select.txt

一个通讯程序源码

NOTE from ...!gatech!emory!tridom!wht:
the following is the mail message I originally received; with
a little tinkering, i got the select call to behave as advertised
in the BSD manual (my xenix is 386 2.3).  Added it to /lib/386/Slibx.a
and all is well for me.  Good luck.


From emory!gatech!hubcap!ncrcae!ncr-sd!crash!elgar!ag Thu Feb  2 13:04:07 EST 1989
Article 4851 of comp.unix.xenix:
Path: tridom!emory!gatech!hubcap!ncrcae!ncr-sd!crash!elgar!ag
>From: [e-mail] (Keith Gabryelski)
Newsgroups: comp.unix.xenix
Subject: select() on SCO XENIX 2.3.
Message-ID: <[e-mail]>
Date: 24 Jan 89 04:54:17 GMT
Reply-To: [e-mail] (Keith Gabryelski)
Organization: Elgar Corporation, San Diego, CA
Lines: 474

A few days ago I was paging through my SCO XENIX 2.3.1 Release Notes
and found, on page 44 (section 16), a section describing 4BSD
enhancements to the current release of SCO XENIX.

select(S) was mentioned specifically.

I checked the rest of the 2.3 manuals to see if there was a clue on
how to access select(); no dice.  I tried my 2.2 dev sys libraries to
see if it had been supported in previous releases and just not
mentioned; negative.

I decided to see if I could get select() to work on my system.  And
that is what this article is about.

I checked /xenix and found:

	% nm xenix | grep select
	nm: xenix: too many symbols to sort		# chuckle
	0020:00015bec  T _select

Bingo!  I also found <sys/select.h> (which has a comment about
the select(2) system call. :-) ).

When a system call is made in a C program on a Unix system (eg
open(2)), it actually links in a file from libc.a (/lib/libc.a) (in
this case `open.o') written in assembly that loads a register with a
system call number and causes an exception to occur.  The `trap'
instruction is used on the 68000, on a vax it's `chmk', and on a 370
it's `svc'.  Control is transfered to the kernel which (in the case of
this particular exception) will index the register into a table
(called the sysent table) to get the address of the actual routine in
kernel memory to call (_open).

At least under SCO XENIX this algorithm is modified somewhat.

When a system call is made in a C program on a SCO XENIX system (eg
open(S)), it links in a file from libc.a (/lib/386/Slibc.a) (in this
case `open.o') written in assembly that loads the register `eax' with
a system call number and jumps to 7:0 which (a guess) is mapped to an
instruction that switches into supervisory mode and jumps to the
routine ioint (??) in the kernel address space.  The interrupt routine
hands the system call number (along with the user given arguments) to
_trap with figures out what to sysent table to use (there are a few
under SCO XENIX) and does the right thing.

The _open routine (in libc.a's open.o) would probably look something
like:

;
; open - open a file for reading or writing
;

	title	open

	.386

SYSNUM	equ 	5			; open's system call number is `5'.
extrn	_errno:dword

public  _open

_TEXT	segment  dword use32 public 'CODE'
	assume   cs: _TEXT
_open	proc near
	mov	eax, SYSNUM		; Get system call number.

	;
	; I don't even pretend to understand masm syntax.  I tried
	; the following line (and variations) without any success.
	;

;	call    far 7:0			; Switch to kernel and call SYSNUM.

	;
	; Don't laugh, it works.
	;

	db 9ah
	dw 0,0
	dw 7

	jb	short _cerror		; below == error.

	xor	eax, eax		; zero return value (no error).
	ret				; done.

_cerror:
	mov	_errno, eax		; Save error code in _errno.
	mov	eax, -1			; Return -1 (as error).
	ret				; done.

