C/C++ :: Read From File / Assign As String And Call Function As Argument

I try to use passing function as argument but I'm stuck. I have two questions: First, I try to call uppercase and open .txt in tfm Second, How can I read characters from in.txt as string and assign to char content[] ?

#include <stdio.h>
void tfm( char str_filename[], void(*pf_convertion)( char content[]));
void uppercase(char content[]); //converts all letters to uppercase
int main(){
puts("-------------------------------");
printf("tfm:
");
tfm("in.txt", uppercase);

[Code] ....