system; it probably won't, and certainly isn't supported. Note also that you can build 5.02 with any older compiler, back to 4.08.1, if you don't want a working interactive system; that's OK, and supported.

When I use a foreign function that takes or returns a float, it gives the wrong answer, or crashes.

  You should use the `-#include` option to bring the correct prototype into scope (see Section 4.10.5, “Options affecting the C compiler (if applicable)”).

My program that uses a really large heap crashes on Windows.

  For utterly horrible reasons, programs that use more than 128Mb of heap won't work when compiled dynamically on Windows (they should be fine statically compiled).

GHC doesn't like filenames containing `+`.

  Indeed not. You could change `+` to `p` or `plus`.

When I open a FIFO (named pipe) and try to read from it, I get EOF immediately.

  This is a consequence of the fact that GHC opens the FIFO in non-blocking mode. The behaviour varies from OS to OS: on Linux and Solaris you can wait for a writer by doing an explicit `threadWaitRead` on the file descriptor (gotten from `Posix.handleToFd`) before the first read, but this doesn't work on FreeBSD (although rumour has it that recent versions of FreeBSD changed the behaviour to match other OSs). A workaround for all systems is to open the FIFO for writing yourself, before (or at the same time as) opening it for reading.

When I `foreign import` a function that returns `char` or `short`, I get garbage back.

  This is a known bug in GHC versions prior to 5.02.2. GHC doesn't mask out the more significant bits of the result. It doesn't manifest with gcc 2.95, but apparently shows up with g++ and gcc 3.0.

My program is failing with `head []`, or an array bounds error, or some other random error, and I have no idea how to find the bug. Can you help?

  Compile your program with `-prof -auto-all` (make sure you have the profiling libraries installed), and run it with `+RTS -xc -RTS` to get a “stack trace” at the point at which the exception was raised. See Section 4.14.4, “RTS options for hackers, debuggers, and over-interested souls” for more details.

How do I increase the heap size permanently for a given binary?

  See Section 4.14.5, ““Hooks” to change RTS behaviour”.

I'm trying to compile my program for parallel execution with the `-parallel`, and GHC complains with an error like “failed to load interface file for Prelude”.

  GHC doesn't ship with support for parallel execution, that support is provided separately by the GPH project.

When is it safe to use `unsafePerformIO`?

  We'll give two answers to this question, each of which may be helpful. These criteria are not rigorous in any real sense (you'd need a formal semantics for Haskell in order to give a proper answer to this question), but should give you a feel for the kind of things you can and cannot do with `unsafePerformIO`.

  - It is safe to implement a function or API using `unsafePerformIO` if you could imagine also implementing the same function or API in Haskell without using `unsafePerformIO` (forget about efficiency, just consider the semantics).

  - In pure Haskell, the value of a function depends only on the values of its arguments (and free variables, if it has any). If you can implement the function using `unsafePerformIO` and still retain this invariant, then you're probably using `unsafePerformIO` in a safe way. Note that you need only consider the observable values of the arguments and result.

  For more information, see this thread.

Why does linking take so long?

  Linking a small program should take no more than a few seconds. Larger programs can take longer, but even linking GHC itself only takes 3-4 seconds on our development machines.

  Long link times have been attributed to using Sun's linker on Solaris, as compared to GNU ld which appears to be much faster. So if you're on a Sun box, try switching to GNU ld. This article from the mailing list has more information.

If I explicitly set the buffering on a Handle to "NoBuffering" I'm not able to enter EOF by typing "Ctrl-D".

  This is a consequence of Unixy terminal semantics. Unix does line buffering on terminals in the kernel as part of the terminal processing, unless you turn it off. However, the Ctrl-D processing is also part of the terminal processing which gets turned off when the kernel line buffering is disabled. So GHC tries its best to get NoBuffering semantics by turning off the kernel line buffering, but as a result you lose Ctrl-D. C'est la vie.

If I print out a string using `putStr`, and then attempt to read some input using `hGetLine`, I don't see the output from the `putStr`.

  The `stdout` handle is line-buffered by default, which means that output sent to the handle is only flushed when a newline (`/n`) is output, the buffer is full, or `hFlush` is called on the Handle. The right way to make the text appear without sending a newline is to use `hFlush`:

        import System.IO
        main = do
          putStr "how are you today? "
          hFlush stdout
          input &- hGetLine
          ...

  You'll probably find that the behaviour differs when using GHCi: the `hFlush` isn't necessary to make the text appear. This is because in GHCi we turn off the buffering on `stdout`, because this is normally what you want in an interpreter: output appears as it is generated.

I can't get finalizers to work properly. My program sometimes just prints `<<loop>>`.

  Chances are that your program is trying to write a message to `stdout` or `stderr` in the finalizer. Handles have finalizers themselves, and since finalizers don't keep other finalized values alive, the `stdout` and `stderr` Handles may be finalized before your finalizer runs. If this happens, your finalizer will block on the handle, and probably end up receiving a `NonTermination` exception (which is printed as `<<loop>>`).

Does GHC implement any kind of extensible records?

  No, extensible records are not implemented in GHC. Hugs implements TRex, one extensible record variant. The problem is that the record design space is large, and seems to lack local optima. And all reasonable variants break backward compatibility. As a result, nothing much happens.

Why do I get errors about missing include files when compiling with `-O` or `-prof`?

  Certain options, such as `-O`, turn on via-C compilation, instead of using the native code generator. Include files named by `-#include` options or in `foreign import` declarations are only used in via-C compilation mode. See Section 8.2.2.1, “Finding Header files” for more details.

How do I compile my program for profiling without overwriting the object files and `hi` files I've already built?

  You can select alternative suffixes for object files and interface files, so you can have several builds of the same code coexisting in the same directory. For example, to compile with profiling, you might do this:

      ghc --make -prof -o foo-prof -osuf p.o -hisuf p.hi Main

  See Section 4.6.4, “Redirecting the compilation output(s)” for more details on the `-osuf` and `-hisuf` options.

I can't use readline under GHCi on Windows

  In order to load the readline package under GHCi on Windows, you need to make a version of the readline library that GHCi can load. Instructions for GHC 6.2.2. are here.

Ctrl-C doesn't work on Windows

  When running GHC under a Cygwin shell on Windows, Ctrl-C sometimes doesn't work. The workaround is to use Ctrl-Break instead.

I get an error message from GHCi about a "duplicate definition for symbol __module_registered"

  An error message like this:

      GHCi runtime linker: fatal error: I found a duplicate definition for symbol
             __module_registered
          whilst processing object file
             /usr/local/lib/ghc-6.2/HSfgl.o

  probably indicates that when building a library for GHCi (`HSfgl.o` in the above example), you should use the `-x` option to `ld`.

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