What's the difference between .lib and .a files?

Question

I'm trying to statically compile something and I'm trying to get a handle on what all these dependencies are. I know that .dll files are for dynamically linked dependencies that will be required by the final output, but what are .a and .lib files and when do you need each of those?

score 59 · Accepted Answer · answered Feb 25 '10 at 22:02

59

.a is an archive of code: compiled but not linked. You would link statically with it during your program's final link step.

.lib can be either the same as .a, or a magical so-called "import library": a thin placeholder which causes you to require a .dll at runtime.

answered Feb 25 '10 at 22:02

crazyscot

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I'm compiling the QT framework with a .a OpenSSL library, but somehow my final executable produced by QT is still requiring an external OpenSSL dll in order to run. Do you have any idea how this could be the case given that I'm using a .a library? – Nantucket Feb 26 '10 at 00:09
How were the contents of the OpenSSL .a compiled? How are you linking your final executable? – crazyscot Feb 26 '10 at 08:52
I am not sure about "require a .dll at runtime". You will not need .dll if your project is using .lib static libraries (unless there are .dll dependencies which you haven't identified correctly). – ha9u63a7 Aug 26 '14 at 07:10
Huh? Even import libraries are ar files, it's merely their contents that differs. Another difference are the paths that may occur for contained "members". – 0xC0000022L Dec 15 '22 at 22:33

score 38 · Answer 2 · answered Feb 25 '10 at 22:01

38

On Unix systems you have the .a files. These are simple archives of object files (.o).

On Windows, there are .lib files, which are quite the same thing, but for Windows instead of Unix.

An additional subtlety is that in order to link some code against a DLL (on Windows), you have to link against a .lib file which contains simple wrappers which invoke the DLL. On Unix system, traditionally, there is no need for such wrappers (the linker is smart enough to generate them on the fly).

answered Feb 25 '10 at 22:01

Thomas Pornin

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Thank you for this answer, found out I was trying to compile with a Windows flag on somewhere! :) – SSH This Dec 04 '12 at 21:56
Is there any difference between .a created in windows and unix ? – samnaction Jan 17 '17 at 12:15
I have an example here without the need of a .lib on a shared (.dll) library in windows: https://github.com/ConsoleTVs/C_Static_Shared_Libraries – Erik Campobadal Nov 11 '18 at 21:28

score 12 · Answer 3 · answered Feb 25 '10 at 22:01

12

Usually, .a is for static libraries under Linux whereas .lib are for the same but on Windows. But of course it is just a convention.

answered Feb 25 '10 at 22:01

jldupont

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score 11 · Answer 4 · answered Jun 13 '14 at 00:22

Something I don't see mentioned here yet is the surprising fact that, at least some of the time, .a files and .lib files are actually the same binary format. Although I couldn't find anything saying as much on the mingw website, I noticed that when trying to get MS Visual C++'s 64-bit compiler cl.exe to link in a .dll file produced using the mingw-w64 g++ compiler, it happily accepted the command line

cl /EHsc /Ipath\to\include gmp_test.cpp path\to\lib\libgmp.dll.a

and the resulting .exe file ran correctly as soon as I put a copy of the corresponding .dll file in the current directory. (It did mumble the warning "Command line warning D9024 : unrecognized source file type 'path\to\lib\gmp-6.0.0\lib\libgmp.dll.a', object file assumed".)

Further evidence is that the Linux file command reported "current ar archive" for several files of each extension (.lib or .a) I tried.

What's the difference between .lib and .a files?

4 Answers4

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