Segmentation fault in C

hi all,

i'm learning C at the moment, and one of the programs given as example converts fahrenheit to celsius.
But since i like to reverse the program, to convert celsius to fahrenheit, i've rewritten it to learn from it.

only problem is, when i run it and enter a value for celsius, i get an segmentation fault.

what did i do wrong? here is the source:

Quote:

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#include <stdio.h>
main()
{
int celsius;
float fahr;
printf("This is very a simple program which allows\n");
printf("you to convert a celsius value to fahrenheit.\n");
printf("\nType the value of celsius you want converted:\n");
scanf("%3d", celsius);
fahr = celsius / (5.0/9.0) + 32;
printf("%3d degrees celsius is %3.2f degrees fahrenheit.", celsius, fahr);
return 0;
}

---

i've tried changing the var fahr to a double, and even an int, but the same problem still persists..

and when i do the calculation with a scientific calculator the result is correct.

scanf("%3d", celsius);
where is the & it should written as
scanf("%d", &celsius);

Yea power_user's correct, but you can use the 3d I think, becuase doesnt that just specify the number of decimal places you want it to?

i2c

ooohhh sh*t, that i've missed that, and yes, the 3d just specifies the maximum input, 3 characters...
so a max of 999 integer...

but i feel very stupid i've forgot the & sign :(

thanks a lot for the help :)

That was always the one that got me when I started coding in C. The damn "&' character when using scanf!! I got used to it after a while though.

it's always the smallest thing

yes indeed, last time i helped someone else out with his code, he had the following string in his code:

for (i = 0; i < 10; i++);

and he wonderd why it didn't work.. he typed a ';' to much, but i saw it right away... this means that just a thing so little, although you know how it works, you just don't see it, and someone else spots it in a second....

strange thing, how the human mind works :)

I've been programming for some years now but pointers, references, badly placed semi-colons and misplaced quotes are still the source of most of my bugs ;)

I once (quite recently), spent about an hour debugging this line:

Code:

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if(i=true)
    blablabla;

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If you can't see what's wrong, it should be == instead of =. Using = causes i to be assigned the value true and the condition is always true. I've been programming for about 10 years now, and I still made this boo boo.

Cheers,
cgkanchi

And the security aspect of this thread is?

Quote:

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And the security aspect of this thread is?

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learning C, to understand how most exploits work and eventually got the knowledge to write them myself, and therefor more knowlegdable about defending a system against it....

lepricaun, let me try that one.

The security aspect doesn't exist. People are talking about there code, it doesn't matter if it is security related. This is the closest forum that you can talk about code. Since the only other code forum is for finished products that you want to show off.

Lepricaun, I have learned from hours, days of debugging C. (I hate that damn langauge) That whenever you can't figure out what the hell is wrong. Just walk away for about 5 min. Have a beer, Blount, just do something. Then come back. Whatever is wrong, will jump out.

I love perl, so much easier, though there are still things that are a pain in the ass.

Quote:

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The security aspect doesn't exist. People are talking about there code, it doesn't matter if it is security related. This is the closest forum that you can talk about code. Since the only other code forum is for finished products that you want to show off.

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my response was more or less a joke, if you read most of the threads, more then 50% doesn't have anything to do with security :)

but that doesn't mean that the reason i gave are my main reasons of learning C.
i've started with c++ about a year ago, but since i got more interested in *nix, i also got more interested in C, that's why i switched to C, cause when you look at a sourcecode of some program, even the kernel, it is all written in C.

Quote:

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Lepricaun, I have learned from hours, days of debugging C. (I hate that damn langauge) That whenever you can't figure out what the hell is wrong. Just walk away for about 5 min. Have a beer, Blount, just do something. Then come back. Whatever is wrong, will jump out.

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this is a VERY good advise, not only with programming in C, but with most problems in general. it doesn't even have to be computer related...

Quote:

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I love perl, so much easier, though there are still things that are a pain in the ass

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Well, i've found that every language has his advantage and disadvantage, and i truly believe that when your good at a language like C, you can learn most other languages (not all of course) in less then a week.
it's just the syntaxis that's different, for the rest the main stays the same..

my biggest problem is developing a program.. making a pseudo source code. e.g:

suppose i wanted to write a portscanner with the possibility to fingerprint and check for known exploits and at the same time keep a database for those exploits, then i wouldn't know where the hell to start..
i wouldn't even know which libraries to use or which functions...

there are dozens of tutorials and books for C on the sysntaxis, but none of them for the above problem :(

as soon as i've got more knowledge / /experience in programming, this kind of tutorial is one of the first things i am going to write...

