C header files

[beny]

What is the location of the directory of C header files that match your running
kernel?

Is anybody who can answer this question? I am on FC6 system.

[keeler1]

Not exactly sure but i believe that it is /lib/libc.so.6. I wrote a program once which seg faulted in the function fclose. When i used the debugger that is where it said that it seg faulted. One thing that would lead me to believe that is not it is the fact that it is not a directory. It is a file. A very long file in unreadable object code. I am fairly certain this is where the object code for the standard library files are kept. Again, someone else who knows for sure probably ought to tell you.

While on the topic of c header files. Would it be possible to put your own header files into the standard library so that you could include them no matter which directory you are writing the code in?

[giulix]

Originally Posted by keeler1

While on the topic of c header files. Would it be possible to put your own header files into the standard library so that you could include them no matter which directory you are writing the code in?

Yes, you can override the standard C header path. But why would you do that ? The directory you are writing the code in (your default directory) is irrelevant.

[keeler1]

Originally Posted by giulix

Yes, you can override the standard C header path. But why would you do that ? The directory you are writing the code in (your default directory) is irrelevant.

I didnt want to overwrite the path, i just want to add more of my own header files to the directory gcc looks for stdio.h ctype.h and all of the other function libraries. I dont want to delete any i just want to add in more. If i added them to the /usr/src/kernels/ --- /include directory, would i then be able to #include the header files just as i would stdio.h and all the other c header files

[giulix]

First, find out which kernel release you're running

uname -r

Then, locate the corresponding directory under /usr/src/kernels

ls -l /usr/src/kernels

If you prefer, here's a shortcut

ls -l /usr/src/kernels/`uname -r`*/include

That is where your kernel headers are. The asterisk is needed to expand the directory name to include your architecture; include is the conventional place where the headers go.

[giulix]

No, you wouldn't. The canonical way is to include your headers by specifying their paths to gcc with the -I option. If you have a directory with your module sources, say ./somemodule in which you store C code and header files, the preferred way is to include the headers with the double quote syntax, i.e.

#include "someheader.h"

which will find it in the current (default) directory. Else, just compile the program with

gcc -I /path_to_include -o program program.c

where path_to_include is the directory which contains your header files.

[keeler1]

So lets say i have a bunch of header files stored in one place but not all. Will gcc still look for some of the header files in the current directory or will it only look in the specified path? I may be testing out a program and do not want to permanently put the header files into that directory just yet because i havent finished editing them. So is there a way to tell gcc to look for header files in multiple paths.

[giulix]

gcc looks for standard header files, e.g. stdio.h and stdlib.h, by default in the /usr/include system directory. You can override this (but there's no need to) with the --sysroot option.
As far as your header files are concerned, you can store them anywhere you like, provided you tell gcc where to find them when you compile your program/modules.
Say you have a program, hello.c, in your home directory /home/keeler, and you have written a header file hello.h which you store in /home/keeler/include, plus you want to include another header, zorro.h, which is in /usr/src/kernels/2.6.18-1.2849.fc6-i686/include/linux.
To compile the program, you'll specify

cd $HOME
gcc -I ./include -I /usr/src/kernels/2.6.18-1.2849.fc6-i686/include/linux -o hello hello.c

Naturally, the program will also contain #includes of standard headers like stdio.h and stdlib.h, but you don't have to specify where to find those, provided the ones you want to include are the ones in /usr/include, as this is the standard path where the compiler will look for them.

[keeler1]

Thanks a lot for you help. One more question though. If you had some header files stored in a directory and specified that correct path and another header file stored in the directory you are currently in and compiling the program in do you have to add another -I flag with the single '.' to have it use those header files which are in your current directory or does gcc look for header files in the current directory by defualt. I know that if no -I flag is given then gcc will use header files from the current directory. Does giving it a -I flag change that at all?

[giulix]

No, gcc does not look for header files anywhere except the system default directory (/usr/include if not overridden by --sysroot). So, either you specify the full path where to find the header file

#include </home/keeler/header.h>

(bad practice: unportable), or you use the double quote syntax (special case where you have the header file stored in the default directory). In any other case, the -I option is required to tell the compiler where to find the (header) files.

[beny]

Is anybody who can tell me where can I find the sources of the kernel 2.6.18-1.2798.fc6xen?
I made: yum -y install kernel-devel kernel-headers, but it installed to me another kernel sources:2.6.18-1.2868.fc6 !
Please help me!

[giulix]

Question answered here