move file pointer

[monion03]

Hello,

We have a problem. We would like to set the pointer of a file in c. We read about fseek but this functions moves the file pointer according to a number of bytes. Unfortunately we don't know the exact byte of the desired pointer position.

Actually, we have a file with just one integer per line and we want to read a specific number of rows of this file.

How can we move the pointer, let's say at line 3?

Thank you in advance.

 

[Salem]

Unless you've already read the file once, you can't seek to a particular line of a file.

For example, to build an index of where all the lines are

while ( !feof(fp) ) {
  pos[line++] = ftell(fp);
  fgets( buff, sizeof buff, fp );
}

Later on, you can then do something like

fseek( fp, pos[3], SEEK_SET );
fgets( buff, sizeof buff, fp );

--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[cpjust]

If your file has a fixed-length record size (i.e. line size) then you could do what you need to do without having to read every line in the file; otherwise, you'll have to use Salem's method.