Path: utzoo!attcan!uunet!husc6!mailrus!iuvax!pur-ee!a.cs.uiuc.edu!uxc.cso.uiuc.edu!uxe.cso.uiuc.edu!mcdonald
From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: Partial application in C
Message-ID: <225800040@uxe.cso.uiuc.edu>
Date: 1 Jul 88 14:23:00 GMT
References: <509@philmds.UUCP>
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Nf-ID: #R:philmds.UUCP:509:uxe.cso.uiuc.edu:225800040:000:1593
Nf-From: uxe.cso.uiuc.edu!mcdonald    Jul  1 09:23:00 1988



>Every segment on the 286 (in 286 mode, of course) has a descriptor. 
>Each descriptor has a byte, called the Access Rights Byte, that
>contains protection bits for the segment.  Bit 4 is the 'executable'
>flag; if 0, this is a Data Segment Descriptor, if 1, this is a
>Executable Code Segment Descriptor.

>Now, if this is a Data Segment Descriptor, bit 1 describes whether
>this segment is read-only (0), or read-write (1).  If, on the other
>tentacle, this is an Executable Code Segment Descriptor, bit 1
>describes whether the segment can be read (1) or not (0).  You cannot
>write to an Executable Code Segment!  The system, of course, loads a
>program as data segments and then modifies the segment descriptors to
>make them executable.

And, for all but brain-dead compilers and/or operating systems,
a cast of a data pointer to a code pointer (or vice-versa) should
still work. The compiler either does it directly, or, in the case
of aggressively over-protective operating systems, calls a system
routine which generates a segment descriptor of the proper type which
points to the same location in memory. I would presume that the ANSI
standard would describe in detail the requirements for this. If
a facility such as this is missing, C would be a seriously restricted
language: no Forth written in C, no Turbo Pascal or Turbo C, no
incremental compilation! OS/2, for example, has a system call to do this;
whether the compiler automatically makes that call I doubt.
I believe it is called DOSCreateAlias or DOSCreateCodeAlias. What is
the equivalent call in UNIX?

Doug McDonald