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Date: Mon, 31 Jan 2011 11:54:53 +0000
From: Dave Korn <dave DOT korn DOT cygwin AT gmail DOT com>
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Subject: Re: On "write" command
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On 31/01/2011 01:52, Larry Hall (Cygwin) wrote:
> On 1/29/2011 8:20 AM, Angelo Graziosi wrote:
>> I would ask if on Cygwin I can use the 'write' command:
>>
>> write - send a message to another user
>>
>> $ write USER [ttyname]
>>
>> I remember I have used it some years ago, but not remember if it was on
>> GNU/Linux or Cygwin.
>>
>> I did a research on http://cygwin.com/packages and didn't find it, but
>> perhaps I did the wrong search..
> 
> No, that's the correct place to search.  If it's not showing up, none of
> the current packages provide 'write'.

  It's part of util-linux, but it's configured out of the build on Cygwin,
because it relies on a working utmp file to operate.  Which we don't have.
Hmm, I'm in the middle of a gcc testrun right now and can't rebuild my DLL,
but I noticed a few "interesting" points about the code:

winsup/cygwin/include/utmpx.h

> /* Must be kept in sync with struct utmp as defined in sys/utmp.h! */
> struct utmpx
> {
>  short	ut_type;
>  pid_t	ut_pid;
>  char	ut_line[UT_LINESIZE];
>  char   ut_id[UT_IDLEN];
>  time_t ut_time;
>  char	ut_user[UT_NAMESIZE];
>  char	ut_host[UT_HOSTSIZE];
>  long	ut_addr;
>  struct timeval ut_tv;
> };

winsup/cygwin/include/sys/utmp.h

> struct utmp
> {
>  short	ut_type;
>  pid_t	ut_pid;
>  char	ut_line[UT_LINESIZE];
>  char  ut_id[UT_IDLEN];
>  time_t ut_time;
>  char	ut_user[UT_NAMESIZE];
>  char	ut_host[UT_HOSTSIZE];
>  long	ut_addr;
> };

  That could explain why successive calls to getutent() appear to "lose frame"
across the contents of utmp.  Also,

winsup/cygwin/syscalls.cc

> /* Note: do not make NO_COPY */
> static struct utmp utmp_data_buf[16];
> static unsigned utix = 0;
> #define nutdbuf (sizeof (utmp_data_buf) / sizeof (utmp_data_buf[0]))
> #define utmp_data ({ \
>   if (utix > nutdbuf) \
>     utix = 0; \
>   utmp_data_buf + utix++; \
> })

  I haven't slept all night, but isn't that going to overrun by one?  Anyway
with the fix to utmp.h to add ut_tv (and a #include <sys/time.h>), write.exe
compiles and works...

    cheers,
      DaveK


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