writing a FreeBSD C library

[Oliver Mahmoudi]

Hello all y'all Kernel Hackers,

Trying to get a deeper understanding of the FreeBSD kernel I am currently I
am studying C library files.
For this reason I wrote a very simple library, just to understand how it
works.

Below are my source codes for:

1. the header
2. the library file
3. a simple C program with which I intend to test my library function

############## start of simple library header ########################


#ifndef _s_lib_h_
#define _s_lib_h_

#define SOME_INT    0x0001

/* prototype */

void myprnf(char *);

#endif /* _s_lib_h_ */

##################### end of header  ############################


################# start of library file lb.c ##########################

#include <stdio.h>
#include "slib.h"


static void _myprf(char *);

void
myprnf(char *farg)
{
    char *narg = farg;

    _myprf(narg);
}

static void
_myprf(char *sarg)
{
    char *pstr = sarg;

    printf("%s\n", pstr);
}

############## end of library file ###############################


################ start of source file ############################

#include "slib.h"

int
main()
{
    char *somestr = "hello world";

    myprnf(somestr);
    return(1-1);
}


################# end of source file ###########################


I compiled the library file in the following way.

% gcc -I../include -Wall -c lb.c
% ar rsv mylib.a lb.o

The compilation finished with no warnings  so I assume that there are no
errors in the library itself.

Trying to compile my source file - let's call it source.c - I always get the
following error message:

% gcc -o testfile source.c
/var/tmp//ccQoff1S.o(.text+0x19): In function `main':
: undefined reference to `myprintf'
%

In other words, gcc doesn't seem to find my library. I tried to mv my
library file in /lib, /libexec
/usr/lib, /usr/libdata and /usr/libexec but none of the above directories
would directory would
let me compile my source file.

Now I am wondering where do I need to put mylib.a file to succeed?

Also, this is a static library. What do I need to do under FreeBSD to
compile this library into a dynamic
library and where would I put this file?

Lastly, most if not all the system calls can be found under /sys/kern/*.c.
When writing a FreeBSD C program
gcc makes use of the corresponding headers, e.g. unistd.h. and the standard
library libc. Is it possible
to peep a look at the source file for libc, i.e. is it included in the
source tree? Where?


Thanks

Oliver


There are many many more kernel related questions on my mind but I think
that this is enought for one
email.
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[Florian Loeber]

Hi,

you have to link your executable to your library.
The command-line option is -l.

% gcc -o testfile -lmylib source.c

Without it, your program doesn't know that this library exists
(somewhere, /usr/lib, ...)

Regards,
Florian
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[Gabor Kovesdan-3]

Oliver Mahmoudi escribió:
> I compiled the library file in the following way.
>
> % gcc -I../include -Wall -c lb.c
> % ar rsv mylib.a lb.o
>  
You can study bsd.lib.mk and bsd.prog.mk in /usr/share/mk. With these
two includes you can deal easily with your C programs/libraries. It will
serve you very well later.

> The compilation finished with no warnings  so I assume that there are no
> errors in the library itself.
>
> Trying to compile my source file - let's call it source.c - I always get the
> following error message:
>
> % gcc -o testfile source.c
> /var/tmp//ccQoff1S.o(.text+0x19): In function `main':
> : undefined reference to `myprintf'
> %
>
>  

That's easy, your library isn't linked to your program. First, compile
the source file but _do not link_:
gcc -c source.c
This will result in a source.o file. Then link the object files
(source.o and the static library) with ld:
ld -o testfile source.o mylib.a
> In other words, gcc doesn't seem to find my library. I tried to mv my
> library file in /lib, /libexec
> /usr/lib, /usr/libdata and /usr/libexec but none of the above directories
> would directory would
> let me compile my source file.
>
>  
With static libraries you can just specify the full path when linking.
> Now I am wondering where do I need to put mylib.a file to succeed?
>
> Also, this is a static library. What do I need to do under FreeBSD to
> compile this library into a dynamic
> library and where would I put this file?
>  
Just using the proper parameters when compiling the library. It is a
Linux article but you'll find some explications here:
http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

Again, the bsd.*.mk Makefile includes are pretty handful.
> Lastly, most if not all the system calls can be found under /sys/kern/*.c.
> When writing a FreeBSD C program
> gcc makes use of the corresponding headers, e.g. unistd.h. and the standard
> library libc. Is it possible
> to peep a look at the source file for libc, i.e. is it included in the
> source tree? Where?
>  
In /usr/src/lib/libc provided you have the source code installed in
/usr/src.
> There are many many more kernel related questions on my mind but I think
> that this is enought for one
> email.
>  
I don't want to desperate you but these are very basic things. You
should get a deeper knowledge of the basics first, before you try to do
some kernel-related things. Otherwise, you will find it way too
difficult and you won't enjoy. Anyway, the book of Marshall K. McKusick
and George V. Neville-Neil is a good source of learning kernel stuff:
http://www.amazon.com/Design-Implementation-FreeBSD-Operating-System/dp/0201702452

Regards,

--
Gabor Kovesdan
FreeBSD Volunteer

EMAIL: gabor@... .:|:. gabor@...
WEB:   http://people.FreeBSD.org/~gabor .:|:. http://kovesdan.org

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[Oliver Mahmoudi]

Thank you for your emails.
Neither one of the methods that you two suggested brought about the desired
solution, but I have solved it.

using gcc for the plain source with the -I switch gives:
% gcc -o aprog aprog.c -I ~/mylib/
/var/tmp//ccHrDiyd.o(.text+0x19): In function `main':
: undefined reference to `myprnf'

creating an object file first and then linking with ld gives me:
% ld -o aprog aprog.o mylib.a
ld: warning: cannot find entry symbol _start; defaulting to 0000000008048080
mylib.a(lb.o)(.text+0x33): In function `_myprf':
: undefined reference to `puts'

whereas placing mylib.a before the -o switch and then linking with ld gives:
% ld mylib.a -o aprog aprog.o
ld: warning: cannot find entry symbol _start; defaulting to 0000000008048080
aprog.o(.text+0x19): In function `main':
: undefined reference to `myprnf'

which is essentially the same message it gives when compiling and linking
with gcc in one step. The fact that the order of the arguments matters is
also mentioned somewhere in gcc(1) and ld(1).

