Re: Mentor for C self study wanted
On Thu, 2007-10-25 at 23:19 +0200, Harald Schmalzbauer wrote: Am Donnerstag, 25. Oktober 2007 20:22:26 schrieb Aryeh M. Friedman: Absolutely. (I just didn't mention it before because obviously Harald already has a beginner's book on the C programming language.) Herald does in fact have one that sucks (it does a terrible job on type sizes for example [doesn't mention that they may very on different Well, probably it's not that bad. First, thanks to all for your help. KR has been laying arround here for at least 2 years. I hated it. It instructs me to use functions like printf without explaining it, and the examples are not really motivating. So every time I tried to write something on my own I was stoped by the simple printf, for example. YOU DARE NOT WORSHIP AT THE CHURCH OF KR No, but seriously, it took me a good three years of picking up KR and trying to get through the first chapter before I eventually started seeing why it's referred to as *the* book. I'm sure it's a very good book as a reference, but it couldn't motivate me as a real C beginner (not a bloody programming beginner though). But if it doesn't work for you, that's okay :) It *is* a good reference, though. There's annotation for the standard libraries in an appendix. So I bought two new books, the first, which I've started with, is Markt+Technik, jetzt lerne ich C (ISBN-13 978-3-8272-4210-5). Indeed, it hasn't made clear that short and int are different, they just explained short and long (and double long) and I thought short is a synonym for int. But it explains in some short sentences the most important behaviour/requirements for the functions we use. It still leaves me alone when it comes to compilers, but after only three evenings so far I think that I made real progress. Writing a simple practice just works :) And I already know that float x; x=10/3 is 3 not 3.. I can't remember reading that in the KR in the first quarter of the book. I'm still quiet happy with it. The next book is O'Reillys C in a nutshell (ISBN 3-89721-344-3). I'll open it if I have structs and pointers practiced... If you've got the cash to spare, try out O'Reilly's Practical C Programming. That book is very friendly, contains a tonne of great examples, and a lot of on the spot quizzes. I was sold on it when I read the rear cover. It says something like: KR teaches there are nine rules of precedence in C. We teach two: the first is that addition comes before subtraction, the second is use parentheses for everything else. That kind of simplicity isn't something you see everywhere. And of course I'll replay the KR if I have some more basics. Thanks, -Harry machines])... since he is paying me a small amount to help him in detail I am going to recommend KR to him (with the caution is is meant for experienced programmers) ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
Harald Schmalzbauer wrote: #include stdio.h void main() That's not a C program. :-) The return value of the main function of a valid C program must be int. And of course, your main function should end with return 0; or exit(0); (the latter requires #include stdlib.h at the top). By the way, I recommend you get a copy of the C standard and use it for reference. You can buy a digital copy (PDF) at http://webstore.ansi.org/ (Search for 9899-1999), it's $30. Alternatively ask Google for C99 draft to get a free copy of a draft of the standard, which isn't very different from the final standard. You can also buy a hardcopy of the standard (i.e. a book), but it was ~ $300 last time I looked. Best regards Oliver -- Oliver Fromme, secnetix GmbH Co. KG, Marktplatz 29, 85567 Grafing b. M. Handelsregister: Registergericht Muenchen, HRA 74606, Geschäftsfuehrung: secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün- chen, HRB 125758, Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd When your hammer is C++, everything begins to look like a thumb. -- Steve Haflich, in comp.lang.c++ ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
