Programming Questions
=====================

How do I contribute a package?
------------------------------

If you are willing to be a package maintainer, great.  We urgently need volunteers to prepare and maintain packages, because the priority of the Cygwin Team is Cygwin itself.

There will be a separate web page where all the details are documented, but this is not prepared yet.  Meanwhile, pore through the cygwin-apps mailing archives (start at http://cygwin.com/lists.html), and subscribe.  Charles Wilson posted a short recipe of what's involved, using texinfo as an example, at http://cygwin.com/ml/cygwin-apps/2000-11/msg00055.html.  This should give you an idea of what is required.

You should announce your intentions to the general cygwin list, in case others were thinking the same thing.

How do I contribute to Cygwin?
------------------------------

If you want to contribute to Cygwin itself, see http://cygwin.com/contrib.html.

Why are compiled executables so huge?!?
---------------------------------------

By default, gcc compiles in all symbols.  You'll also find that gcc creates large executables on UNIX.

If that bothers you, just use the 'strip' program, part of the binutils package.  Or compile with the -s option to gcc.

Where is glibc?
---------------

Cygwin does not provide glibc.  It uses newlib instead, which provides much (but not all) of the same functionality.  Porting glibc to Cygwin would be difficult.

Where is Objective C?
---------------------

Objective C is not distributed with the Cygwin version of gcc, and there are no plans to do so.  The gcc package maintainer had difficulty building it, and once built there were problems using it.  It appears that there is only minimual support for the Objective C front-end in the main GCC distribution, anyway.

Why is make behaving badly?
---------------------------

First of all, if you are using make -j[N], then stop.  It doesn't work well.

Otherwise, read on...

Make has two operating modes, UNIX and WIN32.  You need to make sure that you are operating in the right mode.

In UNIX mode, make uses sh.exe as a subshell.  The path list separator is ':', '\' is the escape character, POSIX paths are expected, and Cygwin mounts will be understood.  Use this for Makefiles written for UNIX.

In WIN32 mode, make uses the "native" command shell (cmd.exe or command.com), with all the restrictions that implies.  The path list separator is ';', the path separator is '\', "copy" and "del" work, but the Cygwin mount table is not understood.  Use this for nmake-style Makefiles.

The default mode for the Net Release of make (the one installed by setup.exe) is UNIX.  The default mode for commercial releases to Redhat (formerly Cygnus) customers is WIN32.

You can override the default by setting the environment variable MAKE_MODE to "UNIX" (actually case is not significant) or "WIN32" (actually anything other than "UNIX").  You can also specify the options --unix or --win32 on the make command line.

Why the undefined reference to WinMain@16?
------------------------------------------

Try adding an empty main() function to one of your sources.

Or, perhaps you have -lm too early in the link command line.  It should be at the end:

         bash$ gcc hello.c -lm
         bash$ ./a.exe
         Hello World!

works, but

         bash$  gcc -lm hello.c
         /c/TEMP/ccjLEGlU.o(.text+0x10):hello.c: multiple definition of `main'
         /usr/lib/libm.a(libcmain.o)(.text+0x0):libcmain.c: first defined here
         /usr/lib/libm.a(libcmain.o)(.text+0x6a):libcmain.c: undefined reference to `WinMain@16'
         collect2: ld returned 1 exit status

This is an artifact of libm.a being a symbolic link to libcygwin.a.

How do I use Win32 API calls?
-----------------------------

(Please note: This section has not yet been updated for the latest net release.)

It's pretty simple actually.  Cygwin tools require that you explicitly link the import libraries for whatever Win32 API functions that you are going to use, with the exception of kernel32, which is linked automatically (because the startup and/or built-in code uses it).

