Can we write an EOF character ourselves?

Well, EOF is just a value returned by the function defined in the C stdio.h header file. Its actually returned to all the reading functions by the OS, so its system dependent. When OS reaches the end of file, it sends it to the function, which in its return value than places most commonly (-1), but not always. So, to summarize, EOF is not character, but constant returned by the OS. EDIT: Well, you need to know more about filesystem, look at this.

Hi, to your second question:

once again, you should look better into filesystems. FAT is a very nice example because you can find many articles about it, and its principles are very similar to NTFS. Anyway, once again, EOF is NOT a character. You cannot place it in file directly. If you could do so, imagine the consequences, even "dumb" image file could not be read by the system.

Why? Because OS works like very complex structure of layers. One of the layers is the filesystem driver. It makes sure that it transfers data from every filesystem known to the driver. It provides a bridge between applications and the actual system of storing files into HDD.

To be exact, FAT filesystem uses the so-called FAT table - it is a table located close to the start of the HDD (or partition) address space, and it contains map of all clusters (little storage cells). OK, so now, when you want to save some file to the HDD, OS (filesystem driver) looks into FAT table, and searches for the value "0x0". This "0x0" value says to the OS that cluster which address is described by the location of that value in FAT table is free to write.

So it writes into it the first part of the file. Then, it looks for another "0x0" value in FAT, and if found, it writes the second part of the file into cluster which it points to. Then, it changes the value of the first FAT table record where the file is located to the physical address of the next in our case second part of the file.

When your file is all stored on HDD, now there comes the final part, it writes desired EOF value, but into FAT table, not into the "data part" of the HDD. So when the file is read next time, it knows this is the end, don´t look any further.

So, now you see, if you would want to manually write EOF value into the place it doesn't belong to, you have to write your own driver which would be able to rewrite the FAT record, but this is practically impossible to do for beginners.

I came here while going through the Kernighan & Ritchie C exercises.

Ctrl+D sends the character that matches the EOF constant from stdio.h.

(Edit: this is on Mac OS X; thanks to @markmnl for pointing out that the Windows 10 equivalent is Ctrl+Z)

There is no EOF character. EOF by definition "is unequal to any valid character code". Often it is -1. It is not written into the file at any point.

There is a historical EOF character value (CTRL+Z) in DOS, but it is obsolete these days.

To answer the follow-up question of Apoorv: The OS never uses the file data to determine file length (files are not 'null terminated' in any way). So you cannot trick the OS. Perhaps old, stupid programs won't read after CTRL+Z character. I wouldn't assume that any Windows application (even Notepad) would do that. My guess is that it would be easier to trick them with a null (\0) character.