On Thu, Oct 25, 2007 at 05:02:00PM +0200, Oliver Fromme wrote: Harald Schmalzbauer wrote: #include stdio.h void main() That's not a C program. :-) The return value of the main function of a valid C program must be int. And of course, your main function should end with return 0; or exit(0); (the latter requires #include stdlib.h at the top). By the way, I recommend you get a copy of the C standard and use it for reference. You can buy a digital copy (PDF) at http://webstore.ansi.org/ (Search for 9899-1999), it's $30. Alternatively ask Google for C99 draft to get a free copy of a draft of the standard, which isn't very different from the final standard. You can also buy a hardcopy of the standard (i.e. a book), but it was ~ $300 last time I looked. That may be the price if you order a paper copy directly from ANSI/ISO, but the C standard has also been published as a regular book (ISBN 978-0-470-84573-8) which is not quite so expensive. For a beginner the standard itself is probably a bit too heavy-going. The book usually recommended is 'The C programming language, Second edition' by Kernighan and Ritchie. ( http://cm.bell-labs.com/cm/cs/cbook/ ) -- Insert your favourite quote here. Erik Trulsson [EMAIL PROTECTED] ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
In response to Oliver Fromme [EMAIL PROTECTED]: Harald Schmalzbauer wrote: #include stdio.h void main() That's not a C program. :-) The return value of the main function of a valid C program must be int. And of course, your main function should end with return 0; or exit(0); (the latter requires #include stdlib.h at the top). By the way, I recommend you get a copy of the C standard and use it for reference. You can buy a digital copy (PDF) at http://webstore.ansi.org/ (Search for 9899-1999), it's $30. Alternatively ask Google for C99 draft to get a free copy of a draft of the standard, which isn't very different from the final standard. You can also buy a hardcopy of the standard (i.e. a book), but it was ~ $300 last time I looked. If we're recommending books, I can't say enough good things about the Kernighan and Richie C book: http://www.amazon.com/C-Programming-Language-2nd/dp/0131103628/ref=pd_bbs_2/105-1904293-7155604?ie=UTF8s=booksqid=1193326006sr=1-2 -- Bill Moran http://www.potentialtech.com ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
Erik Trulsson wrote: Oliver Fromme wrote: By the way, I recommend you get a copy of the C standard and use it for reference. You can buy a digital copy (PDF) at http://webstore.ansi.org/ (Search for 9899-1999), it's $30. Alternatively ask Google for C99 draft to get a free copy of a draft of the standard, which isn't very different from the final standard. You can also buy a hardcopy of the standard (i.e. a book), but it was ~ $300 last time I looked. That may be the price if you order a paper copy directly from ANSI/ISO, Yes. but the C standard has also been published as a regular book (ISBN 978-0-470-84573-8) which is not quite so expensive. OK, I didn't know about that one. Personally I chose to order the PDF copy (which was $15 a few years ago, IIRC, now it's $30) after working for some time with the free draft. For a beginner the standard itself is probably a bit too heavy-going. Right, it's certainly not suitable as a tutorial or as an introduction for a beginner. But it's useful as a reference when you need to know the official details about a certain part of the language (e.g. where do sequence points occur, exactly?). The book usually recommended is 'The C programming language, Second edition' by Kernighan and Ritchie. ( http://cm.bell-labs.com/cm/cs/cbook/ ) Absolutely. (I just didn't mention it before because obviously Harald already has a beginner's book on the C programming language.) Best regards Oliver -- Oliver Fromme, secnetix GmbH Co. KG, Marktplatz 29, 85567 Grafing b. M. Handelsregister: Registergericht Muenchen, HRA 74606, Geschäftsfuehrung: secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün- chen, HRB 125758, Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd Documentation is like sex; when it's good, it's very, very good, and when it's bad, it's better than nothing. -- Dick Brandon ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
On Thu, 25 Oct 2007 17:22:11 +0200 Erik Trulsson [EMAIL PROTECTED] wrote: For a beginner the standard itself is probably a bit too heavy-going. The book usually recommended is 'The C programming language, Second edition' by Kernighan and Ritchie. ( http://cm.bell-labs.com/cm/cs/cbook/ ) Yes, that's the *definitive* book! Add to it The C Answer Book by Tondo/Gimpel (title in german: Das C Loesungsbuch), with goes along with KR 2nd Ed. and you have everything you need. -cpghost. -- Cordula's Web. http://www.cordula.ws/ ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