For example, to use graphics functions (GDI) you must link with gdi32 like this:

gcc -o foo.exe foo.o bar.o -lgdi32

or (compiling and linking in one step):

gcc -o foo.exe foo.c bar.c -lgdi32

The following libraries are available for use in this way:

advapi32  largeint  ole32     scrnsave  vfw32
cap       lz32      oleaut32  shell32   win32spl
comctl32  mapi32    oledlg    snmp      winmm
comdlg32  mfcuia32  olepro32  svrapi    winserve
ctl3d32   mgmtapi   opengl32  tapi32    winspool
dlcapi    mpr       penwin32  th32      winstrm
gdi32     msacm32   pkpd32    thunk32   wow32
glaux     nddeapi   rasapi32  url       wsock32
glu32     netapi32  rpcdce4   user32    wst
icmp      odbc32    rpcndr    uuid
imm32     odbccp32  rpcns4    vdmdbg
kernel32  oldnames  rpcrt4    version

The regular setup allows you to use the option -mwindows on the command line to include a set of the basic libraries (and also make your program a GUI program instead of a console program), including user32, gdi32 and, IIRC, comdlg32.

Note that you should never include -lkernel32 on your link line unless you are invoking ld directly.  Do not include the same import library twice on your link line.  Finally, it is a good idea to put import libraries last on your link line, or at least after all the object files and static libraries that reference them.

The first two are related to problems the linker has (as of b18 at least) when import libraries are referenced twice.  Tables get messed up and programs crash randomly.  The last point has to do with the fact that gcc processes the files listed on the command line in sequence and will only resolve references to libraries if they are given after the file that makes the reference.

How do I compile a Win32 executable that doesn't use Cygwin?
------------------------------------------------------------

The -mno-cygwin flag to gcc makes gcc link against standard Microsoft DLLs instead of Cygwin.  This is desirable for native Windows programs that don't need a UNIX emulation layer.

This is not to be confused with 'MinGW' (Minimalist GNU for Windows), which is a completely separate effort.  That project's home page is http://www.mingw.org/index.shtml.

Can I build a Cygwin program that does not require cygwin1.dll at runtime?
--------------------------------------------------------------------------

No.  If your program uses the Cygwin API, then your executable cannot run without cygwin1.dll.  In particular, it is not possible to statically link with a Cygwin library to obtain an independent, self-contained executable.

If this is an issue because you intend to distribute your Cygwin application, then you had better read and understand http://cygwin.com/licensing.html, which explains the licensing options.  Unless you purchase a special commercial license from Red Hat, then your Cygwin application must be Open Source.

Can I link with both MSVCRT*.DLL and cygwin1.dll?
-------------------------------------------------

No, you must use one or the other, they are mutually exclusive.

How do I make the console window go away?
-----------------------------------------

The default during compilation is to produce a console application. It you are writing a GUI program, you should either compile with -mwindows as explained above, or add the string "-Wl,--subsystem,windows" to the GCC commandline.

Why does make complain about a "missing separator"?
---------------------------------------------------

This problem usually occurs as a result of someone editing a Makefile with a text editor that replaces tab characters with spaces.  Command lines must start with tabs.  This is not specific to Cygwin.

Why can't we redistribute Microsoft's Win32 headers?
----------------------------------------------------

Subsection 2.d.f of the `Microsoft Open Tools License agreement' looks like it says that one may not "permit further redistribution of the Redistributables to their end users".  We take this to mean that we can give them to you, but you can't give them to anyone else, which is something that Cygnus (err... Red Hat) can't agree to.  Fortunately, we have our own Win32 headers which are pretty complete.

How do I link against .lib files?
---------------------------------

(Please note: This section has not yet been updated for the latest net release.)

1. Build a C file with a function table.  Put all functions you intend to use in that table.  This forces the linker to include all the object files from the .lib.  Maybe there is an option to force LINK.EXE to include an object file. 2. Build a dummy 'LibMain'. 3. Build a .def with all the exports you need. 4. Link with your .lib using link.exe.

or

1. Extract all the object files from the .lib using LIB.EXE. 2. Build a dummy C file referencing all the functions you need, either with a direct call or through an initialized function pointer. 3. Build a dummy LibMain. 4. Link all the objects with this file+LibMain. 5. Write a .def. 6. Link.

You can use these methods to use MSVC (and many other runtime libs) with Cygwin development tools.

Note that this is a lot of work (half a day or so), but much less than rewriting the runtime library in question from specs...

