I'm reading a tutorial about C development with the SDL library on MingW and Windows. (The tutorial is actually about C++ but I'm assuming installation and building is the same).
The tutorial gives the following command for building the program:
g++ 01_hello_SDL.cpp -IC:\mingw_dev_lib\include\SDL2 -LC:\mingw_dev_lib\lib -w -Wl,-subsystem,windows -lmingw32 -lSDL2main -lSDL2 -o 01_hello_SDL
In this command we use -L and -l to tell g++ where to find .a files (not .DLL or .so files) for linking. However, as far as I understand - this command is supposed to dynamically link the library. And as we know, .a files are static library files. What am I missing?
Why are we not linking with the .DLL file of the library, but instead we link with these .a files? When executing, the .DLL file will have to be present near the executable, but the .a won't have to as far as I understand. Again, what am I missing?
-l is not only for dynamic libraries. It can also be used to link static libraries.
On Linux, it is normal to directly link with a .so file (equivalent to a .dll). On Windows, it is still possible to do this (I think), but it is more normal to link with a .lib file called an import library, which wraps the .dll (you link to the .lib and the .lib links to the .dll). Since you are not using the Microsoft toolchain, it's possible that your toolchain still uses import libraries, but calls them .a files instead of .lib files.
Related
Basically when using SDL2 library why do I need to use both SDL2.dll (dynamic) file and .a (static) for compilation?
For an example to compile a simple program I need to tell gcc where lib files (static .a) are located to compile the program. Why not just use the .dll file instead?
My first thought is that .a files are needed for the compiler to check if program can compile with the library and .dll is only needed when running the program (and program is not statically linked in the end), but still that wouldn't explain why do i need the .a files instead of just .dll file.
example:
gcc -I src/include src/lib -o main.exe main.c -lmingw32 -lSDL2main -lSDL2
There are two .a files: libSDL2.a and libSDL2.dll.a (not counting libSDL2main.a, which is always static).
The first one is a true static library. -lSDL2 doesn't prefer it by default, it prefers libSDL2.dll.a. And if you force it to use the former, because the latter is unavailable, the resulting app won't depend on SDL2.dll.
The second one is an import library. In the old days, MinGW couldn't link against a .dll directly, and you had to use those. Modern MinGW can link .dlls directly, and those should in theory be unnecessary.
I'd still recommend using the import library if it's available, just because it's more common and more widely tested.
I am using gcc 8.1.0 on Windows. To install it I set up Code::Blocks on my computer and updated the environment variable list by adding the path to the gcc.exe program within the installation folder of CodeBlocks. The file editor I used was the built-in editor in Visual Studio. The terminal to compile was the power shell from Visual Studio as well.
In the library development folder I have the files mul.c and mul.h. Their content is irrelevant.
To compile the library I use the command:
gcc -c mul.c
When I run it, it creates a file object mul.o and not mul.lib. I needed to use the option -o mul.lib to successfully create the desired extension file. After placing the header, the .lib file and the main.c in the same parent folder I am obvioudly able to build the executable by running.
gcc main.c -I./include -L/static -lmul -o my_program.exe
I have two questions:
Why does gcc produces a .o if I am in a Windows environment?
I followed a tutorial that compile the static library under Linux and it names it libmul.o, in this way the -lmul option is able to retrieve the library. But if I call my generated static library libul.lib it generates the error:
C:/Program Files/CodeBlocks/MinGW/bin/../lib/gcc/x86_64-w64-ingw32/8.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lmul
collect2.exe: error: ld returned 1 exit status
Are these a normal behaviours by gcc, or is it side effect of making gcc available just by updating the Windows environmental variables list?
Thank you to the community in advance.
GCC comes from the *nix world where libraries have the .a extension. When using GCC+MinGW this remains the case.
Shared libraries in MinGW are .dll files but their libraries for linking are .dll.a files.
The advantage of .a files is that a lot of sources build out of the box on Windows with MinGW, especially when using MSYS2 shell.
If you use -l it will look for .a (or .dll.a for shared build) file adding the lib prefix and the extension automatically.
So -lmul will look for libmul.a (static, e.g. when --static linker flag is given) or libmul.dll.a (shared).
By the way, you are using quite an old GCC 8.1.0.