Absolutely. (I just didn't mention it before because obviously Harald already has a beginner's book on the C programming language.) Herald does in fact have one that sucks (it does a terrible job on type sizes for example [doesn't mention that they may very on different machines])... since he is paying me a small amount to help him in detail I am going to recommend KR to him (with the caution is is meant for experienced programmers) -- Aryeh M. Friedman FloSoft Systems Developer, not Business, Friendly http://www.flosoft-systems.com ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
On 2007-10-25 Bill Moran wrote: In response to Oliver Fromme [EMAIL PROTECTED]: Harald Schmalzbauer wrote: #include stdio.h void main() That's not a C program. :-) The return value of the main function of a valid C program must be int. And of course, your main function should end with return 0; or exit(0); (the latter requires #include stdlib.h at the top). By the way, I recommend you get a copy of the C standard and use it for reference. You can buy a digital copy (PDF) at http://webstore.ansi.org/ (Search for 9899-1999), it's $30. Alternatively ask Google for C99 draft to get a free copy of a draft of the standard, which isn't very different from the final standard. You can also buy a hardcopy of the standard (i.e. a book), but it was ~ $300 last time I looked. If we're recommending books, I can't say enough good things about the Kernighan and Richie C book: http://www.amazon.com/C-Programming-Language-2nd/dp/0131103628/ref=pd_bbs_2/105-1904293-7155604?ie=UTF8s=booksqid=1193326006sr=1-2 No doubt the most valuable book on programming I've ever read. Not only it teaches programming but also a style of thinking and designing. -- Bahman Movaqar You can't write poems about trees when the woods are full of policemen. -Bertolt Brecht ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
Am Donnerstag, 25. Oktober 2007 20:22:26 schrieb Aryeh M. Friedman: Absolutely. (I just didn't mention it before because obviously Harald already has a beginner's book on the C programming language.) Herald does in fact have one that sucks (it does a terrible job on type sizes for example [doesn't mention that they may very on different Well, probably it's not that bad. First, thanks to all for your help. KR has been laying arround here for at least 2 years. I hated it. It instructs me to use functions like printf without explaining it, and the examples are not really motivating. So every time I tried to write something on my own I was stoped by the simple printf, for example. I'm sure it's a very good book as a reference, but it couldn't motivate me as a real C beginner (not a bloody programming beginner though). So I bought two new books, the first, which I've started with, is Markt+Technik, jetzt lerne ich C (ISBN-13 978-3-8272-4210-5). Indeed, it hasn't made clear that short and int are different, they just explained short and long (and double long) and I thought short is a synonym for int. But it explains in some short sentences the most important behaviour/requirements for the functions we use. It still leaves me alone when it comes to compilers, but after only three evenings so far I think that I made real progress. Writing a simple practice just works :) And I already know that float x; x=10/3 is 3 not 3.. I can't remember reading that in the KR in the first quarter of the book. I'm still quiet happy with it. The next book is O'Reillys C in a nutshell (ISBN 3-89721-344-3). I'll open it if I have structs and pointers practiced... And of course I'll replay the KR if I have some more basics. Thanks, -Harry machines])... since he is paying me a small amount to help him in detail I am going to recommend KR to him (with the caution is is meant for experienced programmers) -- OmniSEC - UNIX und Windows Netzwerke - Sicher Harald Schmalzbauer Flintsbacher Str. 3 80686 München +49 (0) 89 18947781 +49 (0) 160 93860101 USt-IdNr.: DE253184753 signature.asc Description: This is a digitally signed message part.
Re: Mentor for C self study wanted
On 2007-10-23 23:24, Harald Schmalzbauer [EMAIL PROTECTED] wrote:
Thanks all,
here was my example, just for completeness, I found mentors for my
needs.
