Why is it -2 and not -1? This is a part of a code that has to write a string from a file to another in reverse. Can you help me understand why it is -2 and not -1?
while ( n >= 0) {
read(fdin, &c, 1);
write(fdout, &c, 1);
n=lseek(fdin,-2,SEEK_CUR);
}
The read part is the one that reverses the characters. Since each read of 1 character moves the file position forward by one character, we must go backward 2 characters to advance backwards.
Suppose the file is only 2 characters long, having contents AB:
AB
In the beginning in a part of the code you didn't show here, the file pointer is positioned one before the end of file, so it is pointing at B.
|
v
AB
Now we read one character - B is read. The file pointer on fdin is advanced to the end of file.
|
v
AB
If we seek backwards by one character, we'll end up at B again. But if we seek backwards 2 character, it will point to A:
|
v
AB
And we end up reading first B then A.
Finally when we try to seek beyond the beginning of the file, lseek will return (off_t)-1 to signify an error condition and the while loop condition becomes false and the loop is exited.