Thread: Expected output is missing from using fgets()

Because cin >> str; is like scanf("%s") in C.
It uses white-space as a delimiter.

If you want to read a whole line in C++, then use cin.getline()
Also in C++, use std::string to store things, not char arrays.

Mingling C and C++ I/O is a bad idea. It didn't bite you this time, but it's just better not to do it.

What happened here is this: std::istream::operator>> reads until the first delimiter (any whitespace). So on line 14, you get one word. The following space is left in the input buffer, and then, on line 16, you read the remainder, up to 40 characters, starting at the beginning of your buffer (str), and overwriting what you just read on line 14.

You also have a recipe for undefined behavior here, because you allocate enough space for 40 characters, and then you tell fgets to read up to 40 characters into a buffer, and then print that buffer. The problem comes if there are actually 40 characters to read, in which case there will be no null terminator. When you print it back out, std::ostream::operator<< will try to print until it finds the null terminator, which could be anywhere, even outside the allocated space for your program, or nowhere at all, causing your program to crash.

> The problem comes if there are actually 40 characters to read, in which case there will be no null terminator.
This isn't true.
fgets() always appends a \0.
It leaves the rest of the input on the input stream.
fgets(3): input of char/strings - Linux man page

The problem is that the formatted input with the overloaded operator>> leaves the trailing whitespace in the input buffer. If this whitespace includes a newline, then subsequently reading up to the end of the line as with fgets would give you what appears to be an empty (or only consisting of whitespace) string.

There are two general approaches to fixing this:

• Read line by line, parsing the line if necessary. This approach could be easier if you used std::string instead with std::getline.
• Discard characters after reading with operator>> until the newline is encountered, e.g., by using cin.ignore

By the way, instead of using fgets, use cin.getline to read the line. fgets is inherited from the C standard library, whereas cin.getline would be more consistent with what you have been using for I/O.