decrement file pointer by 1

I've never had a problem with fseek().

How about posting your code and giving us a better description of what you're seeing.

One thing to remember is that if you're trying to move the file pointer back so that it's positioned before the new-line character, you have to back up TWO characters for Windows/DOS files.


Kent

Hi Kdo,
I am counting number of lines in a text file. But if there are comments in the line starting with "#" I need to ignore them.
Here is the code, please let me know. The code freezes at fseek...
Rgds,

int countlogs(FILE *fp1)
{
      int count;
        char buff[256];
        long int offset=-2;
        char ch;
        while(!feof(fp1))
        {
               ch=fgetc(fp1);

               if(ch=='#')
                    fgets(buff, 256, fp1);  
                else
                    fseek(fp1, offset, SEEK_CUR);              
              fgets(buff, 256, fp1);
                count++;
        }
        return count;
}

The fseek() is probably working just fine.  Looking at the code you're going to increment count whether the line starts with '#' or not.

Try this for you loop:

  while (fgets (buff, 256, fp1))
    if (*buff != '#')
      ++count;



Kent

Hi Kent,
fgets gets a line...
Only if the first char in the line is '#' I need to ignore the line. So we need to call function fgetc.
All the  other lines I need to count.
Actually the code is freezing after it runs fseek.
Rgds,

Yep.

Think about what your loop is doing:

        while(!feof(fp1))
        {
               ch=fgetc(fp1);

               if(ch=='#')
                    fgets(buff, 256, fp1);  
                else
                    fseek(fp1, offset, SEEK_CUR);              
             fgets(buff, 256, fp1);
                count++;
        }


At the start of a line you read a character.  If the character is a '#', indicating a comment, you set the file pointer back before the '#' character and continue.  When you next read a character, you read the same '#' again and reset the file pointer again.  You've created the infinite loop.  :)

That's why I suggested the short loop in my previous post.  It reads an entire line, and if the line is not a comment the counter is incremented.


Kent

#include <stdio.h>
int main()
{
      int count=0;
        char buff[256];
        long int offset=-2;
        char ch;
        FILE *fp1;
        fp1=fopen("logtab1", "r");
       
        while(!feof(fp1))
        {
               ch=fgetc(fp1);

               if(ch=='#')
                    fgets(buff, 256, fp1);  
               else
               {
                    fgets(buff, 256, fp1);
                    count++;
               }
        }      
        fclose(fp1);
        printf("COUNT=%d\n", count);
        return count;
}


Logtab file
----------
#this is a test
TESTEMERG      1      Emergency test message
TESTALERT      2      Alert test message
TESTALERT      3      Alert test message
TESTALERT      4      Alert test message


Here count prints 5. It should be four. Could you please let me know why....
Rgds,

Sure.  After the line "ch=fgetc(fp1);" insert the following line:

  printf ("count = %d.  Checking %c\n", count, ch);


I think that should show you what's happening.  :)

Kent