Disallow input at certain times - c

I have got a text based game in c that uses scanf.
There are a few times when the player is supposed to type in things, however, while he isn't, the cursor stays in the game, letting the user type in anything he wants, which ruins future scanfs and the story.
Is there a way to disallow input unless there is a scanf waiting for a response?

I think it would be helpful to step back and think about all the moving parts that exist in the execution environment of your program.
When executed, your program becomes a distinct process running in the multitasking environment of the OS. The terminal is a separate process with an associated GUI window, and which may be running locally or remotely (e.g. someone could theoretically run your game from a remote location by connecting over a network via ssh). The user interacts with the terminal program through their keyboard and screen.
Now, it is actually the terminal process (working closely with the OS kernel) that is responsible for most of the nuances of user input. It is the terminal that prints just-typed characters to its GUI window as soon as it receives them, and it is the terminal that maintains an input buffer of characters that have been typed but that have not yet been read by a foreground process.
Conveniently, terminals allow their behavior to be controlled by a set of configuration settings, and these settings can be changed programmatically during the run-time of the connected program. The C-level API that we can use to read and write these settings is called termios.
There's a great article on terminals I highly recommend: The TTY demystified. For the purposes of this question, the section Configuring the TTY device is most useful. It doesn't demonstrate the termios library directly, but shows how to use the stty utility which uses the termios library internally.
(Note that, although the links I've been giving so far are focused on Linux, they are applicable to all Unix-like systems, which includes Mac OS X.)
Unfortunately there's no way to completely "disallow" input with a single switch, but we can achieve the same effect by toggling a couple of terminal settings and manually discarding buffered input at the right times.
The two terminal settings we need to concern ourselves with are ECHO and ICANON. Both settings are normally on by default.
By turning off ECHO, we can prevent the terminal from printing just-typed characters to the terminal window when it receives them. Hence, while the program is running, any characters the user types will seem to be ignored completely, although they will still be buffered internally by the terminal.
By turning off ICANON, we ensure that the terminal will not wait for an enter keypress to submit a complete line of input before returning input to the program, e.g. when the program makes a read() call. Rather, it will return whatever characters it currently has buffered in its internal input buffer, thereby making it possible for us to discard them immediately and carry on with execution.
The full process will look like this:
1: Disable input, meaning turn off ECHO and ICANON.
2: Run some gameplay with output, not requiring any user input.
3: Enable input, meaning discard any buffered terminal input and then turn on ECHO and ICANON.
4: Read user input.
5: Repeat from step 1. Subsequent gameplay can now make use of the latest user input.
There is a complication in step 3 related to discarding buffered input. We can implement this discarding operation by simply reading input from stdin via read() with a fixed-length buffer until there's no more input to be read. But if there's no input ready to be read at all for the discarding operation, then the first call would block until the user types something. We need to prevent this blocking.
I believe there are two ways this could be done. There's such a thing called a non-blocking read, which can be set up with termios or fcntl() (or by opening a second file descriptor to the same endpoint with the O_NONBLOCK flag, I think) which would cause read() to return immediately with errno set to EAGAIN if it would block. The second way is to poll the file descriptor with poll() or select() to determine if there's data ready to be read; if not, we can avoid the read() call completely.
Here's a working solution that uses select() to avoid blocking:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <termios.h>
struct termios g_terminalSettings; // global to track and change terminal settings
void disableInput(void);
void enableInput(void);
void discardInputBuffer(void);
void discardInputLine(void);
void setTermiosBit(int fd, tcflag_t bit, int onElseOff );
void turnEchoOff(void);
void turnEchoOn(void);
void turnCanonOff(void);
void turnCanonOn(void);
int main(void) {
// prevent input immediately
disableInput();
printf("welcome to the game\n");
// infinite game loop
int line = 1;
int quit = 0;
while (1) {
// print dialogue
for (int i = 0; i < 3; ++i) {
printf("line of dialogue %d\n",line++);
