This is a C FAQ: 12.5 How can I read one character at a time, without waiting for the RETURN key?

To quote:

Depending on which operating system you're using and what libraries you have available, you may be able to use one (or more!) of the following techniques:

• If you can use the curses library, you can call cbreak [footnote] (and perhaps noecho), after which calls to getch will return characters immediately.
• If all you're trying to do is read a short password without echo, you may be able to use a function called getpass, if it's available. (Another possibility for hiding typed passwords is to select black characters on a black background.)
• Under classic versions of Unix, use ioctl and the TIOCGETP and TIOCSETP (or TIOCSETN) requests on file descriptor 0 to manipulate the sgttyb structure, defined in <sgtty.h> and documented in tty(4). In the sg_flags field, set the CBREAK (or RAW) bit, and perhaps clear the ECHO bit.
• Under System V Unix, use ioctl and the TCGETAW and TCSETAW requests on file descriptor 0 to manipulate the termio structure, defined in <termio.h>. In the c_lflag field, clear the ICANON (and perhaps ECHO) bits. Also, set c_cc[VMIN] to 1 and c_cc[VTIME] to 0.
• Under any operating system (Unix or otherwise) offering POSIX compatibility, use the tcgetattr and tcsetattr calls on file descriptor 0 to manipulate the termios structure, defined in <termios.h>. In the c_lflag field, clear the ICANON (and perhaps ECHO) bits. Also, set c_cc[VMIN] to 1 and c_cc[VTIME] to 0.
• In a pinch, under Unix, use system (see question 19.27) to invoke the stty command to set terminal driver modes (as in the preceding three items).
• Under MS-DOS, use getch or getche, or the corresponding BIOS interrupts.
• Under VMS, try the Screen Management (SMG$) routines, or curses, or issue low-level $QIO's with the IO$_READVBLK function code (and perhaps IO$M_NOECHO, and others) to ask for one character at a time. (It's also possible to set character-at-a-time or ``pass through'' modes in the VMS terminal driver.)
• Under other operating systems, you're on your own.

There is no standard function for this.

There are some custom solutions e.g. https://www.cs.uleth.ca/~holzmann/C/system/ttyraw.c But your mileage may vary.

However if this kind of dynamic use of the terminal is core to your program I would recommend using ncurses or something similar.