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]
[-y|--output-delaylib 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] [--add-stdcall-underscore]
[-k|--kill-at] [-A|--add-stdcall-alias]
[-p|--ext-prefix-alias prefix]
[-x|--no-idata4] [-c|--no-idata5]
[--use-nul-prefixed-import-tables]
[-I|--identify library-file-name] [--identify-strict]
[-i|--interwork]
[-n|--nodelete] [-t|--temp-prefix prefix]
[-v|--verbose]
[-h|--help] [-V|--version]
[--no-leading-underscore] [--leading-underscore]
[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 (an `import library'). This file can be created by giving the -l option to dlltool when it is creating or reading in a .def file.
If the -y option is specified, dlltool generates a delay-import library that can be used instead of the normal import library to allow a program to link to the dll only as soon as an imported function is called for the first time. The resulting executable will need to be linked to the static delayimp library containing __delayLoadHelper2(), which in turn will import LoadLibraryA and GetProcAddress from kernel32.
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
dlltool may also be used to query an existing import library to determine the name of the DLL to which it is associated. See the description of the -I or --identify option.
The command line options have the following meanings:
This does not change the naming of symbols provided by the import library
to programs linked against it, but only the entries in the import table
(ie the .idata section).
.idata4 section. This is for compatibility
with certain operating systems.
.idata4 and .idata5 by zero an
element. This emulates old gnu import library generation of
dlltool. By default this option is turned off.
.idata5 section. This is for compatibility
with certain operating systems.
stdout, the name(s)
of the associated DLL(s). This can be performed in addition to any
other operations indicated by the other options and arguments.
dlltool fails if the import library does not exist or is not
actually an import library. See also --identify-strict.