sleep(1);
} // end for
// input loop
enableInput();
int input;
while (1) {
printf("choose a number in 1:3 (-1 to quit)\n");
int ret = scanf("%d",&input);
discardInputLine(); // clear any trailing garbage (can do this immediately for all cases)
if (ret == EOF) {
if (ferror(stdin)) { fprintf(stderr, "[error] scanf() failed: %s", strerror(errno) ); exit(1); }
printf("end of input\n");
quit = 1;
break;
} else if (ret == 0) { // invalid syntax
printf("invalid input\n");
} else if (input == -1) { // quit code
quit = 1;
break;
} else if (!(input >= 1 && input <= 3)) { // invalid value
printf("number is out-of-range\n");
} else { // valid
printf("you entered %d\n",input);
break;
} // end if
} // end while
if (quit) break;
disableInput();
} // end while
printf("goodbye\n");
return 0;
} // end main()
void disableInput(void) {
turnEchoOff(); // so the terminal won't display all the crap the user decides to type during gameplay
turnCanonOff(); // so the terminal will return crap characters immediately, so we can clear them later without waiting for a LF
} // end disableInput()
void enableInput(void) {
discardInputBuffer(); // clear all crap characters before enabling input
turnCanonOn(); // so the user can type and edit a full line of input before submitting it
turnEchoOn(); // so the user can see what he's doing as he's typing
} // end enableInput()
void turnEchoOff(void) { setTermiosBit(0,ECHO,0); }
void turnEchoOn(void) { setTermiosBit(0,ECHO,1); }
void turnCanonOff(void) { setTermiosBit(0,ICANON,0); }
void turnCanonOn(void) { setTermiosBit(0,ICANON,1); }
void setTermiosBit(int fd, tcflag_t bit, int onElseOff ) {
static int first = 1;
if (first) {
first = 0;
tcgetattr(fd,&g_terminalSettings);
} // end if
if (onElseOff)
g_terminalSettings.c_lflag |= bit;
else
g_terminalSettings.c_lflag &= ~bit;
tcsetattr(fd,TCSANOW,&g_terminalSettings);
} // end setTermiosBit()
void discardInputBuffer(void) {
struct timeval tv;
fd_set rfds;
while (1) {
// poll stdin to see if there's anything on it
FD_ZERO(&rfds);
FD_SET(0,&rfds);
tv.tv_sec = 0;
tv.tv_usec = 0;
if (select(1,&rfds,0,0,&tv) == -1) { fprintf(stderr, "[error] select() failed: %s", strerror(errno) ); exit(1); }
if (!FD_ISSET(0,&rfds)) break; // can break if the input buffer is clean
// select() doesn't tell us how many characters are ready to be read; just grab a big chunk of whatever is there
char buf[500];
ssize_t numRead = read(0,buf,500);
if (numRead == -1) { fprintf(stderr, "[error] read() failed: %s", strerror(errno) ); exit(1); }
printf("[debug] cleared %d chars\n",numRead);
} // end while
} // end discardInputBuffer()
void discardInputLine(void) {
// assumes the input line has already been submitted and is sitting in the input buffer
int c;
while ((c = getchar()) != EOF && c != '\n');
} // end discardInputLine()
I should clarify that the discardInputLine() feature I included is completely separate from the discarding of the input buffer, which is implemented in discardInputBuffer() and called by enableInput(). Discarding of the input buffer is an essential step in the solution of temporarily disallowing user input, while discarding the remainder of the input line that is left unread by scanf() is not exactly essential. But I think it does make sense to prevent residual line input from being scanned on subsequent iterations of the input loop. It's also necessary to prevent infinite loops if the user entered invalid input, so for that reason we can probably call it essential.
Here's a demo of me playing around with the input:
welcome to the game
line of dialogue 1
line of dialogue 2
line of dialogue 3
[debug] cleared 12 chars
choose a number in 1:3 (-1 to quit)
0
number is out-of-range
choose a number in 1:3 (-1 to quit)
4
number is out-of-range
choose a number in 1:3 (-1 to quit)
asdf
invalid input
choose a number in 1:3 (-1 to quit)
asdf 1 2 3
invalid input
choose a number in 1:3 (-1 to quit)
0 1
number is out-of-range
choose a number in 1:3 (-1 to quit)
1 4
you entered 1
line of dialogue 4
line of dialogue 5
line of dialogue 6
choose a number in 1:3 (-1 to quit)
2
you entered 2
line of dialogue 7
line of dialogue 8
line of dialogue 9
[debug] cleared 256 chars
[debug] cleared 256 chars
[debug] cleared 256 chars
[debug] cleared 256 chars
[debug] cleared 256 chars
[debug] cleared 256 chars
[debug] cleared 256 chars
[debug] cleared 238 chars
choose a number in 1:3 (-1 to quit)
-1
goodbye
During the first triplet of dialogue I typed 12 random characters which were discarded afterward. Then I demonstrated various types of invalid input and how the program responds to them. During the second triplet of dialogue I didn't type anything, so no characters were discarded. During the final triplet of dialogue I quickly pasted a large block of text into my terminal several times (using a mouse right-click, which is a quick and easy shortcut for pasting into my particular terminal), and you can see it discarded all of it properly, taking several iterations of the select()/read() loop to complete.