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
You are passing %d to scanf() so it expects to find enough 'storage'
in its pointer argument for an 'int'. If 'short' happens to have a
smaller size (as is commonly the case), scanf() will overwrite random
memory locations after 'nnote'. On systems where 'nnote' is stored in
the stack (because it's an automatic/local variable of main()), you are
risking stack corruption (and a SEGFAULT *may* happen).
It's also a very good idea to check the return code of scanf():
int nnote;
if (scanf(%d, nnote) != 1) {
error;
}
switch (nnote)
{
case 1: printf(Die Note %d entspricht sehr gut.,nnote);
break;
case 2: printf(Die Note %d entspricht gut.,nnote);
break;
case 3: printf(Die Note %d entspricht befriedigend.,nnote);
break;
case 4: printf(Die Note %d entspricht ausreichend.,nnote);
break;
case 5: printf(Die Note %d entspricht mangelhaft.,nnote);
break;
case 6: printf(Die Note %d entspricht ungen?gend.,nnote);
break;
default: printf(%d ist keine zul?ssige Schulnote!);
There's no `int' argument to the printf() call of the default clause.
This will either cause printf() to print random garbage, or try to
access memory regions which are unmapped and SEGFAULT.
P.S.:
I found that declaring nnote as int soleves my problem, but I couldn?t
understand why.
Another one was the result of default: nnote was -1077942208 instead
of 9 for example.
It was never assigned to 9 :)
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Re: Mentor for C self study wanted
On Tue, 23 Oct 2007 20:44:52 +0200 Harald Schmalzbauer [EMAIL PROTECTED] wrote: The first one was for example the attached code: Why does it segfault? Mailman ate the attachment... Can't see it here. -cpghost. -- Cordula's Web. http://www.cordula.ws/ ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
In response to cpghost [EMAIL PROTECTED]: On Tue, 23 Oct 2007 20:44:52 +0200 Harald Schmalzbauer [EMAIL PROTECTED] wrote: The first one was for example the attached code: Why does it segfault? Mailman ate the attachment... Can't see it here. I may be out of line, but I think if you're using FreeBSD as your learning platform, that it wouldn't be a problem to ask this list. Although, you'll have to include your code inline to get past the sanitizers. -- Bill Moran http://www.potentialtech.com ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
Am Dienstag, 23. Oktober 2007 22:24:54 schrieb Bill Moran:
In response to cpghost [EMAIL PROTECTED]:
On Tue, 23 Oct 2007 20:44:52 +0200
Harald Schmalzbauer [EMAIL PROTECTED] wrote:
The first one was for example the attached code: Why does it segfault?
Mailman ate the attachment... Can't see it here.
I may be out of line, but I think if you're using FreeBSD as your
learning platform, that it wouldn't be a problem to ask this list.
Harald Schmalzbauer wrote:
Am Dienstag, 23. Oktober 2007 21:43:52 schrieben Sie:
Hello Aryeh,
I'm willing to pay fair fees, but are you interested in micro payment
;) ?
For other reasons I love micro payments.
Serious, I'll have the one or other short question per week (I'm
usually busy, just making spare time lessons from my book (UTC-1 spare
time)).
Just so I know what level to present on what is your background in CS
and programming?
CS??
I'm able to solve problems analytically, but I don't know any language
really well.
I know bourne shell, csh, pascal, and basic. And a tiny bit asm, but
that's been on ZX81.
Although, you'll have to include your code inline to get past the
sanitizers.
Thanks all,
here was my example, just for completeness, I found mentors for my needs.
Thanks a lot to all!
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
switch (nnote)
{
case 1: printf(Die Note %d entspricht sehr gut.,nnote);
break;
case 2: printf(Die Note %d entspricht gut.,nnote);
break;
case 3: printf(Die Note %d entspricht befriedigend.,nnote);
break;
case 4: printf(Die Note %d entspricht ausreichend.,nnote);
break;
case 5: printf(Die Note %d entspricht mangelhaft.,nnote);
break;
case 6: printf(Die Note %d entspricht ungenügend.,nnote);
break;
default: printf(%d ist keine zulässige Schulnote!);
}
printf(\n);
}
P.S.:
I found that declaring nnote as int soleves my problem, but I couldnÄt
understand why.