_open	endp

_TEXT	ends

	end

Under SCO XENIX the sysent table (struct sysent in <sys/systm.h>) looks
something like:

struct sysent
{
    unsigned char  sy_ret;	 /* Type of return value (int, void ...) */
    unsigned char  sy_arg386;	 /* Number of 386 words args on stack */
    unsigned char  sy_nlarg286;	 /* # of 286 large model word args on stack */
    unsigned char  sy_nmarg286;	 /* 286 Small Middle: max # of args */
    unsigned	   sy_argmask;	 /* Argument types on stack. */
	     int   (*sy_call)(); /* System call address in kernel */
}

sy_ret is the type return of the value this system call returns.  `0'
seems to be INT and `6' is probably void.

sy_arg386 is the number of words the arguments for this system call
take on the stack.

sy_nlarg286 and sy_nmarg286 are similar to sy_arg386 but used for
doing 286 stuff.  I don't plan on mentioning the 286 stuff in this
article that much, it just isn't interesting to me.

sy_argmask is the type of args on the stack using the following table:

NUM | SYMBOL  | 386 | 286L | EXPLANATION
 0  |         |     |      | Arg not used.
 1  | DATAP   |	 2  |  1   | Arg is a data pointer; seg + address
 2  | TEXTP   |	 2  |  1   | Arg is a text pointer; seg + address
 3  | CONST   |	 1  |  1   | Arg is an int-sized constant 
 4  | UCONST  |	 1  |  1   | Arg is an unsigned int-sized constant
 5  | LCONST  |  1  |  1   | Arg is a long-sized constant
 6  | FDATAP  |	 1  |  1   | Arg is FAR data pointer.
 7  | SODATAP |  2  |      | 386: 32-bit offset.
    |         |     |  1   | 286: low word is 16 bit data pointer offset,
    |         |	    |      |	  high word is 16 bit selector.
 8  | SOTEXTP |  2  |      | 386: 32-bit offset.
    |         |     |  1   | 286: low word is 16 bit text pointer offset,
    |         |	    |      |	  high word is 16 bit selector.

Each nybble in sy_argmask represents one argument passed to the system
call.  Bits 0-3 represent arg one; 4-7 arg two; 8-12 arg three; etc.
A total of eight arguments (4 bits times 8 args = 32 bits in an int)
can be passed to a function (although MASK, a macro used to make
sysent's sy_argmask field is limited to six arguments).

NUM is the number (put in each nybble) represented by the SYMBOL (in
<sys/systm.h>) that corresponds to the arg type EXPLANATION and takes
[386|286] (depending on the model you are using) words on the user
stack.

So, for the open() system call: sy_argmask is 0x00000331 and sy_arg386
is 0x04.

	open(char *path, int oflag, int mode);
             ^^^^^^      ^^^        ^^^
             DATAP       CONST      CONST

sy_call is the pointer to the function in kernel memory that should
handle this system call request.

The sysent table on my system looks something like:

