Writing my own scanf(), sscanf() and fscanf() functions

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I did an assignment where I had to read in a bunch of bank records from a file, sort them and then display them. When starting it in debug mode in Microsoft Visual Studio, I had trouble specifying the file that it was supposed to read. Every time I specified it, it would tell me that the file could not be read, unless I ran the program from the command prompt, in which case, it found the file, but did not help me much. I spent a long time debugging my program by going back and forth from Visual Studio, a secure file transfer application and a Unix shell where I would compile my program in GCC and run it, looking for indications of what was wrong by examining the output of printf statements I had inserted into the code. Needless to say, I need to familiarize myself with gdb.

 

Anyway, a few bugs caused me huge headaches. One involved a file pointer, where I was passing ofp to a function where I should have passed ifp. The other involved scanf. I ended up writing a program in Visual Studio specifically to debug my usage of the format descriptors. Here is some example code where I am trying to scan what would have been a record from a file if I had not been trying to debug the thing:

 

char * string = "Bob Jones&12/11/1965&854367451&30082.95", * s = string;

 

sscanf(s, "%s %[^&]s&%2i/%2i/%4i & %i & %lf", list.first, list.last,

*************** &list.month, &list.day, &list.year,

*************** &list.accountNumber, &list.balance);

 

That code will properly read the first two strings and then be able to read nothing else. Here is something I wrote (which I had to modify to be able to use with file pointers) that did work:

 

 

char * string = "Bob Jones&12/11/1965&854367451&30082.95", * s = string;

 

sscanf(s, "%s %[^&]s&", list.first, list.last);

 

while (*s++ != '&');

 

sscanf(s, "%2i/%2i/%4i & %i & %lf",

******* &list.month, &list.day, &list.year,

******* &list.accountNumber, &list.balance);

 

Because of problems I was having, for the final implementation, I split the fscanf into a separate statement for each variable with additional fscanf statements to skip over characters. Even then, it was not without issues. If the string was "Bob Jones &12/11/1965&854367451&30082.95," it would store "Jones " instead of "Jones" in list.last. I modified the descriptor to be %[^& ]s, which did not make any change. I modified it to be %[^ &]s and it broke the entire thing. I eventually decided to scan it to a temporary string where it would then be rescanned to its destination.

 

I spent a few hours working on these issues and I do not want any recurrences of it, so I plan to do a few things:

 

1. Learn to use gdb

 

2. Figure out why my program did not find the file under debug mode, but found it without a problem when I ran it through the command prompt.

 

3. Write my own scanf(), sscanf() and fscanf() functions. I am sure that they will make my life easier in the future. However, I have a few questions:

1. I have a copy of The C Programming Language, by Brian W. Kernighan and Dennis M. Ritchie. Will I need any other reference materials or should this be sufficient?
   
2. Would robust sscanf() and fscanf() implementations be written using parse trees or could they be written by implementing cases.
   
3. Will I be violating Brian W. Kernighan's and Dennis M. Ritchie's vision for the scanf(), sscanf() and fscanf() functions if I make whitespace matter inside square brackets?
   
4. Is it possible to do things in such a way that I could divert all function calls to scanf(), sscanf() and fscanf() to my own custom functions without affecting the other stdio.h functions or will I have to use different function names?
   
5. If I manage to make robust implementations of scanf(), sscanf() and fscanf(), what will be my options for giving back to the community?

 

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