Encoder Issue

This is a Windows program, but the question I have is not Windows specific.
I have here a file encoder. You pass the file you want encoded into the file param and you're all set... in a perfect world. The problem is, I cannot get the program to stop writing when the file is done. It writes for the whole buffer size. this is the only way I could get it to work because I don't know how to get file size. Any ideas?
Oh yeah, it writes a byte to tell the decoder how to decode, just to note for you.

Code:

---

#define MyBufferLength 1024000
#define WIN32_LEAN_AND_MEAN
//
//INCLUDES//
//
int WINAPI WinMain(HINSTANCE hInstance,
                                  HINSTANCE hPrevInstance,
                                  LPSTR lpCmdLine,
                                  int nCmdShow)
{
        srand(time(NULL));//Randomize seed (based on computer clock time)
        char mod = rand() % (3 - 25 + 2);//Random Modifier (key to the encoding)

    char* FileBuffer = new char[MyBufferLength];//File read into this buffer
        char* EncodedFile = new char[50];//Name for new encoded file
        char ModChar[1];
        FILE *fd = fopen(lpCmdLine,"r+");
       
        ModChar[0] = mod;
                               
        if( fd == NULL )
        {
                MessageBox(HWND_DESKTOP,"Error opening file","Bah!",NULL);
                return 1;
        }
        fread(FileBuffer,sizeof(char),MyBufferLength,fd);
               
        for(int x = 0; x < MyBufferLength; x++)
        {
                FileBuffer[x] = FileBuffer[x] + mod;
        }

        rewind(fd);
        fwrite(ModChar,sizeof(char),sizeof(char),fd);
        fwrite(FileBuffer,sizeof(char),MyBufferLength/*needs to be length of file*/,fd);
        //END
        delete []FileBuffer;
        return 0;
}

---

You could write until feof() or read the position of the file with:

off_t fileLength = lseek(fd, 0L, SEEK_END);

Ah, thank you. What needs to be included to use that line of code?

#include <stdio.h>

not working... doesn't recognize identifiers.