Another one was the result of default: nnote was -1077942208 instead of 9 for
example.
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Re: Mentor for C self study wanted
Am Dienstag, 23. Oktober 2007 23:24:09 schrieb Harald Schmalzbauer:
[*snip*]
Although, you'll have to include your code inline to get past the
sanitizers.
Thanks all,
here was my example, just for completeness, I found mentors for my needs.
Thanks a lot to all!
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
switch (nnote)
{
case 1: printf(Die Note %d entspricht sehr gut.,nnote);
break;
[snip]
default: printf(%d ist keine zulässige Schulnote!);
}
printf(\n);
}
P.S.:
I found that declaring nnote as int soleves my problem, but I couldnÄt
understand why.
Another one was the result of default: nnote was -1077942208 instead of 9
for example.
Ok, the last one is a typo, I forgot ...ote!,%d);.
But interesting that ther's some output. Constant output
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Re: Mentor for C self study wanted
At 04:24 PM 10/23/2007, Harald Schmalzbauer wrote:
Am Dienstag, 23. Oktober 2007 22:24:54 schrieb Bill Moran:
In response to cpghost [EMAIL PROTECTED]:
On Tue, 23 Oct 2007 20:44:52 +0200
Harald Schmalzbauer [EMAIL PROTECTED] wrote:
The first one was for example the attached code: Why does it segfault?
Mailman ate the attachment... Can't see it here.
I may be out of line, but I think if you're using FreeBSD as your
learning platform, that it wouldn't be a problem to ask this list.
Harald Schmalzbauer wrote:
Am Dienstag, 23. Oktober 2007 21:43:52 schrieben Sie:
Hello Aryeh,
I'm willing to pay fair fees, but are you interested in micro payment
;) ?
For other reasons I love micro payments.
Serious, I'll have the one or other short question per week (I'm
usually busy, just making spare time lessons from my book (UTC-1 spare
time)).
Just so I know what level to present on what is your background in CS
and programming?
CS??
I'm able to solve problems analytically, but I don't know any language
really well.
I know bourne shell, csh, pascal, and basic. And a tiny bit asm, but
that's been on ZX81.
Although, you'll have to include your code inline to get past the
sanitizers.
Thanks all,
here was my example, just for completeness, I found mentors for my needs.
Thanks a lot to all!
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
switch (nnote)
{
case 1: printf(Die Note %d entspricht sehr gut.,nnote);
break;
case 2: printf(Die Note %d entspricht gut.,nnote);
break;
case 3: printf(Die Note %d entspricht befriedigend.,nnote);
break;
case 4: printf(Die Note %d entspricht ausreichend.,nnote);
break;
case 5: printf(Die Note %d entspricht mangelhaft.,nnote);
break;
case 6: printf(Die Note %d entspricht ungenügend.,nnote);
break;
default: printf(%d ist keine zulässige Schulnote!);
}
printf(\n);
}
P.S.:
I found that declaring nnote as int soleves my problem, but I couldnÄt
understand why.
Another one was the result of default: nnote was -1077942208 instead of 9 for
example.
if you check the man page on scanf:
d Matches an optionally signed decimal integer; the next pointer must
be a pointer to int.
You shouldn't try to put a short into an int. Always declare the correct
size for variables. Your segv is because scanf was trying to put an int
where it won't fit.
You will get the same result if you go off the end of an array.
-Derek
--
This message has been scanned for viruses and
dangerous content by MailScanner, and is
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Re: Mentor for C self study wanted
On Tue, 23 Oct 2007 23:24:09 +0200
Harald Schmalzbauer [EMAIL PROTECTED] wrote:
#include stdio.h
void main()
{
short nnote;
^
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
^
I found that declaring nnote as int soleves my problem, but I
couldnÄt understand why.
