#include <unistd.h> off_t tell(int fd);
The tell() function obtains the current value of the file-position indicator for the file descriptor fd.
Upon successful completion, tell() returns the current value of the file-position indicator for fd measured in bytes from the beginning of the file.
Otherwise, it returns −1 and sets errno to indicate the error.
The tell() function will fail if:
The file descriptor fd is not an open file descriptor.
The current file offset cannot be represented correctly in an object of type off_t.
The file descriptor fd is associated with a pipe or FIFO.
The tell() function is equivalent to lseek(fd, 0, SEEK_CUR).
See attributes(5) for descriptions of the following attributes: