Some questions about kernel programming

Dear Friends
 
I have some questions about kernel programming:
 
1. Why I can call some system calls functions into the kernel but
   another not?, for example: I can call printf(), but I can't call
   socket().
 
2. Into kernel I can call the socket "low level" functions
   that this system calls invoke  sosocket(), soconnect(), etc.
   but, How I do replace the send() system call? Perhaps, Can I call
   write() into kernel with same parameters?
   For example :
   /* res =  send(skt, buf, buflen, 0); */
   res =  write (skt, buf, buflen);
 
3. How I can copy a pointer string ( character array ) from user space to
   kernel space using copyin() without the following problem (I can't
   pass the length the explicitly from user land):
 
struct	MySystemCall_args {
	char *	address;
};
 
int MySystemCall( p,uap)
  struct proc *p;
  register struct  MySystemCall_args *uap;
{
  char *the_address;
 
  printf(" ---> uap->address : %s\n", uap->address );
  printf(" ---> (strlen (uap->address) * sizeof(char)) : %d \n",
	(strlen (uap->address) * sizeof(char)) );
  copyin(uap->address, the_address, (strlen (uap->address) * sizeof(char))
);
  printf("the_address: %s \n", the_address );
  printf("strlen (the_address): %d \n", strlen (the_address) );
 
When this code run in mode kernel:
  ---> uap->address : 127.0.0.1
  ---> (strlen (uap->address) * sizeof(char)) : 9
  the_address : 127.0.0.1\M-"\M-Y\M-GX\M-p+\M-@@\M-_\M-*\M-@
  strlen (the_address): 20
 
This crash the kernel later...
 
Thanks for your help.
 
 
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| YONNY CARDENAS B.		Apartado Aereo	22828		   |
| Systems Engineer		Santafe de Bogota D.C.		   |
|				Colombia - South America	   |
| Student M.Sc. 		Tel:   +571 6095477		   |
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