(thanks to Jacob Navia (root@jacob.remcomp.fr) for this explanation)

How do I rebuild the tools on my NT box?
----------------------------------------

Install all required components in one directory (we'll call it /src). Ideally, you should check out what you need from CVS (http://cygwin.com/cvs.html).  This is the preferred method for acquiring the sources.  Otherwise, you can install the appropriate source packages from the cygwin distribution.

As of this writing, you need to install at least the cygwin source package and the w32api source package.  The winsup source package should be extracted first and the w32api source directory should be extracted into the resulting 'winsup' directory.  Then rename w32api-whatever to w32api.

It is possible that the cygwin source package may require a newer version of the w32api package since the release of the packages is not always in lock step (another reason to just use CVS).

You must build cygwin in a separate directory from the source. So, create something like a /obj directory.  You'll be performing your build in that directory:

     bash
     cd /obj
     /src/configure --prefix=/install -v > configure.log 2>&1
     make > make.log 2>&1
     make install > install.log 2>&1

Normally, this procedure will also attempt to build the documentation, which additionally requires db2html, and possibly other tools, which are not included in the Cygwin distribution.  You can get db2html as part of docbook, from http://sources.redhat.com/docbook-tools/.

To check a cygwin1.dll, run "make check" in the winsup/cygwin directory. If that works, install everything except the dll (if you can). Then, close down all cygwin programs (including bash windows, inetd, etc.), save your old dll, and copy the new dll to all the places where the old dll was (if there is more than one on your machine).  Then start up a bash window and see what happens.  (Or better, run a cygwin program from the Windows command prompt.)

If you get the error "shared region is corrupted" it means that two different versions of cygwin1.dll are running on your machine at the same time.

How can I compile a powerpc NT toolchain?
-----------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

Unfortunately, this will be difficult.  It hasn't been built for some time (late 1996) since Microsoft has dropped development of powerpc NT.  Exception handling/signals support semantics/args have been changed for x86 and not updated for ppc so the ppc specific support would have to be rewritten.  We don't know of any other incompatibilities. Please send us patches if you do this work!

How can I compile an Alpha NT toolchain?
----------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

We have not ported the tools to Alpha NT and do not have plans to do so at the present time.  We would be happy to add support for Alpha NT if someone contributes the changes to us.

How can I adjust the heap/stack size of an application?
-------------------------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

Pass heap/stack linker arguments to gcc.  To create foo.exe with a heap size of 1024 and a stack size of 4096, you would invoke gcc as:

gcc -Wl,--heap,1024,--stack,4096 -o foo foo.c

How can I find out which dlls are needed by an executable?
----------------------------------------------------------

objdump -p provides this information, but is rather verbose.

cygcheck will do this much more concisely, and operates recursively, provided the command is in your path.

Note there is currently a bug in cygcheck in that it will not report on a program in a Windows system dir (e.g., C:\Windows or C:\WINNT) even if it's in your path.  To work around this, supply the full Win32 path to the executable, including the .exe extension:

     cygcheck c:\\winnt\\system32\\cmd.exe

(Note the windows path separator must be escaped if this is typed in bash.)

How do I build a DLL?
---------------------

(Please note: This section has not yet been updated for the latest net release.)

There's documentation that explains the process on the main Cygwin project web page (http://cygwin.com/).

How can I set a breakpoint at MainCRTStartup?
---------------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

Set a breakpoint at *0x401000 in gdb and then run the program in question.

How can I build a relocatable dll?
----------------------------------

(Please note: This section has not yet been updated for the latest net release.  However, there was a discussion on the cygwin mailing list recently that addresses this issue.  Read http://cygwin.com/ml/cygwin/2000-06/msg00688.html and related messages.)

You must execute the following sequence of five commands, in this order:

     $(LD) -s --base-file BASEFILE --dll -o DLLNAME OBJS LIBS -e ENTRY

     $(DLLTOOL) --as=$(AS) --dllname DLLNAME --def DEFFILE \
             --base-file BASEFILE --output-exp EXPFILE

     $(LD) -s --base-file BASEFILE EXPFILE -dll -o DLLNAME OBJS LIBS -e ENTRY

     $(DLLTOOL) --as=$(AS) --dllname DLLNAME --def DEFFILE \
     	--base-file BASEFILE --output-exp EXPFILE

     $(LD) EXPFILE --dll -o DLLNAME OBJS LIBS -e ENTRY

In this example, $(LD) is the linker, ld.