The solution was to simply compile and link it like so:
% gcc -o testfile aprog.c mylib.a
% ./testfile
hello world
%


This is in essence a synthesis of what you two suggested.


Anyways, thanks again.


Oliver



>
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[Harald Servat-2]

Hello Oliver,

2009/11/4 Oliver Mahmoudi <olivermahmoudi@...>

> Thank you for your emails.
> Neither one of the methods that you two suggested brought about the desired
> solution, but I have solved it.
>
> using gcc for the plain source with the -I switch gives:
> % gcc -o aprog aprog.c -I ~/mylib/
> /var/tmp//ccHrDiyd.o(.text+0x19): In function `main':
> : undefined reference to `myprnf'
>
> creating an object file first and then linking with ld gives me:
> % ld -o aprog aprog.o mylib.a
> ld: warning: cannot find entry symbol _start; defaulting to
> 0000000008048080
> mylib.a(lb.o)(.text+0x33): In function `_myprf':
> : undefined reference to `puts'
>
> whereas placing mylib.a before the -o switch and then linking with ld
> gives:
> % ld mylib.a -o aprog aprog.o
> ld: warning: cannot find entry symbol _start; defaulting to
> 0000000008048080
> aprog.o(.text+0x19): In function `main':
> : undefined reference to `myprnf'
>
> which is essentially the same message it gives when compiling and linking
> with gcc in one step. The fact that the order of the arguments matters is
> also mentioned somewhere in gcc(1) and ld(1).
>
> The solution was to simply compile and link it like so:
> % gcc -o testfile aprog.c mylib.a
> % ./testfile
> hello world
> %
>
>
> This is in essence a synthesis of what you two suggested.
>
>

  I'm afraid that this is not the most common usage of libraries in the unix
world. Libraries, typically, are called lib[SOMETHING].a (if they are static
libraries) or lib[SOMETHING].so* (if they are shared libraries).

  In addition, the -l X option in the gcc compiler looks for libX.[a|so] in
the all specified paths defined by -L, so in your first command
  gcc -o aprog aprog.c -I ~/mylib/
  you're making gcc to look for for something called lib~/mylib/.[a|so]
which I doubt it can be found.

  So, I suggest you to:
  1.- Name your "mylib.a" into "libmylib.a" (or other name that begins with
lib),
  2.- add -L. to your 1st gcc invocation (in order to instruct gcc to look
at the current directory, i.e., "."), and
  3.- add -lmylib (if you called your library libmylib.a) to the gcc

  Your compile instruction, then, should look like
  gcc -o aproc aprog.c -L. -lmylib

Regards.

>
> Anyways, thanks again.
>
>
> Oliver
>
>
>
> >
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--
_________________________________________________________________
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[Dag-Erling Smørgrav]

Harald Servat <redcrash@...> writes:
>   In addition, the -l X option in the gcc compiler looks for libX.[a|so] in
> the all specified paths defined by -L, so in your first command
>   gcc -o aprog aprog.c -I ~/mylib/
>   you're making gcc to look for for something called lib~/mylib/.[a|so]
> which I doubt it can be found.

You're confusing -l with -I...  but the rest of your email is correct.

DES
--
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[Harald Servat-2]

Oh, yes! You're right DES. They look the same to me here in the web-browser
:)

Oliver, regarding the Dag-Erling correction, the -I option in gcc refers to
include header files (typically files ended with .h), not for naming
libraries as I mentioned.

Regards.

2009/11/4 Dag-Erling Smørgrav <des@...>

> Harald Servat <redcrash@...> writes:
> >   In addition, the -l X option in the gcc compiler looks for libX.[a|so]
> in
> > the all specified paths defined by -L, so in your first command
> >   gcc -o aprog aprog.c -I ~/mylib/
> >   you're making gcc to look for for something called lib~/mylib/.[a|so]
> > which I doubt it can be found.
>
> You're confusing -l with -I...  but the rest of your email is correct.
>
> DES
> --
> Dag-Erling Smørgrav - des@...
>

--
_________________________________________________________________
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[KAYVEN RIESE]

On Wed, 4 Nov 2009, Harald Servat wrote:

> Oh, yes! You're right DES. They look the same to me here in the web-browser
> :)

Oh, no. shoulda used a serif font!  {:P

>
> Oliver, regarding the Dag-Erling correction, the -I option in gcc refers to
> include header files (typically files ended with .h), not for naming
> libraries as I mentioned.
>
> Regards.
>
> 2009/11/4 Dag-Erling Smørgrav <des@...>
>
>> Harald Servat <redcrash@...> writes:
>>>   In addition, the -l X option in the gcc compiler looks for libX.[a|so]
>> in
>>> the all specified paths defined by -L, so in your first command
>>>   gcc -o aprog aprog.c -I ~/mylib/
>>>   you're making gcc to look for for something called lib~/mylib/.[a|so]
>>> which I doubt it can be found.
>>
>> You're confusing -l with -I...  but the rest of your email is correct.
>>
>> DES
>> --
>> Dag-Erling Smørgrav - des@...
>>
>
>
>
> --
> _________________________________________________________________
> Fry: You can see how I lived before I met you.
> Bender: You lived before you met me?!
> Fry: Yeah, lots of people did.
> Bender: Really?!
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