On Linux and HP-UX machines, use
to disable display of inputs from keyboard on terminal
stty -echo
to enable display of inputs from keyboard on terminal
stty echo

Related

Accepting a single character in C?

What I intend to do is to get the character entered and used it as a pattern. I've tried using getchar() but it won't work. I've hear of using scanf but it skips and stops whenever I press "shift" for the special characters on my keyboard.
int i, j, n;
char c;
c = getchar();
printf("Enter value of n: ");
scanf("%d", &n);
printf("Enter a Character: ");
getchar();
for(i=1; i<=n; i++)
{
for(j=1; j<=i; j++)
{
printf("%c", c);
}
printf("\n");
}
You need to assign the value returned by getchar to the variable c, and you had a redundent call to getchar that's why it skips reading the desired input:
int i, j, n;
char c;
printf("Enter value of n: ");
scanf("%d", &n);
printf("Enter a Character: ");
scanf(" %c", &c);
for(i=1; i<=n; i++)
{
for(j=1; j<=i; j++)
{
printf("%c", c);
}
printf("\n");
}
You can use %c with scanf:
scanf("%d %c", &n, %c);
This eliminates the need for the two getchar calls.
The space is required; it tells scanf to skip whitespace.
The problem you have is that your assumptions on getchar(3) are incorrect. You think getchar() is going to return the next key pressed in the input stream, but you are incorrectly assuming that it will be done without buffering or system processing (the terminal driver gives the program complete lines, or even worse, if you are reading from a file, complete buffer blocks, that have to be buffered so you miss no characters from the input stream)
You are assuming incorrectly that the end of line you need to press for the input to be feeded to the program does not count in the input stream.
What actually happens is:
you feed a complete line (because the kernel driver works that way) so you press your character, and then you see nothing, not after you have pressed the return key.
once you press it, you have more than one character (depending on how many you pressed before hitting the return key) that will stay in the buffer, until they are so consumed by the program. Normally this happens when you have executed more getchar() or scanf() statements.
The idea of this buffering mechanism is to allow a programmer to process character by charcacter large amounts of text, without the overhead of making a system call per character reading (this is a costly operation) so think of getchar() not as a sample function to get new users introduced to the world of programming, but as a hint to experienced programmers to use efficiently without having to think on buffering large amounts of text.
With stdio package, every character counts, so you have to think slowly and minuciously when you feed input to getchar(3).
The next question is: Right, then how can I solve and stop my program until I press some key? The first answer, with the set of tools you have exposed here is, be careful on what you input, instead of asking for any key, ask the user to press the return key, and then, do something like:
printf("Hit <ENTER> to continue"); fflush(stdout); /* so we get the line out, bypassing the buffering mechanism */
int c;
while ((c = getchar()) != EOF && c != '\n') {
/* just ignore the character we have received */
}
/* c == '\n' || c == EOF, so we can continue */
or, if you prefer, you can write a function just to do this (as there can be so many criteria to implement it, nobody included such a function in the standard C library, my apologies for that. ;) )
void wait_for_enter()
{
/* I use stderr, for two reasons:
* o stderr is normally unbuffered, so there's no need to fflush()
* o stdout can be redirected, so the prompt will not be visible in
* case you want to save the output of your program.
*/
fprintf(stderr, "Hit <ENTER> to continue");
int c;
while ((c = getchar()) != EOF && c != '\n') {
/* just ignore the character we have received
* until we get the end of file (ctrl-d at the terminal)
* or a new line */
}
