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Date:      Mon, 26 Oct 2009 21:08:16 +0100
From:      Oliver Mahmoudi <olivermahmoudi@gmail.com>
To:        freebsd-hackers@freebsd.org
Subject:   writing a FreeBSD C library
Message-ID:  <6b4b2d2c0910261308i367569dbg887d7c713bf20ad1@mail.gmail.com>
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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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