C++ istream::read causes undefined behaviour - c++

Question

I using the following code to copy n character from binary file to char* variable :
std::ifstream is ("write.abc", std::ifstream::binary);
if (is) {
// get length of file:
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);
char * buffer = new char [length];
std::cout << "Reading " << length << " characters... ";
// read data as a block:
is.read (buffer,length);
std::cout << "length of buffer: "<<strlen(buffer) <<endl;
std::cout << "Content of buffer: "<<buffer <<endl;
.......
The content of my file:
This is the result of compilation:
My question is the following: I'm waiting to have:
length of buffer: 13
Content of buffer: abcdefghjklmn
Can some one, please, help me to interpret the result?

Answer 1

Your buffer is not terminated - you need to allocate an extra char and set it to '\0', otherwise it's just an unterminated C string, so strlen will most likely return an invalid value, and attempting to print the string will typically produce garbage.
char * buffer = new char [length + 1]; // <<< allocate space for `length`
// characters + terminator
std::cout << "Reading " << length << " characters... ";
// read data as a block:
is.read (buffer,length);
buffer[length] = '\0'; // <<< terminate C-style string
Note that using proper C++-style std::strings instead of old school C-style char * strings avoids this and other common problems, and is generally simpler and a lot more robust. If you're reading binary data rather than text, then consider using std::vector<unsigned char>.

Answer 2

Please note that this is not answering the question. It is a followup to a comment
One solution is to "store" the buffer as a [begin, end) range instead of null terminated C string. This has a big advantage over null terminate string - it can be used with STL algorithms without calling strlen(...) to find end element.
Following some examples:
std::stringstream is("some text\n");
if (is)
{
// get length of file:
is.seekg (0, is.end);
auto length = is.tellg();
is.seekg (0, is.beg);
char* begin = new char [length];
char* end = begin + length;
std::cout<<"Reading "<<length<<" characters...\n";
// read data as a block:
is.read (begin,length);
//print the data:
std::copy(begin, end, std::ostream_iterator<char>(std::cout));
//print the data backwards:
std::copy(std::reverse_iterator<char*>(end), std::reverse_iterator<char*>(begin), std::ostream_iterator<char>(std::cout));
std::cout<<std::endl;
//create string from data:
std::string str(begin, end);
std::cout<<str;
//sum the characters
auto sum = std::accumulate(begin, end, 0);
std::cout<<sum<<std::endl;
//make them uppercase
std::transform(begin, end, begin, toupper);
std::copy(begin, end, std::ostream_iterator<char>(std::cout));
}