[OCLUG-devel] Trouble with strings and files

[Tim Thelin]

James Colannino wrote:

> Hey everyone.  One of the programming excercises in my C book asked me 
> to write a program that will take numbers from a file, place all the 
> numbers divisible by 3 in one file, and all the numbers not divisible 
> by 3 in another file.  I wrote a program that compiles without any 
> fatal errors (albeit I do get some warnings; I will post if asked), 
> but when I run the program with the correct number of arguments, it 
> seg faults.  I ran strace on it to see what it was failing on, but had 
> trouble locating the exact problem.  Below are URLs pointing to both 
> the C source and strace's output:
>
> http://james.colannino.org/mailinglist/strace.output
> http://james.colannino.org/mailinglist/twolist.c
>
> I'm sure there's some blatantly obvious and stupid thing that I'm 
> doing, but for the life of me, I can't figure out what it is.  I'm 
> hoping that someone more experienced can hit me over the head with the 
> correct answer :)  Thanks very much in advance.
>
> James

You probably only want to use strace for debugging when your trying to 
figure out what system calls are being used and with what arguments.  
Otherwise its not too useful as a general debugging tool.  Getting 
familiar with gdb (or one of its graphical front ends) would probably be 
more useful to you.

I believe the issue is your sscanfs for turning the number into a 
string.  You should actually be using sprintf there, not sscanf.  scanf 
is for turning a string something and parsing it into smaller units 
(scaning); printf is used for taking smaller units and turning them into 
a single string.  So you want to take your smaller unit (the number) and 
turn it into the string you want to place in the file with your fputs.

Its probably segfaulting because scanf needs a string as a first arg and 
your passing in a number; remember how strings are just char pointers... 
and numbers can be cast as a char* quite easily... so scanf is using 
your number as a memory address and seg faulting.  One of the compiler's 
warnings probably hinted at this (as a personal rule I always compile 
with -Wall (all warnings) and make sure i've solved every warning before 
continuingor at least made sure i know why i'm ignoring a warning).

Btw you could combine your (soon to be) sprintfs and fputs into a single 
fprintf, and you could combine your fgets and scanf into a single fscanf.

- Tim Thelin