$(DLLTOOL) is dlltool.

$(AS) is the assembler, as.

DLLNAME is the name of the DLL you want to create, e.g., tcl80.dll.

OBJS is the list of object files you want to put into the DLL.

LIBS is the list of libraries you want to link the DLL against.  For example, you may or may not want -lcygwin.  You may want -lkernel32. Tcl links against -lcygwin -ladvapi32 -luser32 -lgdi32 -lcomdlg32 -lkernel32.

DEFFILE is the name of your definitions file.  A simple DEFFILE would consist of ``EXPORTS'' followed by a list of all symbols which should be exported from the DLL.  Each symbol should be on a line by itself. Other programs will only be able to access the listed symbols.

BASEFILE is a temporary file that is used during this five stage process, e.g., tcl.base.

EXPFILE is another temporary file, e.g., tcl.exp.

ENTRY is the name of the function which you want to use as the entry point.  This function should be defined using the WINAPI attribute, and should take three arguments: int WINAPI startup (HINSTANCE, DWORD, LPVOID)

This means that the actual symbol name will have an appended @12, so if your entry point really is named startup, the string you should use for ENTRY in the above examples would be startup@12.

If your DLL calls any Cygwin API functions, the entry function will need to initialize the Cygwin impure pointer.  You can do that by declaring a global variable _impure_ptr, and then initializing it in the entry function.  Be careful not to export the global variable _impure_ptr from your DLL; that is, do not put it in DEFFILE.

     /* This is a global variable.  */
     struct _reent *_impure_ptr;
     extern struct _reent *__imp_reent_data;

     int entry (HINSTANT hinst, DWORD reason, LPVOID reserved)
     {
       _impure_ptr = __imp_reent_data;
       /* Whatever else you want to do.  */
     }

You may put an optional `--subsystem windows' on the $(LD) lines.  The Tcl build does this, but I admit that I no longer remember whether this is important.  Note that if you specify a --subsytem <x> flag to ld, the -e entry must come after the subsystem flag, since the subsystem flag sets a different default entry point.

You may put an optional `--image-base BASEADDR' on the $(LD) lines. This will set the default image base.  Programs using this DLL will start up a bit faster if each DLL occupies a different portion of the address space.  Each DLL starts at the image base, and continues for whatever size it occupies.

Now that you've built your DLL, you may want to build a library so that other programs can link against it.  This is not required: you could always use the DLL via LoadLibrary.  However, if you want to be able to link directly against the DLL, you need to create a library. Do that like this:

$(DLLTOOL) --as=$(AS) --dllname DLLNAME --def DEFFILE --output-lib LIBFILE

$(DLLTOOL), $(AS), DLLNAME, and DEFFILE are the same as above.  Make sure you use the same DLLNAME and DEFFILE, or things won't work right.

LIBFILE is the name of the library you want to create, e.g., libtcl80.a.  You can then link against that library using something like -ltcl80 in your linker command.

How can I debug what's going on?
--------------------------------

You can debug your application using gdb.  Make sure you compile it with the -g flag!  If your application calls functions in MS dlls, gdb will complain about not being able to load debug information for them when you run your program.  This is normal since these dlls don't contain debugging information (and even if they did, that debug info would not be compatible with gdb).

Can I use a system trace mechanism instead?
-------------------------------------------

Yes.  You can use the strace.exe utility to run other cygwin programs with various debug and trace messages enabled.  For information on using strace, see the Cygwin User's Guide or the file winsup/utils/utils.sgml.

Why doesn't gdb handle signals?
-------------------------------

Unfortunately, there is only minimal signal handling support in gdb currently.  Signal handling only works with Windows-type signals. SIGINT may work, SIGFPE may work, SIGSEGV definitely does.  You cannot 'stop', 'print' or 'nopass' signals like SIGUSR1 or SIGHUP to the process being debugged.

The linker complains that it can't find something.
--------------------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

A common error is to put the library on the command line before the thing that needs things from it.

This is wrong gcc -lstdc++ hello.cc. This is right gcc hello.cc -lstdc++.

I use a function I know is in the API, but I still get a link error.
--------------------------------------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

The function probably isn't declared in the header files, or the UNICODE stuff for it isn't filled in.

Can you make DLLs that are linked against libc ?
------------------------------------------------

(Please note: This section has not yet been updated for the latest net release.)

Yes.

Where is malloc.h?
------------------

(Please note: This section has not yet been updated for the latest net release.)

Include stdlib.h instead of malloc.h.

Can I use my own malloc?
------------------------

If you define a function called malloc in your own code, and link with the DLL, the DLL will call your malloc.  Needless to say, you will run into serious problems if your malloc is buggy.

If you run any programs from the DOS command prompt, rather than from in bash, the DLL will try and expand the wildcards on the command line. This process uses malloc before your main line is started. If you have written your own malloc to need some initialization to occur after main is called, then this will surely break.

Moreover, there is an outstanding issue with _malloc_r in newlib.  This re-entrant version of malloc will be called directly from within newlib, by-passing your custom version, and is probably incompatible with it.  But it may not be possible to replace _malloc_r too, because cygwin1.dll does not export it and Cygwin does not expect your program to replace it.  This is really a newlib issue, but we are open to suggestions on how to deal with it.

Can I mix objects compiled with msvc++ and gcc?
-----------------------------------------------

Yes, but only if you are combining C object files.  MSVC C++ uses a different mangling scheme than GNU C++, so you will have difficulties combining C++ objects.

Can I use the gdb debugger to debug programs built by VC++?
-----------------------------------------------------------

No, not for full (high level source language) debugging. The Microsoft compilers generate a different type of debugging symbol information, which gdb does not understand.

However, the low-level (assembly-type) symbols generated by Microsoft compilers are coff, which gdb DOES understand. Therefore you should at least be able to see all of your global symbols; you just won't have any information about data types, line numbers, local variables etc.

Where can I find info on x86 assembly?
--------------------------------------

CPU reference manuals for Intel's current chips are available in downloadable PDF form on Intel's web site:

http://developer.intel.com/design/pro/manuals/

Shell scripts aren't running properly from my makefiles?
--------------------------------------------------------

If your scripts are in the current directory, you must have . (dot) in your $PATH.  (It is not normally there by default.)  Otherwise, you would need to add /bin/sh in front of each and every shell script invoked in your Makefiles.

What preprocessor do I need to know about?
------------------------------------------

We use _WIN32 to signify access to the Win32 API and __CYGWIN__ for access to the Cygwin environment provided by the dll.

We chose _WIN32 because this is what Microsoft defines in VC++ and we thought it would be a good idea for compatibility with VC++ code to follow their example.  We use _MFC_VER to indicate code that should be compiled with VC++.

_WIN32 is only defined when you use either the -mno-cygwin or -mwin32 gcc command line options.  This is because Cygwin is supposed to be a Unix emulation environment and defining _WIN32 confuses some programs which think that they have to make special concessions for a Windows environment which Cygwin handles automatically.

How should I port my Unix GUI to Windows?
-----------------------------------------

There are two basic strategies for porting Unix GUIs to Windows.

The first is to use a portable graphics library such as tcl/tk, X11, or V (and others?).  Typically, you will end up with a GUI on Windows that requires some runtime support.  With tcl/tk, you'll want to include the necessary library files and the tcl/tk DLLs.  In the case of X11, you'll need everyone using your program to have an X11 server installed.

The second method is to rewrite your GUI using Win32 API calls (or MFC with VC++).  If your program is written in a fairly modular fashion, you may still want to use Cygwin if your program contains a lot of shared (non-GUI-related) code.  That way you still gain some of the portability advantages inherent in using Cygwin.

Why not use DJGPP ?
-------------------

DJGPP is a similar idea, but for DOS instead of Win32.  DJGPP uses a "DOS extender" to provide a more reasonable operating interface for its applications.   The Cygwin toolset doesn't have to do this since all of the applications are native WIN32.   Applications compiled with the Cygwin tools can access the Win32 API functions, so you can write programs which use the Windows GUI.

You can get more info on DJGPP by following http://www.delorie.com/.