Buffer filling last empty spaces in char array with unwanted data - c++

Question

FILE* file_;
char buffer[5];
file_ = fopen("Data.txt", "a+");
while (!feof(file_))
{
fread(buffer, sizeof(buffer), 1, file_);
cout << buffer<<" ";
BM(buffer, pat);
}
Data.txt="ABCC1ABCC2XXX"
Output:
ABCC1m
ABCC2m
XXXC2m
How can I make buffer stop before it starts generating chars from previous buffer?(bolded font part)
Wanted output:
ABCC1
ABCC2
XXX

Answer 1

You have two problems. Firstly you are not null-terminating buffer at all, (which is why you are getting the m output. Secondly, you are not null-terminating buffer in the right place when there is a short read.
fread will tell you how many characters it has read, and you need to put the '\0' there. Edit That previous description is not accurate. It tells you how many objects it has read, each of size arg2, and you read upto arg3 of them. You need to change the arguments to fread so that you are reading single characters, and as many of them as there room in the buffer. So:
FILE* file_;
char buffer[5+1];
file_ = fopen("Data.txt", "a+");
while (!feof(file_))
{
const size_t nchars = fread(buffer, 1, sizeof(buffer)-1, file_);
buffer[nchars] = '\0';
cout << buffer<<" ";
BM(buffer, pat);
}
Pedantic note: The second argument to fread could also be written as sizeof(buffer[0]) if buffer is of something other than char/signed char/unsigned char - but those three are defined to have a sizeof 1.

Answer 2

OK, I can't find a decent duplicate - maybe someone else will.
The line
fread(buffer, sizeof(buffer), 1, file_);
potentially fills your buffer completely. You need to keep the return value to know how many bytes were actually written, but assuming your file contained at least five bytes, all five bytes of your buffer array are now initialized.
However, to print buffer as a regular C string, it needs a sixth byte, containing the null terminator.
For example, the C string "12345" is actually represented as the char array {'1', '2', '3', '4', '5', 0}.
You don't have room for a terminator in your buffer, and don't write one, so you can't treat it as a simple C string.
Your options are:
add a terminator manually, as in Martin Bonner's answer
don't add a terminator, but track the size - you can use the C++17
std::string_view bufstr(buffer, nchars);
to keep the pointer and size together (and you can print this normally)
stop using the old C I/O library entirely. The C++ I/O library admittedly doesn't have a much better way to read groups of five characters, but reading whole lines, for example, is much easier to do correctly.