/* c == '\n' || c == EOF, so we can continue */
/* it's assumed that the user pressed the enter key, so the echoed
* enter already did a newline, no need to do it here */
} /* wait_for_enter */
In order to wait for any character and in raw mode, you need first to ensure your input comes from a terminal (you cannot do the following on a normal file), then you have to switch the terminal driver to raw mode, so each character is given immediately to the program and no line editing processing is done, and then set the stdin descriptor to no buffering at all. Only then, you can receive individual characters with getchar(3), one by one, as they are keyed in. I think this is far out of the scope of this question, as the code to do that is far more complex than the above.
EDIT
Following is a complete sample of a program that uses raw input to process characters as they are keyed in.
/* pru.c -- program to show raw input from the terminal.
* Author: Luis Colorado <luiscoloradourcola#gmail.com>
* Date: Fri Sep 20 08:46:06 EEST 2019
* Copyright: (C) 2019 Luis Colorado. All rights reserved.
* License: BSD.
*/
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <termios.h> /* see termios(3) for a description on terminal conf */
#define F(_fmt) __FILE__":%d:%s: " _fmt, __LINE__, __func__
/* this function switches the terminal into raw mode and returns a malloc(3)ed
* terminal configuration, so it can be later restored. BEWARE that the returned
* configuration info must be deallocated by free(3) once it's not needed anymore.
* In case of failure of any system call, the function returns NULL, and errno is
* set to the failing cause. */
struct termios *set_raw(int fd)
{
struct termios *ret = malloc(sizeof *ret), cfg;
if (!ret) return NULL;
int res = tcgetattr(fd, &cfg);
if (res < 0) goto error;
*ret = cfg; /* save it for return */
cfmakeraw(&cfg);
/* set it after all buffered characters in the driver have drained out */
res = tcsetattr(fd, TCSADRAIN, &cfg);
if (res < 0) goto error;
return ret;
error:
free(ret);
return NULL;
} /* set_raw */
/* restores the configuration back to the associated file descriptor */
int restore_cfg(int fd, struct termios *cf)
{
/* set it after all buffered characters in the driver have drained out */
return tcsetattr(fd, TCSADRAIN, cf);
} /* restore_cfg */
int main()
{
struct termios *cfg = set_raw(fileno(stdin));
if (!cfg) {
fprintf(stderr, F("stdin: %s\n"),
strerror(errno));
}
setbuf(stdin, NULL); /* stdin unbuffered */
setbuf(stdout, NULL); /* stdout unbuffered */
/* BEWARE that raw mode doesn't process any characters, so no Ctrl-C(interrupt), Ctrl-D(EOF), etc.
* will be available, only if you read from a file, you'll get EOF, but you'll not be able to produce
* that on the terminal, you'll need to swith to another console and kill the process. */
int c;
while ((c = getchar()) != EOF && c != '\033') { /* ESCAPE key('\033') is a safeguard to end input */
/* print the input char as an hex number */
printf("[%02x]", c);
}
if (cfg) { /* if we were able to set the terminal to raw mode */
/* restore config */
restore_cfg(fileno(stdin), cfg);
/* and free it */
free(cfg);
}
exit(EXIT_SUCCESS);
} /* main */
The full source code can be also downloaded from here.
You can use this program to see how input keys get mapped into characters, as you'll note that when you press the enter key, the raw input is [0d] (ascii char 13, CARRY RETURN) while in normal line mode you get '\n' which is [0a] or ASCII LINE FEED, instead (you can check this if you redirect input from the pru.c text file). Also you'll see that you are unable to specify EOF from the terminal driver with Ctrl-D and that Ctrl-C does not come to help. Well, I have included a safeguard, by ending the program in case you press the ESC key, which generates an ASCII ESCAPE character (\033). This is also commented in the source code.
All of this processing is done by the kernel driver, so all unix implementations get the same line end characters or interpret the control characters the same way.