Another one was the result of default: nnote was -1077942208 instead
of 9 for example.
There's a mismatch here: scanf(%d, ...) expects a pointer to int,
while nnote is a pointer to a short. Normally, an int occupies more
bytes in memory than a short (typically sizeof(int) == 4 on 32bit
platforms, and sizeof(int) == 8 on 64bit platforms; while typically
sizeof(short) == 2).
So scanf(3) tries to store the result into 4 bytes, but you've provided
a pointer to only 2 bytes of memory. Where will the other 2 bytes be
stored by scanf? In your example, short nnote is an automatic variable:
i.e. it's stored on the stack. So the other 2 bytes will be also saved
on the stack, on a place that's not reserved for this. There could be
anything there, like, say, a part of the return address for the
function, or it could be on some page in memory that's read-only or
non-allocated. In either case, the program behaviour is undefined, and
this normally means it dumps core.
So either replace short nnote with int nnote, OR change %d
to the appropriate format string identifier for short int %hd
(look up man scanf for a list of those identifiers), both in
scanf and printf calls.
-cpghost.
--
Cordula's Web. http://www.cordula.ws/
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Re: Mentor for C self study wanted
Derek Ragona wrote:
At 04:24 PM 10/23/2007, Harald Schmalzbauer wrote:
Am Dienstag, 23. Oktober 2007 22:24:54 schrieb Bill Moran:
In response to cpghost [EMAIL PROTECTED]:
On Tue, 23 Oct 2007 20:44:52 +0200
Harald Schmalzbauer [EMAIL PROTECTED] wrote:
The first one was for example the attached code: Why does it
segfault?
Mailman ate the attachment... Can't see it here.
I may be out of line, but I think if you're using FreeBSD as your
learning platform, that it wouldn't be a problem to ask this list.
Harald Schmalzbauer wrote:
Am Dienstag, 23. Oktober 2007 21:43:52 schrieben Sie:
Hello Aryeh,
I'm willing to pay fair fees, but are you interested in micro
payment
;) ?
For other reasons I love micro payments.
Serious, I'll have the one or other short question per week (I'm
usually busy, just making spare time lessons from my book (UTC-1
spare
time)).
Just so I know what level to present on what is your background
in CS
and programming?
CS??
I'm able to solve problems analytically, but I don't know any
language
really well.
I know bourne shell, csh, pascal, and basic. And a tiny bit asm, but
that's been on ZX81.
Although, you'll have to include your code inline to get past the
sanitizers.
Thanks all,
here was my example, just for completeness, I found mentors for my needs.
Thanks a lot to all!
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
switch (nnote)
{
case 1: printf(Die Note %d entspricht sehr gut.,nnote);
break;
case 2: printf(Die Note %d entspricht gut.,nnote);
break;
case 3: printf(Die Note %d entspricht befriedigend.,nnote);
break;
case 4: printf(Die Note %d entspricht ausreichend.,nnote);
break;
case 5: printf(Die Note %d entspricht mangelhaft.,nnote);
break;
case 6: printf(Die Note %d entspricht ungenügend.,nnote);
break;
default: printf(%d ist keine zulässige Schulnote!);
}
printf(\n);
}
P.S.:
I found that declaring nnote as int soleves my problem, but I couldnÄt
understand why.
Another one was the result of default: nnote was -1077942208 instead
of 9 for
example.
if you check the man page on scanf:
d Matches an optionally signed decimal integer; the next pointer must
be a pointer to int.
You shouldn't try to put a short into an int. Always declare the
correct size for variables. Your segv is because scanf was trying to
put an int where it won't fit.
You will get the same result if you go off the end of an array.
-Derek
It's well worth increasing the number of warnings enabled when writing C
code, to catch any errors early on. -Wall catches this sort of mistake.
