Date: Mon, 8 Mar 1999 21:26:33 +0100
From: Frank Heckenbach <frank AT tim DOT gerwinski DOT de>
Message-Id: <124D6181.19990308212633.FOO-19CE.frank@goedel.fjf.gnu.de>
X-Mailer: smtphack 0.3.3 by Jan Andres
To: djgpp AT delorie DOT com, peter AT gerwinski DOT de
Subject: Patch for select()
X-Counter: 883 peter AT gerwinski DOT de
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=iso-8859-1
Reply-To: djgpp AT delorie DOT com

Hello,

while porting some GPC code to DJGPP, I found that select() has
different semantics regarding end of file under DJGPP than under
Un*x. DJGPP's select() returns `not ready' at the end of a regular
file (i.e., no character device under Dos), but select() under Un*x
returns `ready' in this situation (verified under Linux and
Solaris).

DJGPP's behaviour is, of course, due to the behaviour of the Dos
interrupt used. The following patch first checks if the file is a
regular file and returns `ready' if it is. A small test program
(which should write 1) is appended.

Frank

*** src/libc/compat/time/select.c~	Mon Mar  8 02:32:50 1999
--- src/libc/compat/time/select.c	Mon Mar  8 20:32:13 1999
***************
*** 84,89 ****
--- 84,103 ----
  
    __dpmi_regs regs;
  
+   /* If it's a disk file, always return 1, since according to Un*x
+      semantics, select() returns ``ready'' at EOF (and before EOF,
+      anyway). */
+   regs.x.ax = 0x4400;
+   regs.x.bx = fd;
+   __dpmi_int (0x21, &regs);
+   if (regs.x.flags & 1)
+   {
+     errno = __doserr_to_errno (regs.x.ax);
+     return -1;
+   }
+   if (!(regs.x.dx & 0x80))
+     return 1;
+ 
    regs.x.ax = 0x4406;
    regs.x.bx = fd;
    __dpmi_int (0x21, &regs);


Test program:

#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>

int main ()
{
  fd_set s;
  struct timeval t = { 0, 0 };
  int f = open("foobar", O_RDWR | O_CREAT | O_APPEND, 0666);
  if (f < 0) abort ();
  FD_ZERO (&s);
  FD_SET (f, &s);
  printf ("%i\n", select (f + 1, &s, NULL, NULL, &t));
  return 0;
}