Ending a Loop with EOF (without enter)

i am currently trying to end a while loop with something like this:
#include <stdio.h>
int main()
{
while(getchar() != EOF)
{
if( getchar() == EOF )
break;
}
return 0;
}
When i press CTRL+D on my Ubuntu, it ends the loop immediately. But on Windows i have to press CTRL+Z and then press ENTER to close the loop. Can i get rid of the ENTER on Windows?
The getchar behavior
For linux the EOF char is written with ctrl + d, while on Windows it is written by the console when you press enter after changing an internal status of the CRT library through ctrl + z (this behaviour is kept for retrocompatibility with very old systems). If I'm not wrong it is called soft end of file. I don't think you can bypass it, since the EOF char is actually consumed by your getchar when you press enter, not when you press ctrl + z.
As reported here:
In Microsoft's DOS and Windows (and in CP/M and many DEC operating systems), reading from the terminal will never produce an EOF. Instead, programs recognize that the source is a terminal (or other "character device") and interpret a given reserved character or sequence as an end-of-file indicator; most commonly this is an ASCII Control-Z, code 26. Some MS-DOS programs, including parts of the Microsoft MS-DOS shell (COMMAND.COM) and operating-system utility programs (such as EDLIN), treat a Control-Z in a text file as marking the end of meaningful data, and/or append a Control-Z to the end when writing a text file. This was done for two reasons:
Backward compatibility with CP/M. The CP/M file system only recorded the lengths of files in multiples of 128-byte "records", so by convention a Control-Z character was used to mark the end of meaningful data if it ended in the middle of a record. The MS-DOS filesystem has always recorded the exact byte-length of files, so this was never necessary on MS-DOS.
It allows programs to use the same code to read input from both a terminal and a text file.
Other information are also reported here:
Some modern text file formats (e.g. CSV-1203[6]) still recommend a trailing EOF character to be appended as the last character in the file. However, typing Control+Z does not embed an EOF character into a file in either MS-DOS or Microsoft Windows, nor do the APIs of those systems use the character to denote the actual end of a file.
Some programming languages (e.g. Visual Basic) will not read past a "soft" EOF when using the built-in text file reading primitives (INPUT, LINE INPUT etc.), and alternate methods must be adopted, e.g. opening the file in binary mode or using the File System Object to progress beyond it.
Character 26 was used to mark "End of file" even if the ASCII calls it Substitute, and has other characters for this.
If you modify your code like that:
#include <stdio.h>
int main() {
while(1) {
char c = getchar();
printf("%d\n", c);
if (c == EOF) // tried with also -1 and 26
break;
}
return 0;
}
and you test it, on Windows you will see that the EOF (-1) it is not written in console until you press enter. Beore of that a ^Z is printed by the terminal emulator (I suspect). From my test, this behavior is repeated if:
you compile using the Microsoft Compiler
you compile using GCC
you run the compiled code in CMD window
you run the compiled code in bash emulator in windows
Update using Windows Console API
Following the suggestion of #eryksun, I successfully written a (ridiculously complex for what it can do) code for Windows that changes the behavior of conhost to actually get the "exit when pressing ctrl + d". It does not handle everything, it is only an example. IMHO, this is something to avoid as much as possible, since the portability is less than 0. Also, to actually handle correctly other input cases a lot more code should be written, since this stuff detaches the stdin from the console and you have to handle it by yourself.
The methods works more or less as follows:
get the current handler for the standard input
create an array of input records, a structure that contains information about what happens in the conhost window (keyboard, mouse, resize, etc.)
read what happens in the window (it can handle the number of events)
iterate over the event vector to handle the keyboard event and intercept the required EOF (that is a 4, from what I've tested) for exiting, or prints any other ascii character.
This is the code:
#include <windows.h>
#include <stdio.h>
#define Kev input_buffer[i].Event.KeyEvent // a shortcut
int main(void) {
HANDLE h_std_in; // Handler for the stdin
DWORD read_count, // number of events intercepted by ReadConsoleInput
i; // iterator
INPUT_RECORD input_buffer[128]; // Vector of events
h_std_in = GetStdHandle( // Get the stdin handler
STD_INPUT_HANDLE // enumerator for stdin. Others exist for stdout and stderr
);
while(1) {
ReadConsoleInput( // Read the input from the handler
h_std_in, // our handler
input_buffer, // the vector in which events will be saved
128, // the dimension of the vector
&read_count); // the number of events captured and saved (always < 128 in this case)
for (i = 0; i < read_count; i++) { // and here we iterate from 0 to read_count
switch(input_buffer[i].EventType) { // let's check the type of event
case KEY_EVENT: // to intercept the keyboard ones
if (Kev.bKeyDown) { // and refine only on key pressed (avoid a second event for key released)
// Intercepts CTRL + D
if (Kev.uChar.AsciiChar != 4)
printf("%c", Kev.uChar.AsciiChar);
else
return 0;
}
break;
default:
break;
}
}
}
return 0;
}
while(getchar() != EOF)
{
if( getchar() == EOF )
break;
}
return 0;
Here it is inconsistent.
If getchar() != EOF it will enter the loop, otherwise (if getchar() == EOF) it will not enter the loop. So, there is no reason to check getchar() == EOF inside the loop.
On the other hand, you call getchar() 2 times, you wait to enter 2 characters instead of only 1.
What did you try to do ?