Quote:

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suppose i wanted to write a portscanner with the possibility to fingerprint and check for known exploits and at the same time keep a database for those exploits, then i wouldn't know where the hell to start..
i wouldn't even know which libraries to use or which functions...

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My development cycle for C goes something like this:
1. Jot down (either on paper or notepad/word processor) the purpose of the program.
2. Jot down all the functions that you want the program to have.
3. Decide the order in which you implement the functions.
4. Start writing code.

If it were an OOP language like C++ or Java, I'd also design the classes that I want in great detail.

Cheers,
cgkanchi

thats reasonable, but that means you would already know which functions you would use for that particular program, and i doubt that there are standard functions for portscanning, fingerprinting or something like that, so you would have to know how to write them... and i wouldn't even know where to start writing such a function..

Quote:

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Originally posted here by whizkid2300
lepricaun, let me try that one.

The security aspect doesn't exist. People are talking about there code, it doesn't matter if it is security related. This is the closest forum that you can talk about code. Since the only other code forum is for finished products that you want to show off.

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Then put it in GCC where, IMHO, it belongs.

I know that lots of stuff isn't security related but that doesn't mean it shouldn't be.

Quote:

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thats reasonable, but that means you would already know which functions you would use for that particular program, and i doubt that there are standard functions for portscanning, fingerprinting or something like that, so you would have to know how to write them... and i wouldn't even know where to start writing such a function..

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No, I mean your functions. The functions that you're going to write. Say, to write your port scanner/fingerprinter, your step 2 might be:

1. TCP Connect Scan
2. OS Identification
3. FIN Scan
4. SYN Scan
....

and step 3 might be...
1. TCP Connect Scan
2. SYN Scan
3. OS Identification
4. FIN Scan

Cheers,
cgkanchi

ok, i understand what you mean, but i'm not advanced enough yet to program using sockets, so i doubt i will write a portscanner very soon :P

but thanks for the replies .

Agh... You got to love perl. To write a portscanner in perl. Get yourself a module, and little code and your done.

(Kind of one of the reasons that I stopped using C so much. It took friggin forever to do anything, when I could write the same thing in perl, with much less time.)

I will say, though. I am going to have to go back and relearn C. I have to use it in school, and that will be interesting, trying to use a language that you have forgot.

Oh and Steve things related to programming more relate to here, than to GCC. In Gcc most people want even notice the thread. I don't even go in there most of the time. It works out better in Programming Security, until, or unless they make a forum for just Coding BS. (Which I doubt.)

I created a thread in site suggestions asking for a code questions (or something like that forum). A few people gave it the thumbs up, but the moderators, admins, etc. just ignored it. So until that forum does exist, it looks like it's either code review or programming security for threads like this, and I frown at putting code related questions in code review.

ac

i'll have to agree with that, the only other option is going to another forum if posts about code aren't appreciated or read, so let's hope it doesn't come this far, i really like this forum :D

b.t.w., i've just registered an account to this forum, but i haven't posted anything there yet, but perhaps i will soon, we'll see...

That's a bit like one of the letters to the editor in the uk "Private Eye" satire magazine. A kind of threat to hand in your subscription. :P if it doesn't make sense, ignore it.

ac

hahahha, that's not what i meant, but isn't it best to stick to one forum where you can go with all your problems, instead of needing 2 or 3? and for me that forum would surely be AO.

Quote:

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Originally posted here by whizkid2300
Agh... You got to love perl. To write a portscanner in perl. Get yourself a module, and little code and your done.

(Kind of one of the reasons that I stopped using C so much. It took friggin forever to do anything, when I could write the same thing in perl, with much less time.)

I will say, though. I am going to have to go back and relearn C. I have to use it in school, and that will be interesting, trying to use a language that you have forgot.

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Actually I sometimes use Perl as the pseudocode for my C program. Perl has a very forgiving syntax and you can use a C-style syntax in Perl. If you stick to the 'basic' modules in Perl you can build and test the functionality of your program. It's usually easy to convert your Perl script to C, just add all the things Perl does automagicly (declaring variables, memory etc.).