erasing every letter before my cursor

sup /prog/

i'm reading a textfile (in C with getc()) an i want to erase all letter who are before my cursor, how can i do that?

>>12
Are you some kind of retard?

$ man ftell
FSEEK(3)               FreeBSD Library Functions Manual               FSEEK(3)

NAME
     fgetpos, fseek, fseeko, fsetpos, ftell, ftello, rewind -- reposition a
     stream

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     int
     fseek(FILE *stream, long offset, int whence);

     long
     ftell(FILE *stream);
...

$ man truncate
TRUNCATE(2)               FreeBSD System Calls Manual              TRUNCATE(2)

NAME
     truncate, ftruncate -- truncate or extend a file to a specified length

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     truncate(const char *path, off_t length);
...

Maybe try installing an actual operating system?