## 13 dlltool

dlltool is used to create the files needed to create dynamic link libraries (DLLs) on systems which understand PE format image files such as Windows. A DLL contains an export table which contains information that the runtime loader needs to resolve references from a referencing program.

The export table is generated by this program by reading in a .def file or scanning the .a and .o files which will be in the DLL. A .o file can contain information in special .drectve sections with export information.

> Note: dlltool is not always built as part of the binary utilities, since it is only useful for those targets which support DLLs.

         dlltool [-d|--input-def def-file-name]
                 [-b|--base-file base-file-name]
                 [-e|--output-exp exports-file-name]
                 [-z|--output-def def-file-name]
                 [-l|--output-lib library-file-name]
                 [--export-all-symbols] [--no-export-all-symbols]
                 [--exclude-symbols list]
                 [--no-default-excludes]
                 [-S|--as path-to-assembler] [-f|--as-flags options]
                 [-D|--dllname name] [-m|--machine machine]
                 [-a|--add-indirect] [-U|--add-underscore] [-k|--kill-at]
                 [-A|--add-stdcall-alias]
                 [-p|--ext-prefix-alias prefix]
                 [-x|--no-idata4] [-c|--no-idata5] [-i|--interwork]
                 [-n|--nodelete] [-t|--temp-prefix prefix]
                 [-v|--verbose]
                 [-h|--help] [-V|--version]
                 [object-file ...]

dlltool reads its inputs, which can come from the -d and -b options as well as object files specified on the command line. It then processes these inputs and if the -e option has been specified it creates a exports file. If the -l option has been specified it creates a library file and if the -z option has been specified it creates a def file. Any or all of the -e, -l and -z options can be present in one invocation of dlltool.

When creating a DLL, along with the source for the DLL, it is necessary to have three other files. dlltool can help with the creation of these files.

The first file is a .def file which specifies which functions are exported from the DLL, which functions the DLL imports, and so on. This is a text file and can be created by hand, or dlltool can be used to create it using the -z option. In this case dlltool will scan the object files specified on its command line looking for those functions which have been specially marked as being exported and put entries for them in the .def file it creates.

In order to mark a function as being exported from a DLL, it needs to have an -export:<name_of_function> entry in the .drectve section of the object file. This can be done in C by using the asm() operator:

           asm (".section .drectve");
           asm (".ascii \"-export:my_func\"");

           int my_func (void) { ... }

The second file needed for DLL creation is an exports file. This file is linked with the object files that make up the body of the DLL and it handles the interface between the DLL and the outside world. This is a binary file and it can be created by giving the -e option to dlltool when it is creating or reading in a .def file.

The third file needed for DLL creation is the library file that programs will link with in order to access the functions in the DLL. This file can be created by giving the -l option to dlltool when it is creating or reading in a .def file.

dlltool builds the library file by hand, but it builds the exports file by creating temporary files containing assembler statements and then assembling these. The -S command line option can be used to specify the path to the assembler that dlltool will use, and the -f option can be used to pass specific flags to that assembler. The -n can be used to prevent dlltool from deleting these temporary assembler files when it is done, and if -n is specified twice then this will prevent dlltool from deleting the temporary object files it used to build the library.

Here is an example of creating a DLL from a source file dll.c and also creating a program (from an object file called program.o) that uses that DLL:

           gcc -c dll.c
           dlltool -e exports.o -l dll.lib dll.o
           gcc dll.o exports.o -o dll.dll
           gcc program.o dll.lib -o program

The command line options have the following meanings:

-d filename
--input-def filename
  Specifies the name of a .def file to be read in and processed.
-b filename
--base-file filename
  Specifies the name of a base file to be read in and processed. The contents of this file will be added to the relocation section in the exports file generated by dlltool.
-e filename
--output-exp filename
  Specifies the name of the export file to be created by dlltool.
-z filename
--output-def filename
  Specifies the name of the .def file to be created by dlltool.
-l filename
--output-lib filename
  Specifies the name of the library file to be created by dlltool.
--export-all-symbols
  Treat all global and weak defined symbols found in the input object files as symbols to be exported. There is a small list of symbols which are not exported by default; see the --no-default-excludes option. You may add to the list of symbols to not export by using the --exclude-symbols option.
--no-export-all-symbols
  Only export symbols explicitly listed in an input .def file or in .drectve sections in the input object files. This is the default behaviour. The .drectve sections are created by dllexport attributes in the source code.
--exclude-symbols list
  Do not export the symbols in list. This is a list of symbol names separated by comma or colon characters. The symbol names should not contain a leading underscore. This is only meaningful when --export-all-symbols is used.
--no-default-excludes
  When --export-all-symbols is used, it will by default avoid exporting certain special symbols. The current list of symbols to avoid exporting is DllMain@12, DllEntryPoint@0, impure_ptr. You may use the --no-default-excludes option to go ahead and export these special symbols. This is only meaningful when --export-all-symbols is used.
-S path
--as path
  Specifies the path, including the filename, of the assembler to be used to create the exports file.
-f options
--as-flags options
  Specifies any specific command line options to be passed to the assembler when building the exports file. This option will work even if the -S option is not used. This option only takes one argument, and if it occurs more than once on the command line, then later occurrences will override earlier occurrences. So if it is necessary to pass multiple options to the assembler they should be enclosed in double quotes.
-D name
--dll-name name
  Specifies the name to be stored in the .def file as the name of the DLL when the -e option is used. If this option is not present, then the filename given to the -e option will be used as the name of the DLL.
-m machine
-machine machine
  Specifies the type of machine for which the library file should be built. dlltool has a built in default type, depending upon how it was created, but this option can be used to override that. This is normally only useful when creating DLLs for an ARM processor, when the contents of the DLL are actually encode using Thumb instructions.
-a
--add-indirect
  Specifies that when dlltool is creating the exports file it should add a section which allows the exported functions to be referenced without using the import library. Whatever the hell that means!
-U
--add-underscore
  Specifies that when dlltool is creating the exports file it should prepend an underscore to the names of the exported functions.
-k
--kill-at
  Specifies that when dlltool is creating the exports file it should not append the string @ <number>. These numbers are called ordinal numbers and they represent another way of accessing the function in a DLL, other than by name.
-A
--add-stdcall-alias
  Specifies that when dlltool is creating the exports file it should add aliases for stdcall symbols without @ <number> in addition to the symbols with @ <number>.
-p
--ext-prefix-alias prefix
  Causes dlltool to create external aliases for all DLL imports with the specified prefix. The aliases are created for both external and import symbols with no leading underscore.
-x
--no-idata4
  Specifies that when dlltool is creating the exports and library files it should omit the `.idata4` section. This is for compatibility with certain operating systems.
-c
--no-idata5
  Specifies that when dlltool is creating the exports and library files it should omit the `.idata5` section. This is for compatibility with certain operating systems.
-i
--interwork
  Specifies that dlltool should mark the objects in the library file and exports file that it produces as supporting interworking between ARM and Thumb code.
-n
--nodelete
  Makes dlltool preserve the temporary assembler files it used to create the exports file. If this option is repeated then dlltool will also preserve the temporary object files it uses to create the library file.
-t prefix
--temp-prefix prefix
  Makes dlltool use prefix when constructing the names of temporary assembler and object files. By default, the temp file prefix is generated from the pid.
-v
--verbose
  Make dlltool describe what it is doing.
-h
--help
  Displays a list of command line options and then exits.
-V
--version
  Displays dlltool's version number and then exits.