Rajen said:
Suppose the field length is 25 characters.
After entering the 25th character, it should be available to process.
Program should not wait for the user to press enter/return key.
This is in the FAQ.
19.1: How can I read a single character from the keyboard without
waiting for the RETURN key? How can I stop characters from
being echoed on the screen as they're typed?
A: Alas, there is no standard or portable way to do these things in
C. Concepts such as screens and keyboards are not even
mentioned in the Standard, which deals only with simple I/O
"streams" of characters.
At some level, interactive keyboard input is usually collected
and presented to the requesting program a line at a time. This
gives the operating system a chance to support input line
editing (backspace/delete/rubout, etc.) in a consistent way,
without requiring that it be built into every program. Only
when the user is satisfied and presses the RETURN key (or
equivalent) is the line made available to the calling program.
Even if the calling program appears to be reading input a
character at a time (with getchar() or the like), the first call
blocks until the user has typed an entire line, at which point
potentially many characters become available and many character
requests (e.g. getchar() calls) are satisfied in quick
succession.
When a program wants to read each character immediately as it
arrives, its course of action will depend on where in the input
stream the line collection is happening and how it can be
disabled. Under some systems (e.g. MS-DOS, VMS in some modes),
a program can use a different or modified set of OS-level input
calls to bypass line-at-a-time input processing. Under other
systems (e.g. Unix, VMS in other modes), the part of the
operating system responsible for serial input (often called the
"terminal driver") must be placed in a mode which turns off line-
at-a-time processing, after which all calls to the usual input
routines (e.g. read(), getchar(), etc.) will return characters
immediately. Finally, a few systems (particularly older, batch-
oriented mainframes) perform input processing in peripheral
processors which cannot be told to do anything other than line-
at-a-time input.
Therefore, when you need to do character-at-a-time input (or
disable keyboard echo, which is an analogous problem), you will
have to use a technique specific to the system you're using,
assuming it provides one. Since comp.lang.c is oriented towards
those topics that the C language has defined support for, you
will usually get better answers to other questions by referring
to a system-specific newsgroup such as comp.unix.questions or
comp.os.msdos.programmer, and to the FAQ lists for these groups.
Note that the answers are often not unique even across different
variants of a system; bear in mind when answering system-
specific questions that the answer that applies to your system
may not apply to everyone else's.
However, since these questions are frequently asked here, here
are brief answers for some common situations.
Some versions of curses have functions called cbreak(),
noecho(), and getch() which do what you want. If you're
specifically trying to read a short password without echo, you
might try getpass(). Under Unix, you can use ioctl() to play
with the terminal driver modes (CBREAK or RAW under "classic"
versions; ICANON, c_cc[VMIN] and c_cc[VTIME] under System V or
POSIX systems; ECHO under all versions), or in a pinch, system()
and the stty command. (For more information, see <sgtty.h> and
tty(4) under classic versions, <termio.h> and termio(4) under
System V, or <termios.h> and termios(4) under POSIX.) 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.
(As an aside, note that simply using setbuf() or setvbuf() to
set stdin to unbuffered will *not* generally serve to allow
character-at-a-time input.)
If you're trying to write a portable program, a good approach is
to define your own suite of three functions to (1) set the
terminal driver or input system into character-at-a-time mode
(if necessary), (2) get characters, and (3) return the terminal
driver to its initial state when the program is finished.
(Ideally, such a set of functions might be part of the C
Standard, some day.) The extended versions of this FAQ list
(see question 20.40) contain examples of such functions for
several popular systems.
See also question 19.2.
References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1; POSIX
Sec. 7.
19.2: How can I find out if there are characters available for reading
(and if so, how many)? Alternatively, how can I do a read that
will not block if there are no characters available?
A: These, too, are entirely operating-system-specific. Some
versions of curses have a nodelay() function. Depending on your
system, you may also be able to use "nonblocking I/O", or a
system call named "select" or "poll", or the FIONREAD ioctl, or
c_cc[VTIME], or kbhit(), or rdchk(), or the O_NDELAY option to
open() or fcntl(). See also question 19.1.
