[OCLUG-devel] Trouble with strings and files

[James Colannino]

Tim Thelin wrote:
> 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.

Extremely helpful.  Thank you :)

James
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