ftell---return position in a stream or file#include <stdio.h> long ftell(FILE *fp);Description
FILE can have a "position" that records how much
of the file your program has already read. Many of the stdio functions
depend on this position, and many change it as a side effect.
The result of ftell is the current position for a file
identified by fp. If you record this result, you can later
use it with fseek to return the file to this
position.
In the current implementation, ftell simply uses a character
count to represent the file position; this is the same number that
would be recorded by fgetpos.
Returns
ftell returns the file position, if possible. If it cannot do
this, it returns -1L. Failure occurs on streams that do not support
positioning; the global errno indicates this condition with the
value ESPIPE.
Portability
ftell is required by the ANSI C standard, but the meaning of its
result (when successful) is not specified beyond requiring that it be
acceptable as an argument to fseek. In particular, other
conforming C implementations may return a different result from
ftell than what fgetpos records.
No supporting OS subroutines are required.