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From: gdtltr@freezer.it.udel.edu (Gary Duzan)
Newsgroups: comp.lang.c
Subject: Re: Array bounds checking with C????
Message-ID: <29051@nigel.ee.udel.edu>
Date: 31 Aug 90 05:38:13 GMT
References: <7611@ucdavis.ucdavis.edu> <26196@mimsy.umd.edu> <988@christopher-robin.cs.bham.ac.uk>
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Reply-To: gdtltr@freezer.it.udel.edu (Gary Duzan)
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In article <988@christopher-robin.cs.bham.ac.uk> cjr@christopher-robin.UUCP (Chris Ridd <RiddCJ>) writes:
=>In article <26196@mimsy.umd.edu> chris@mimsy.umd.edu (Chris Torek) writes:
=>>In article <7611@ucdavis.ucdavis.edu> kuan@iris.ucdavis.edu
=>>(Frank [Who me?] Kuan) writes:
=>>>Why is it that most C compilers don't seem to [check array bounds]?
=>>
=>>Mostly because it is hard.  Given `int *p', is `p[-1] = 3' valid?
=>>That depends on the value of p....
=>>
=>>There is a company called Saber that produces a product called
=>>Saber-C that does this and more.  It works quite well, although last
=>>I had heard it still objected to `&arr[sizeof arr/sizeof *arr]',
=>>which is Officially Legal.  (Fortunately you can turn off each
=>>individual objection.)
=>
=>  Why is this?  I never could figure out why accessing the first
=>element *past* the end of an array should be legal.
=>
   Correct me if I am wrong, but I don't believe accessing the element after
is legal, but the pointer is still legal. In other words:

int x,foo[foolen],*fooptr;

x=foo[foolen]; /* Illegal */

fooptr=foo+foolen; /* Legal, points one int past end of foo */
x=*fooptr;         /* Illegal */
--fooptr;          /* Legal, points to last element of foo */

                                        Gary Duzan
                                        Time  Lord
                                    Third Regeneration



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