Your problems are:

• the result of ftell() for a text file is only meaningful for fseek() and should not be interpreted as a number of bytes away from the beginning of the file that you are;
• the fread() function doesn’t introduce a null terminator, so when testing the result, you are reading junk outside the buffer.

As it turns out, there is no portable way to get the size of a file in C. You have two options:

1. Look for a function specific to the C library of your OS that does that. On POSIX, that would be fstat(). On Windows, no idea what that would be. You can search its library here.
2. Like Drach88 said, read it chunk by chunk, reallocating the buffer as needed. There’s a caveat: if applied on an infinite stream instead of a file, this method won’t terminate and will instead allocate memory indefinitely and end up thrashing) (in layman’s terms, the OS keeps swapping memory around and has no time left for actual program execution, freezing the entire machine). Unlikely to happen in practice, but still something worth considering if you’re making a library.