Strange Behavior?
Pete Holsberg
pjh at mccc.edu
Fri May 10 02:14:25 AEST 1991
In article <1991May8.204009.1694 at cubmol.bio.columbia.edu> ping at cubmol.bio.columbia.edu (Shiping Zhang) writes:
=In article <1991May8.020720.20170 at mccc.edu> pjh at mccc.edu (Peter J. Holsberg) writes:
=>Here is an extract from a program a student wrote. Note that he has
=>declared "input" incorrectly. The strange behavior is that, when choice
=>"1" is made, the print function outputs all but the first line that was
=>entered. Can anyone explain that in terms of what scanf() is doing to
=>memory near "input"? (This is on a 386, if endianism matters.)
=
=[...]
=
=> char sentence [MAX][SIZE]; /* an array of strings */
=> char *point[MAX]; /* an array of pointers to char */
=> char *orginal[MAX]; /* an array holding the orginal sequence */
=> char input; /* SHOULD HAVE BEEN int input!!! */
=
=[...]
=
=> printf("Make a choice: ");
=> scanf ("%d" , &input); /* value converted to decimal integer
=
=I think it results from declaring input as char and using it as int
=in scanf(). When scanf() writes 1 into the location of input, it puts
=into 0's those bits that belong to the first byte(s) of sentence[0],
=terminating sentence[0] at its first byte. I could be wrong, though.
I'm sure you're right. But what I really want to know is why the
compiler assigns memory that way. It would seem to me that it would
assign memory "downward," so that the attempt to write a 4 byte value
into a 1 byte location would result in the "corruption" of the 3 cells
*following* the one allocated to "input."
Pete
--
Prof. Peter J. Holsberg Mercer County Community College
Voice: 609-586-4800 Engineering Technology, Computers and Math
UUCP:...!princeton!mccc!pjh 1200 Old Trenton Road, Trenton, NJ 08690
Internet: pjh at mccc.edu Trenton Computer Festival -- 4/20-21/91
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