Detecting Ctrl-D in C

I am trying to detect the Ctrl+D user input, which I know returns EOF. Right now, I know the code waits for input from the stdin stream, but is there a way to let the program continue until the Ctrl+D command is in stdin? The program should continue running past the if statement if Ctrl+D isn't inputted.
char buffer[];
if (fgets(buffer, 10, stdin) == NULL{
//write to file
}
You want to stop your program when the user presses Ctrl+D without actually reading stdin? In this case, you should consider using Ctrl+C instead. But first I will write something about non-blocking I/O, since this is what you are asking for.
There is no way to achieve nonblocking I/O in standard C. However, you could use POSIX-functions like select or fcntl in combination with read. There are other questions about it on StackOverflow which should provide all information you need. This question for example.
If you want to handle Ctrl+C instead, you can use thesignal function:
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
volatile bool shouldRun = true;
void sighandler(int) {
shouldRun = false;
}
int main(int argc, char *argv[]) {
if (signal(SIGINT, &sighandler) == SIG_ERR) {
fprintf(stderr, "Could not set signal handler\n");
return EXIT_FAILURE;
}
printf("Program started\n");
while (shouldRun) {
// Do something...
}
printf("Program is shutting down.\n");
return EXIT_SUCCESS;
}
Note that signal handlers (i.e. sighandler) might interrupt your thread at any moment. This means they are prone to race conditions. You must even avoid acquiring any locks within a signal handler. This means just calling printf within a signal handler can cause a deadlock. Just setting boolean flags as shown in the example is fine, though. There are solutions like signal masks and the self pipe trick to circumvent these limitations, but they should not be necessary here.
Since the machine generates EOF on Ctrl+D, you should be checking fgets() for NULL, as fgets() is obliged to return NULL on end of file.
line = fgets(l, BUFFSIZE, stdin)
if (line == NULL)
continue;
On most operating systems, stdin is buffered one line at a time, and any attempt to read it (without going into low-level nasties) will stop until either a line or EOF is available. If you don't mind this, and just want to check for EOF without reading-in any waiting input if EOF is not present, you could use ungetc:
#include <stdio.h>
int check_for_EOF() {
if (feof(stdin)) return 1;
int c = getc(stdin);
if (c == EOF) return 1;
ungetc(c, stdin);
}
int main() {
printf("Start typing:\n");
while (!check_for_EOF()) {
int bytes_typed = 0;
while (getchar() != '\n') bytes_typed++;
printf("You typed a line of %d bytes\n", bytes_typed);
}
printf("You typed EOF\n");
}
You are only guaranteed one character of push-back from ungetc, although most implementations give you much more. And it works only if you're not going to seek the stream later (which is the case with stdin). Notice also that I'm calling it "bytes typed", not "characters typed": Chinese, Japanese and Korean characters for example cannot fit into the char type of most C implementations, and it would depend how the console encodes them when you type (if you have a CJK input method set up or can copy/paste some, you can try it on the above program and see).
It is too much to post here and you are not specific what you have currently and what you want. So here gives you a general idea of how to do it:
Put that if statement inside a forked process or other thread
Send a posix signal to your (parent) process when the key is captured
Add signal handler in your program
If you just wanna terminate the program when C-d is entered, just send a SIGKILL in step 2 and ignore step 3.
If you do not know any term above, Google is your friend

