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here I have several problem for static libraries need your help.

From some books I learned that, a static library (.a in Linux) contains a set of compiled objects, when it's linked into an executable, the link tool will only take out those objects that are actually referenced.

So if the .a contains 1.o, 2.o and 3.o and my application only uses functions in 1.o, then only 1.o will be built into the executable, is this correct?

Then let's go further, say we have 2 .a libs, first contains 1.o, 2.o and 3.o and second contains 3.o 4.o and 5.o. If my application only uses functions in 1.o 2.o 3.o and 4.o then only these these 4 .o will be built into the executable, is it correct?

I raised this question because I'm building some .a files to use with MSVC. These .a libraries are built in MinGW and then should be compatible with MSVC. I could include these libs into the MSVC project and build my program successfully. But the generated executable is 5 MB (the total size of all .a should be about 8MB) even when my program is an empty one (only have an empty main function).

Does this mean, .a when used in MSVC or .lib (static libs for Win) will be built into executable as a whole, but not in the way it behaves under Linux?

and also I have a question for below content

enter image description here

If I could use -static to link to static version of lib tiff, then why it needs to link to other libs? Should a static lib already contains all the code it needs?

Thanks

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  • How is linux tag relevant? Commented Mar 13, 2014 at 10:47
  • Avoid static libraries when possible on Linux. Commented Mar 13, 2014 at 11:35

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A static library won't have all code it needs. Imagine a library that uses printf() to output error messages. This library will still depend on the static version of libc, it won't include the code for printf itself.

In your case, since Tiff supports various internal representation formats, one of which is jpeg, the static libtiff wants you to link the static libjpeg.

There is no fundamental difference in this between windows and linux. When you do a static link against libtiff and libjpeg, only the libjpeg functions that are actually needed by libtiff get linked, but not, for example, the parts that handle the JFIF Jpeg wrapper.

EDIT - answer to comments

There's a lot of stuff going on before your main() gets called. It's not that much on linux/unix, where the OS delivers arguments the way main() wants them, but on Windows, a different function, normally called WinMain() gets control when the program starts. This WinMain() is hidden in the library. It gets the whole command line in one string and has to parse arguments to pass them to main(), which means checking for spaces, which is probably implemented using isspace(), which pulls in ctype, which pulls in lots of locale-dependent stuff, and so on. So much of your 5 MB might be code that serves to make your program work in windows just like it would in unix. Also, if you're compiling with debug options on, these debug symbols take a lot of space as well.

Keltar's comment about a static library calling a dynamic one is right as well - but that adds a complication you don't need very often. There's more or less 2 reasons for static linkage:

  • You want your program be able to run even when something goes wrong with dynamic libraries. If i accidentially rm /lib/libc.so.* i will be glad if i have statically linked versions of mount and cp to copy it from somewhere else. Thus, installers and "emergency programs" are often linked static
  • You want to make sure your program uses a specific version of libraries, that was current on your system when you compiled it, not the dynalink version that might be installed on some system 5 years later.

Both these reasons don't make much sense if some but not all libraries are static.

There's exceptions though: imagine you need a specific feature in libtiff. You browse the documentation and it says nothing about the feature. You check the source code and find that it implements specific_feature(), with a big "This is experimental and might go away in a future version" comment. If you decide you need that feature now, you might want to link libtiff statically to protect your program from failing when future versions of libtiff don't implement the function any more. Of course, you'll still want the dynamic versions of libjpeg and libc. I'll leave the decision if this is good practise to you.

Windows is a special case as it always uses kernel.dll and user32.dll as there are no static versions, even if the rest of your program is linked statically.

So while a libtiff.a could require a libjpeg.so, and a libtiff.lib could require a libjpeg.dll, there's generally not much reason to do so.

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Thank Guntram, then could you help to explain the first question, why my program gets so big even when it doesn't call any function from those .a libs?
It is most likely includes libgcc (mingw specific). And, to clarify, static library don't really have to depend on other static libraries, they could be dynamic ones (at least on linux - could someone tell the difference for windows version?)

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