bidirectional io: how do you shorten file length?

I was asked a question about fstream a few days:

How do you shorten the file length of a bidirectional io? A requirement is that we can only open the stream once.

I tried to write the following code, but it failed.
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#include <iostream>
#include <string>
#include <fstream>

/** Original content of a.txt is
aba

After running the program, I want the modified content to be
aa

This means that I have to shorten the file size by 1 byte.
*/

using namespace std;

int main()
{
HANDLE hFile=CreateFile("F:\\1.txt",GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,0, &hFile);
char buf[MAX_PATH]={0};
DWORD readLen=0;
int size=ReadFile(hFile,buf,MAX_PATH,&readLen,NULL);
string s=string(buf);
cout<<s<<endl; // s is "aba"
s.replace(s.find("b"), 1, "");
cout<<s<<endl; // s is "aa" now
SetFilePointer(hFile, 0, NULL, FILE_BEGIN);
SetEndOfFile(hFile);
WriteFile(hFile,s.c_str(),s.length(),&readLen,NULL);
CloseHandle(hFile);

return 0;
}