C - how to handle user input in a while loop

I'm new to C and I have a simple program that takes some user input inside a while loop, and quits if the user presses 'q':
while(1)
{
printf("Please enter a choice: \n1)quit\n2)Something");
*choice = getc(stdin);
// Actions.
if (*choice == 'q') break;
if (*choice == '2') printf("Hi\n");
}
When I run this and hit 'q', the program does quit correctly. However if I press '2' the program first prints out "Hi" (as it should) but then goes on to print the prompt "Please choose an option" twice. If I enter N characters and press enter, the prompt gets printed N times.
This same behaviour happens when I use fgets() with a limit of 2.
How do I get this loop working properly? It should only take the first character of input and then do something once according to what was entered.
EDIT
So using fgets() with a larger buffer works, and stops the repeated prompt issue:
fgets(choice, 80, stdin);
This kind of helped: How to clear input buffer in C?
When you getc the input, it's important to note that the user has put in more than one character: at the very least, the stdin contains 2 chars:
2\n
when getc gets the "2" the user has put in, the trailing \n character is still in the buffer, so you'll have to clear it. The simplest way here to do so would be to add this:
if (*choice == '2')
puts("Hi");
while (*choice != '\n' && *choice != EOF)//EOF just in case
*choice = getc(stdin);
That should fix it
For completeness:
Note that getc returns an int, not a char. Make sure to compile with -Wall -pedantic flags, and always check the return type of the functions you use.
It is tempting to clear the input buffer using fflush(stdin);, and on some systems, this will work. However: This behavior is undefined: the standard clearly states that fflush is meant to be used on update/output buffers, not input buffers:
C11 7.21.5.2 The fflush function, fflush works only with output/update stream, not input stream
However, some implementations (for example Microsoft) do support fflush(stdin); as an extension. Relying on it, though, goes against the philosophy behind C. C was meant to be portable, and by sticking to the standard, you are assured your code is portable. Relying on a specific extension takes away this advantage.
What seems to be a very simple problem is actually pretty complicated. The root of the problem is that terminals operate in two different modes: raw and cooked. Cooked mode, which is the default, means that the terminal does not read characters, it reads lines. So, your program never receives any input at all unless a whole line is entered (or an end of file character is received). The way the terminal recognizes an end of line is by receiving a newline character (0x0A) which can be caused by pressing the Enter key. To make it even more confusing, on a Windows machine pressing Enter causes TWO characters to be generated, (0x0D and 0x0A).
So, your basic problem is that you want a single-character interface, but your terminal is operating in a line-oriented (cooked) mode.
The correct solution is to switch the terminal to raw mode so your program can receive characters as the user types them. Also, I would recommend the use of getchar() rather than getc() in this usage. The difference is that getc() takes a file descriptor as an argument, so it can read from any stream. The getchar() function only reads from standard input, which is what you want. Therefore, it is a more specific choice. After your program is done it should switch the terminal back to the way it was, so it needs to save the current terminal state before modifying it.
Also, you should handle the case that the EOF (0x04) is received by the terminal which the user can do by pressing CTRL-D.
Here is the complete program that does these things:
#include <stdio.h>
#include <termios.h>
main(){
tty_mode(0); /* save current terminal mode */
set_terminal_raw(); /* set -icanon, -echo */
interact(); /* interact with user */
tty_mode(1); /* restore terminal to the way it was */
return 0; /* 0 means the program exited normally */
}
void interact(){
while(1){
printf( "\nPlease enter a choice: \n1)quit\n2)Something\n" );
switch( getchar() ){
case 'q': return;
case '2': {
printf( "Hi\n" );
break;
}
case EOF: return;
}
}
}
/* put file descriptor 0 into chr-by-chr mode and noecho mode */
set_terminal_raw(){
struct termios ttystate;
tcgetattr( 0, &ttystate); /* read current setting */
ttystate.c_lflag &= ~ICANON; /* no buffering */
ttystate.c_lflag &= ~ECHO; /* no echo either */
ttystate.c_cc[VMIN] = 1; /* get 1 char at a time */
tcsetattr( 0 , TCSANOW, &ttystate); /* install settings */
}
/* 0 => save current mode 1 => restore mode */
tty_mode( int operation ){
static struct termios original_mode;
if ( operation == 0 )
tcgetattr( 0, &original_mode );
else
return tcsetattr( 0, TCSANOW, &original_mode );
}
As you can see, what seems to be a pretty simple problem is quite tricky to do properly.
A book I can highly recommend to navigate these matters is "Understanding Unix/Linux Programming" by Bruce Molay. Chapter 6 explains all the things above in detail.
The reason why this is happening is because stdin is buffered.
When you get to the line of code *choice = getc(stdin); no matter how many characters you type, getc(stdin) will only retrieve the first character. So if you type "foo" it will retrieve 'f' and set *choice to 'f'. The characters "oo" are still in the input buffer. Moreover, the carriage return character that resulted from you striking the return key is also in the input buffer. Therefore since the buffer isn't empty, the next time the loop executes, rather than waiting for you to enter something, getc(stdin); will immediately return the next character in the buffer. The function getc(stdin) will continue to immediately return the next character in the buffer until the buffer is empty. Therefore, in general it will prompt you N number of times when you enter a string of length N.
You can get around this by flushing the buffer with fflush(stdin); immediately after the line *choice = getc(stdin);
EDIT: Apparently someone else is saying not to use fflush(stdin); Go with what he says.

