Thread: How do you move around a text file in C?

If you know the length - you can use fsetpos for example to skip known number of bytes

Nope, you can't do arbitrary seeking on a text file. It has to be opened as a binary file.

And of course you have a quote from the reference guide to support your opinion?

Why yes, since you asked so nicely. Here's the quote that says fsetpos() won't work because you have to have been to the position first and called fgetpos():

The fsetpos function sets the mbstate_t object (if any) and file position indicator
for the stream pointed to by stream according to the value of the object pointed to by
pos, which shall be a value obtained from an earlier successful call to the fgetpos
function on a stream associated with the same file.

And here's the quote that says you can't do arbitrary seeking on a text file with the only other option, fseek():

For a text stream, either offset shall be zero, or offset shall be a value returned by
an earlier successful call to the ftell function on a stream associated with the same file
and whence shall be SEEK_SET.

Oh, and those quotes are from the C standard, not just any old reference guide that happens to agree with me.

yeah, but this is about these 2 functions...

They won't work though. You can't jump to somewhere you haven't been yet and fsetpos() requires a successful call to fgetpos(). The only way to jump somewhere you haven't been is with fseek() and fseek() doesn't work for arbitrary jumps on a text file.

Originally Posted by Noir

fseek() doesn't work for arbitrary jumps on a text file.

No, it doesn't work on stream which are opened in text mode. Not the same thing. You can open a text file in binary mode, or a binary file in text mode, if you want.

That was bad wording on my part. I meant "text stream". Sorry for the confusion.

Originally Posted by buggerboy_3

Hello! I'm new to C and i'm trying to figure out how to move down a text file and print out only certain lines.

I know that fgets reads one line from a file but how do you skip 4 lines and then print out the 5th line?

You don't "skip" lines in a text file, you read them in, BUT YOU ONLY MAY PRINT EVERY 4th OR 5TH LINE (whatever you want).

You can use a while loop, like this:

Code:

while (there is another line to be read)  
   increment line_count;
   if ((line_count % 4 == 0) && (line_count > 0))  /* selects every 4th line */
      print that line;
loop

So you're not "jumping over", your program is just selecting every 4th line to do something with. The other lines are just ignored.

Adak