How could I write a EOF into a file?

The end of a file is EOF, but if I want to insert EOF into somewhere else of a file, how could I do??

There are only a few file formats that use an EOF character marker.

You should probably post more information. The standard EOF marker for text type files has an ASCII calue of 26 (ctrl-Z).

if My file dosen't have a format or extension name, then does it
have a EOF?
For example, my file is "111" and the content of the file is as follow:

string1
string2
...
stringn

Best way to check is with a raw hex file editor....

What do you mean? Can you give a example?