c++ read binary data from istream

I have an istream and I have to read it into a buffer. I could not find a way to figure out the read_len once eof is encountered? I cannot use get because my file does not have delimeters.

It seems that the only option is to read it character by character, is it really the only option?

char buffer[128];
while(is.good()) {
     is.read(buffer, sizeof(buffer));
     size_t read_len = sizeof(buffer);
     if (is.eof()) {
         read_len = xxxx;
     }
     process(buffer, read_len);
}

>Solution :

You could check istream::gcount() which "Returns the number of characters extracted by the last unformatted input operation".

Example:

    while(is) {
         is.read(buffer, sizeof buffer);
         auto read_len = is.gcount();  // <- 
         if(read_len > 0)
             process(buffer, read_len);
         else
             break;
    }

You could also use istream::readsome() – but note:
"The behavior of this function is highly implementation-specific." which may or may not be an issue.