Compiling your code gives the following output:
gcc -Wall test.c -o test
test.c:4: warning: return type of 'main' is not 'int'
test.c: In function 'main':
test.c:9: warning: format '%d' expects type 'int *', but argument 2 has
type 'short int *'
--
Bruce Cran
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Re: Mentor for C self study wanted
cpghost wrote: There's a mismatch here: scanf(%d, ...) expects a pointer to int, while nnote is a pointer to a short. Normally, an int occupies more bytes in memory than a short (typically sizeof(int) == 4 on 32bit platforms, and sizeof(int) == 8 on 64bit platforms; while typically sizeof(short) == 2). I think short and int stay the same on both 32 and 64 bit platforms, while it's only long that gets bumped to 8 bytes. At least that seems to be what happens on FreeBSD amd64. -- Bruce ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
On Tue, 23 Oct 2007 23:36:40 +0100 Bruce Cran [EMAIL PROTECTED] wrote: cpghost wrote: There's a mismatch here: scanf(%d, ...) expects a pointer to int, while nnote is a pointer to a short. Normally, an int occupies more bytes in memory than a short (typically sizeof(int) == 4 on 32bit platforms, and sizeof(int) == 8 on 64bit platforms; while typically sizeof(short) == 2). I think short and int stay the same on both 32 and 64 bit platforms, while it's only long that gets bumped to 8 bytes. At least that seems to be what happens on FreeBSD amd64. Hmmm... yep, you're right, I'm wrong! I've switched compilers too often recently. Yes, on gcc sizeof(int) == 4 on both 32bit and 64bit. Thanks for pointing this out: I stay corrected. ;) -- Cordula's Web. http://www.cordula.ws/ ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Mentor for C self study wanted
Am Dienstag, 23. Oktober 2007 23:24:09 schrieb Harald Schmalzbauer:
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
man 3 scanf (most important thing to look at with any such problem is the
C-library documentation, which is excellent on FreeBSD) says that for %d
the passed pointer has to be a pointer to integer, which nnote is not.
nnote is a pointer to short, which points to 2 bytes, whereas a pointer to
integer is a pointer to 4 bytes of storage.
Generally, nnote is reserved by the compiler on the stack (as it's a local
variable) with two bytes (but this depends on your platform), and nnote
points to the beginning of this area.
As you are probably running on a little-endian architecture, the layout that
scanf presumes is (from low to high):
--- increasing addresses
lsbyte 2 3 msbyte
^
|-- nnote points here
of which only the first two are interpreted as nnote by the rest of the
program; the upper two are different stack content (probably a return address
to the C initialization code calling main(), or a pushed stack pointer, or
such, as your procedure defines no other locals, see below).
Now, when scanf assigns the four bytes, it'll properly enter the lower two
bytes of the integer into lsbyte 2 (which is nnote, in the same byte
order), but overwrite two bytes that are above it.
When main() finishes, the (now broken) saved address (of which 3 msbyte is
the lower half) is popped, which leads to the SIGSEGV you're seeing.
In case you were on big-endian, the result would be different (i.e., the order
would be reversed, so that nnote would always be zero or minus one in case
you entered small integral values in terms of absolute value), but
effectively, the return address would be overwritten as well, breaking it.
This is effectively what can be called a buffer-overflow.
Just to finish this: the proper format would be %hd, for which the flag h
signifies that the pointer is a pointer to a short int, also documented in
man 3 scanf.
Why aren't you seeing this behaviour with printf (i.e., why can you pass a
short but still specify %d)? Because C defines that functions that take a
variable number of arguments (of which printf is one such) get each argument
as type long (the type that's at least as big as a pointer on the current
platform), so when passing a short as argument to a var-args function, the
C-compiler inserts code which makes sure that the value is promoted to a long
in the argument stack for printf. scanf is also a varargs function, but
you're not passing the value of nnote, but rather a pointer to it, which
(should) already be as wide as a long.