As I write this current version is 12.2.0. Check https://winlibs.com/ for a standalone download (instructions on how to configure in Code::Blocks are on the site) or use MSYS2's package manager pacman.
I have a JNI project, which I have to make work on Windows (I am working on Linux). This project actually depends on third-party library file which is static (archived i.e .a files). I am trying to create a JNI shared library file using i686-w64-mingw32-g++ and including -static followed by static third-party library name. Following is the command I am using
i686-w64-mingw32-g++ -v -L./ -L/home/user/jre1.8.0_40/lib/amd64/ -I/user/all/apps/Linux2/x86_64/gcc/4.8.2/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.8.2/include -shared -o test.dll test.cpp -lstdc++ -static -thirdparty
In-spite of placing the third party library in the current working directory, I keep getting error
/user/all/apps/Linux2/src/mxe/2013_12_03/usr/bin/../lib/gcc/i686-w64-mingw32/4.8.1/../../../../i686-w64-mingw32/bin/ld: cannot find -thirdparty
Please note : I included -I/user/all/apps/Linux2/x86_64/gcc/4.8.2/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.8.2/include to avoid the error cannot find jni.h which I hit before including the path.
I also tried to compile using gcc, in place of g++.
Do I need to create .dll of this third-party library(currently it is archived .a containing .obj files)?
Being a newbie in cross compilers, I might be doing something wrong. Please correct me and any suggestions with this will be very helpful. Thank you.
-Wl,--export-all-symbols -Wl,--add-stdcall-alias -v adding this solved my problem
I'm currently trying to port a C application to AIX and am getting confused. What are .a and .so files and how are they used when building/running an application?
Archive libraries (.a) are statically linked i.e when you compile your program with -c option in gcc. So, if there's any change in library, you need to compile and build your code again.
The advantage of .so (shared object) over .a library is that they are linked during the runtime i.e. after creation of your .o file -o option in gcc. So, if there's any change in .so file, you don't need to recompile your main program.
But make sure that your main program is linked to the new .so file with ln command.
This will help you to build the .so files.
http://www.yolinux.com/TUTORIALS/LibraryArchives-StaticAndDynamic.html
.a are static libraries. If you use code stored inside them, it's taken from them and embedded into your own binary. In Visual Studio, these would be .lib files.
.so are dynamic libraries. If you use code stored inside them, it's not taken and embedded into your own binary. Instead it's just referenced, so the binary will depend on them and the code from the so file is added/loaded at runtime. In Visual Studio/Windows these would be .dll files (with small .lib files containing linking information).
.a files are usually libraries which get statically linked (or more accurately archives), and
.so are dynamically linked libraries.
To do a port you will need the source code that was compiled to make them, or equivalent files on your AIX machine.
They are used in the linking stage. .a files are statically linked, and .so files are sort-of linked, so that the library is needed whenever you run the exe.
You can find where they are stored by looking at any of the lib directories... /usr/lib and /lib have most of them, and there is also the LIBRARY_PATH environment variable.
Wikipedia is a decent source for this info.
To learn about static library files like .a read Static libarary
To learn about shared library files like .so read Library_(computing)#Shared_libraries On this page, there is also useful info in the File naming section.
Are library files .o or .exe files in C?
Neither; generally .o files are object files and .exe files are fully-linked binaries (on Windows).
Static libraries in Linux are .a
Dynamic libraries in Linux are .so
Static libraries in Windows are .lib
Dynamic libraries in Windows are .dll
It's more operating system dependent than language dependent.
In Windows, they are likely to be .dll files.
In Linux, they are likely to be .a or .so files.
In OS X, they are likely to be .a, .so or .dylib files.
Neither. It also depends on the platform. Also, the file extension is only convention and libraries can have any other or no extension at all.
The answer is libraries are neither *.o or *.exe. Also the naming convention depends on the Platform you are compiling.
A *.so file is a shared lib. *.a is a static library on the Linux platform.
You can specify options at compile time to build the libraries.
Here you can check more about shared libraries and compilation and build options for the same.
In linux, library files are an archive of one or more .o files. Linux uses the 'ar' program ( think 'tar' without the tape ), to create the archive. After bundling them together, you then use the ranlib program to add some indexing.
ar rc mylib.a source1.o source2.o source3.o
ranlib mylib.a