Syscal Num | ret| 386| L  | SM |    Arg Types    | System Call 
sysent:
    00     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _nosys
    01     | 00 | 02 | 01 | 05 | 0 0 0 0 0 0 0 1 | _rexit
    02     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _fork	
    03     | 00 | 04 | 03 | 03 | 0 0 0 0 0 4 1 3 | _read	
    04     | 00 | 04 | 03 | 03 | 0 0 0 0 0 4 1 3 | _write
    05     | 00 | 04 | 03 | 03 | 0 0 0 0 0 3 3 1 | _open	
    06     | 00 | 01 | 01 | 03 | 0 0 0 0 0 0 0 3 | _close
    07     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _wait	
    08     | 00 | 03 | 02 | 03 | 0 0 0 0 0 0 3 1 | _creat	
    09     | 00 | 04 | 02 | 03 | 0 0 0 0 0 0 1 1 | _link	
    0a     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 1 | _unlink	
    0b     | 00 | 04 | 02 | 03 | 0 0 0 0 0 0 1 1 | _exec
    0c     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 1 | _chdir	
    0d     | 00 | 00 | 00 | 05 | 0 0 0 0 0 0 0 0 | _gtime
    0e     | 00 | 04 | 03 | 03 | 0 0 0 0 0 3 3 1 | _mknod       
    0f     | 00 | 03 | 02 | 03 | 0 0 0 0 0 0 3 1 | _chmod       
    10     | 00 | 04 | 03 | 03 | 0 0 0 0 0 3 3 1 | _chown       
    11     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 7 | _brk
    12     | 00 | 04 | 02 | 03 | 0 0 0 0 0 0 1 1 | _stat
    13     | 00 | 04 | 03 | 05 | 0 0 0 0 0 3 5 3 | _seek 
    14     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _getpid      
    15     | 00 | 05 | 03 | 03 | 0 0 0 0 0 3 1 1 | _smount      
    16     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 1 | _sumount     
    17     | 00 | 01 | 01 | 03 | 0 0 0 0 0 0 0 4 | _setuid      
    18     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _getuid      
    19     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 5 | _stime       
    1a     | 00 | 05 | 04 | 03 | 0 0 0 0 3 1 3 3 | _ptrace      
    1b     | 00 | 01 | 01 | 04 | 0 0 0 0 0 0 0 3 | _alarm       
    1c     | 00 | 03 | 02 | 03 | 0 0 0 0 0 0 1 3 | _fstat       
    1d     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _pause       
    1e     | 00 | 04 | 02 | 03 | 0 0 0 0 0 0 1 1 | _utime       
    1f     | 00 | 03 | 02 | 03 | 0 0 0 0 0 0 1 3 | _stty        
    20     | 00 | 03 | 02 | 03 | 0 0 0 0 0 0 1 3 | _gtty        
    21     | 00 | 03 | 02 | 03 | 0 0 0 0 0 0 3 1 | _saccess     
    22     | 00 | 01 | 01 | 03 | 0 0 0 0 0 0 0 3 | _nice        
    23     | 00 | 00 | 00 | 00 | 0 0 0 0 0 0 0 0 |
    24     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _sync        
    25     | 00 | 02 | 02 | 03 | 0 0 0 0 0 0 3 3 | _kill        
    26     | 00 | 00 | 01 | 00 | 0 0 0 0 0 0 0 0 |              
    27     | 00 | 00 | 02 | 00 | 0 0 0 0 0 0 0 0 |              
    28     | 00 | 00 | 03 | 00 | 0 0 0 0 0 0 0 0 |              
    29     | 00 | 02 | 02 | 03 | 0 0 0 0 0 0 3 3 | _dup         
    2a     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _pipe        
    2b     | 00 | 02 | 01 | 05 | 0 0 0 0 0 0 0 1 | _times       
    2c     | 06 | 08 | 05 | 03 | 0 0 0 1 4 8 4 1 | _profil      
    2d     | 00 | 01 | 01 | 03 | 0 0 0 0 0 0 0 3 | _lock        
    2e     | 00 | 01 | 01 | 03 | 0 0 0 0 0 0 0 4 | _setgid      
    2f     | 00 | 00 | 00 | 03 | 0 0 0 0 0 0 0 0 | _getgid      
    30     | 00 | 03 | 02 | 02 | 0 0 0 0 0 0 2 3 | _ssig        
    31     | 00 | 00 | 01 | 03 | 0 0 0 0 0 0 0 3 | _msgsys      
    32     | 06 | 07 | 04 | 03 | 0 0 0 0 5 5 1 3 | _sysi86      
    33     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 1 | _sysacct     
    34     | 00 | 00 | 01 | 06 | 0 0 0 0 0 0 0 3 | _shmsys      
    35     | 00 | 00 | 01 | 03 | 0 0 0 0 0 0 0 3 | _semsys      
    36     | 00 | 04 | 03 | 03 | 0 0 0 0 0 7 3 3 | _ioctl       
    37     | 00 | 00 | 04 | 00 | 0 0 0 0 0 0 0 0 |              
    38     | 00 | 00 | 00 | 00 | 0 0 0 0 0 0 0 0 | _nosys       
    39     | 00 | 00 | 05 | 00 | 0 0 0 0 0 0 0 0 |              
    3a     | 00 | 00 | 00 | 00 | 0 0 0 0 0 0 0 0 | _nosys       
    3b     | 00 | 06 | 03 | 03 | 0 0 0 0 0 1 1 1 | _exece       
    3c     | 00 | 01 | 01 | 03 | 0 0 0 0 0 0 0 3 | _umask       
    3d     | 00 | 02 | 01 | 03 | 0 0 0 0 0 0 0 1 | _chroot      
    3e     | 00 | 00 | 06 | 00 | 0 0 0 0 0 0 0 0 |              
    3f     | 00 | 00 | 07 | 00 | 0 0 0 0 0 0 0 0 |              
    40     | 00 | 00 | 00 | 00 | 0 0 0 0 0 0 0 0 | _nosys       
    41     | 00 | 00 | 00 | 00 | 0 0 0 0 0 0 0 0 | _nosys       
    42     | 00 | 00 | 00 | 00 | 0 0 0 0 0 0 0 0 | _nosys