Finally, looking at (parts of) the assembly that gcc generates (on a
little-endian i386 machine):
.globl main
.type main, @function
main:
leal4(%esp), %ecx
andl$-16, %esp
pushl -4(%ecx)
pushl %ebp
; Set up the pointer to the local frame (EBP on i386). All locals are
; relative to EBP in a function.
movl%esp, %ebp
; ECX is the first (hidden) local.
pushl %ecx
subl$20, %esp
subl$12, %esp
pushl $.LC0
callprintf
addl$16, %esp
subl$8, %esp
; Load the effective address of EBP-6, i.e., nnote, into EAX, which
; is pushed for scanf. scanf will thus write its output on EBP-6 up to
; EBP-3, where EBP-4 and EBP-3 are part of the value that's been
; pushed in the pushl %ecx above.
leal-6(%ebp), %eax
pushl %eax
pushl $.LC1
callscanf
...
; Restore the value at EBP-4 (i.e., the ECX that was pushed above) into
; ECX at function exit. This value has been corrupted by the integer
; assignment due to scanf.
movl-4(%ebp), %ecx
leave
; Restore the stack pointer from the (invalidated) %ecx, i.e. produce a
; bogus stack pointer.
leal-4(%ecx), %esp
ret
This produces a segfault, after the return to the C initialization code,
simply because the stack pointer is totally bogus.
P.S.:
I found that declaring nnote as int soleves my problem, but I couldnÄt
understand why.
Everything clear now? ;-)
--
Heiko Wundram
Product Application Development
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Re: Mentor for C self study wanted
Bruce Cran wrote:
cpghost wrote:
There's a mismatch here: scanf(%d, ...) expects a pointer to int,
while nnote is a pointer to a short. Normally, an int occupies more
bytes in memory than a short (typically sizeof(int) == 4 on 32bit
platforms, and sizeof(int) == 8 on 64bit platforms; while typically
sizeof(short) == 2).
I think short and int stay the same on both 32 and 64 bit platforms,
while it's only long that gets bumped to 8 bytes. At least that seems
to be what happens on FreeBSD amd64.
--
Bruce
No... you're only safe using int32, int64, etc. Just for grins try
compiling a program like this:
#include stdio.h
int main() {
printf(%d\n, sizeof(int));
return 0;
}
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Re: Mentor for C self study wanted
On Tue, 23 Oct 2007 23:24:09 +0200
Harald Schmalzbauer [EMAIL PROTECTED] wrote:
#include stdio.h
void main()
{
short nnote;
// Numerischen Notenwert einlesen
printf(Bitte numerischen Schulnotenwert eingeben: );
scanf(%d,nnote);
switch (nnote)
{
case 1: printf(Die Note %d entspricht sehr gut.,nnote);
break;
case 2: printf(Die Note %d entspricht gut.,nnote);
break;
case 3: printf(Die Note %d entspricht befriedigend.,nnote);
break;
case 4: printf(Die Note %d entspricht ausreichend.,nnote);
break;
case 5: printf(Die Note %d entspricht mangelhaft.,nnote);
break;
case 6: printf(Die Note %d entspricht ungenügend.,nnote);
break;
default: printf(%d ist keine zulässige Schulnote!);
^ ^
No matching int for %d here. It'll print garbage. Change to:
default: printf(%hd ist keine...!, nnote);
}
printf(\n);
}
P.S.:
I found that declaring nnote as int soleves my problem, but I
couldnÄt understand why.
Another one was the result of default: nnote was -1077942208 instead
of 9 for example.
The reason for this is that the number of arguments after printf's
format string MUST match the number of %-place holders (unless
you're using exotic stuff like %n, of course). If printf misses
some arguments, it will fetch them from a place that is
implementation dependant (and that almost always means:
you'll get garbage).
Sorry for overlooking your second question... ;)
-cpghost.
--
Cordula's Web. http://www.cordula.ws/
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