C++ - fgets() ignores subsequent inputs if Enter is pressed

Question

I am trying to create an emulator for something, and in the main loop for the processor I wanted to implement a simple way to step the CPU one loop at a time (prompted by pressing Enter each loop) so I can see what instructions are being executed each step. In addition, it allows you to enter a number instead of just Enter to change the default step amount from 1 to something else (so it will skip x number of cycles and then return to 1 at a time afterwards.

The issue is that it works fine when I enter a number (skip that amount of cycles and then prompts me again each cycle), but when I just press Enter rather than entering a number I want it to default to 1 step. Instead, pressing Enter causes it to just run through the whole program without ever prompting me again. How do I make Enter == 1?

void CPU_loop()
{
    ...


    static int step = 1;
    char cmd[10];
    if(step == 1)
    {
        if(fgets(cmd, 10, stdin) != NULL) // If you entered something other than Enter; doesn't work
        {
            step = std::atoi(cmd); // Set step amount to whatever you entered
        }
    }
    else
    {
        --step;
    }

    ...
}

Answer 1

When you press enter directly, it does not default to 1, but instead you are passing the string "\n" to std::atoi(), std::atoi() cannot be used to perform sanity check on it's input, you can use a different function for that like std::strtol() or, you can simply add

if (step == 0)
    step = 1;

because when, std::atoi() takes a "\n" as input, it returns 0. Read the documentation to further understand it.

Quoting the documentation

Integer value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, the return value is undefined. ​If no conversion can be performed, 0​ is returned.

One more thing, you could do it the c++ way using streams for input to avoid all this.