How can I prevent scanf() to wait forever for an input character?

Question

I want to fulfill the following things in a console application:

1. If user inputs a character, the application will do the corresponding task. For example, if us

Answer 1

But the problem is the program will trap at the line of scanf() function forever to wait for an input character,

Remove the semicolon after while.

Answer 2

Try using the select() function. Then you can wait for 10 seconds until you can read from stdin without blocking. If select() returns with zero, perform the default action. I don't know if this works on windows, it's POSIX standard. If you happen to develop on unix/linux, try man select

I just wrote a working example using select:

#include <stdlib.h>
#include <stdio.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#define MAXBYTES 80

int main(int argc, char *argv[])
{
        fd_set readfds;
        int    num_readable;
        struct timeval tv;
        int    num_bytes;
        char   buf[MAXBYTES];
        int    fd_stdin;

        fd_stdin = fileno(stdin);

        while(1) {
                FD_ZERO(&readfds);
                FD_SET(fileno(stdin), &readfds);

                tv.tv_sec = 10;
                tv.tv_usec = 0;

                printf("Enter command: ");
                fflush(stdout);
                num_readable = select(fd_stdin + 1, &readfds, NULL, NULL, &tv);
                if (num_readable == -1) {
                        fprintf(stderr, "\nError in select : %s\n", strerror(errno));
                        exit(1);
                }
                if (num_readable == 0) {
                        printf("\nPerforming default action after 10 seconds\n");
                        break;  /* since I don't want to test forever */
                } else {
                        num_bytes = read(fd_stdin, buf, MAXBYTES);
                        if (num_bytes < 0) {
                                fprintf(stderr, "\nError on read : %s\n", strerror(errno));
                                exit(1);
                        }
                        /* process command, maybe by sscanf */
                        printf("\nRead %d bytes\n", num_bytes);
                        break; /* to terminate loop, since I don't process anything */
                }
        }

        return 0;
}

Note: the poll() example below is OK too, no problem. For the rest I chose to read the available bytes into a buffer (up to MAXBYTES). It can be (s)scanned afterwards. (scanf() just isn't too much my friend, but that's a personal taste matter).

Answer 3

As others have said, the best way to do truly async IO is with select(...).

But a quick and dirty way to do what you want is with getline(...) which will return the number of bytes read every time (not hanging on IO) and returns -1 on no bytes read.

The following is from the getline(3) man page:

// The following code fragment reads lines from a file and writes them to standard output.  
// The fwrite() function is used in case the line contains embedded NUL characters.

char *line = NULL;
size_t linecap = 0;
ssize_t linelen;
while ((linelen = getline(&line, &linecap, fp)) > 0)
    fwrite(line, linelen, 1, stdout);

Answer 4

Try alarm(3)

#include <stdio.h>
#include <unistd.h>
int main(void)
{
   char buf [10];
   alarm(3);
   scanf("%s", buf);
   return 0;
}

Answer 5

Here is a working example of how to do this with poll (probably the most 'correct' way on Linux):

#include <unistd.h>
#include <poll.h>
#include <stdio.h>

int main()
{
    struct pollfd mypoll = { STDIN_FILENO, POLLIN|POLLPRI };
    char string[10];

    if( poll(&mypoll, 1, 2000) )
    {
        scanf("%9s", string);
        printf("Read string - %s\n", string);
    }
    else
    {
        puts("Read nothing");
    }

    return 0;
}

The timeout is the third argument to poll and is in milliseconds - this example will wait for 2 seconds for input on stdin. Windows has WSAPoll, which should work similarly.