True, offset in fseek() function can be a negative number. It makes the file pointer to 

move backwards from the current position.

Declaration: retval = fseek( fp, offset, from );

Where:

FILE *fp; = points to the file on which I/O is to be repositioned.

long offset; = is an integer giving the number of bytes to move forward or backward in

 the file. This may be positive or negative.

int from; = is one of the manifests SEEK_SET, SEEK_CUR, or SEEK_END.

int retval; = is non-zero if the seek operation was invalid (e.g. on a file not opened with 

a "b" option); otherwise, the return value is zero.