Confusion about how a getchar() loop works internally

I've included an example program using getchar() below, for reference (not that anyone probably needs it), and feel free to address concerns with it if you desire. But my question is:
What exactly is going on when the program calls getchar()?
Here is my understanding (please clarify or correct me):
When getchar is called, it checks the STDIN buffer to see if there is any input.
If there isn't any input, getchar sleeps.
Upon wake, getchar checks to see if there is any input, and if not, puts it self to sleep again.
Steps 2 and 3 repeat until there is input.
Once there is input (which by convention includes an 'EOF' at the end), getchar returns the first character of this input and does something to indicate that the next call to getchar should return the second letter from the same buffer? I'm not really sure what that is.
When there are no more characters left other than EOF, does getchar flush the buffer?
The terms I used are probably not quite correct.
#include <stdio.h>
int getLine(char buffer[], int maxChars);
#define MAX_LINE_LENGTH 80
int main(void){
char line[MAX_LINE_LENGTH];
int errorCode;
errorCode = getLine(line, sizeof(line));
if(errorCode == 1)
printf("Input exceeded maximum line length of %d characters.\n", MAX_LINE_LENGTH);
printf("%s\n", line);
return 0;
}
int getLine(char buffer[], int maxChars){
int c, i = 0;
while((c = getchar()) != EOF && c != '\n' && i < maxChars - 1)
buffer[i++] = c;
buffer[i++] = '\0';
if(i == maxChars)
return 1;
else
return 0;
}
Step 2-4 are slightly off.
If there is no input in the standard I/O buffer, getchar() calls a function to reload the buffer. On a Unix-like system, that normally ends up calling the read() system call, and the read() system call puts the process to sleep until there is input to be processed, or the kernel knows there will be no input to be processed (EOF). When the read returns, the code adjusts the data structures so that getchar() knows how much data is available. You description implies polling; the standard I/O system does not poll for input.
Step 5 uses the adjusted pointers to return the correct values.
There really isn't an EOF character; it is a state, not a character. Even though you type Control-D or Control-Z to indicate 'EOF', that character is not inserted into the input stream. In fact, those characters cause the system to flush any typed characters that are still waiting for 'line editing' operations (like backspace) to change them so that they are made available to the read() system call. If there are no such characters, then read() returns 0 as the number of available characters, which means EOF. Then getchar() returns the value EOF (usually -1 but guaranteed to be negative whereas valid characters are guaranteed to be non-negative (zero or positive)).
So basically, rather than polling, is it that hitting Return causes a certain I/O interrupt, and then when the OS receives this, it wakes up any processes that are sleeping for I/O?
Yes, hitting Return triggers interrupts and the OS kernel processes them and wakes up processes that are waiting for the data. The terminal driver is woken by the kernel when interrupt occurs, and decides what to do with the character(s) that were just received. They may be stashed for further processing (canonical mode) or made available immediately (raw mode), etc. Assuming, of course, that the input is a terminal; if the input is from a disk file, it is simpler in many ways — or if it is a pipe, or …
Nominally, it isn't the terminal app that gets woken by the interrupt; it is the kernel that wakes first, then the shell running in the terminal app that is woken because there's data for it to read, and only when there's output does the terminal app get woken.
I say 'nominally' because there's an outside chance that in fact the terminal app does mediate the I/O via a pty (pseudo-tty), but I think it happens at the kernel level and the terminal application is involved fairly late in the process. There's a huge disconnect really between the keyboard where you type and the display where what you type appears.
See also Canonical vs non-canonical terminal input.

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