how to setting for my own header file and library?

bvkim · 05-08-2010, 12:19 AM

I have 3 files: main.c, mylib.h, mylib.c

Now I want to put mylib.h into : /usr/include/mydir/mylib.h
And I create a static library: libmylib.a, and put into the folder: /usr/lib/mydir/libmylib.a

Then I compile: $ gcc -o main main.c
Then I got linking error

Code:

main.c:(.text+0x3e): undefined reference to `extract_v1'
main.c:(.text+0x7b): undefined reference to `modify_v1'
collect2: ld returned 1 exit status

So I try with -l options: $ gcc -o main main.c -lmylib
I still got error

Code:

/usr/bin/ld: cannot find -lmylib
collect2: ld returned 1 exit status

Any idea about this?

Also, I want to ask about: how to create my own header and library to put into /usr/include/mylib/ and /usr/lib/mylib/, so when I use function in my program, the compiler will automatically link to library.
It's like when you #include <stdio.h>, and you compile: $ gcc -o program program.c, you don't need to specify any linking folder or library.

Simon Bridge · 05-08-2010, 02:11 AM

How did you go about creating the library?
Did you put /usr/include/mydir/ in the search path for libraries?
How to make and use libraries in gcc ... for djgpp, but clear. Also see gcc documentation.

smeezekitty · 05-08-2010, 02:14 AM

Code?

hda7 · 05-08-2010, 07:07 AM

To get gcc to find your library, you must add the folder it is in to gcc's search path:

Code:

gcc -o main main.c -L/usr/lib/mydir -lmylib

Because you are linking a static library, you don't have to change LD_LIBRARY_PATH (for runtime loading) only the link path.

theNbomr · 05-08-2010, 10:38 AM

To summarize, and clarify: gcc needs to be told what libraries to link, and this is done with the '-l' (lower-case ell) flag followed by the name of the library (but without the prefix 'lib'; go figure). It also needs to be told where to look for any libraries that it wants to use, and this is done with one or more '-L' arguments, each of which specify a path, and which collectively comprise an ordered list of paths to search for libraries. The '-I' (uppercase eye) argument is used analogously to the '-L' argument by the compiler to find header files.
Having said this, the compiler has built-in lists of standard places to look for things, and also what standard libraries should be linked. You can only get the compiler to look in other places by telling it to do so, using the switches mentioned above. So, you could put your libraries and header files in one of the 'standard' places, but this is generally considered bad form. I suppose you could re-build your toolchain, and add to the lists of standard places, but this sounds like a lot more work than just adding some commandline switches to your Makefile. Moreover, the CFLAGS & LDFLAGS variables in your Makefile serve as a form of documentation. They spell out what libraries are used and where they can be found. Header files that are created as part of a project are usually stored in the same directory as the C source code. These kinds of things are conventions that other programmers learn to expect, and going to another arrangement would probably not be a good idea, except in special circumstances.
--- rod.