Input within a time limit in Standard C - c

Question

I'm currently doing my assignment and it's compulsory to use C-Free 5.0. Just need your help to solve this piece of puzzle. I want to implement a time limit for the user to input an answer before it expires. I've tried this code but it got block at scanf() function. Is there any other method like an unblocking input or something. I've tried to implement '#include <sys/select.h>' but this program doesn't have that library.
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
int main()
{
char st[10];
printf ("Please enter a line of text : ");
time_t end = time(0) + 5; //5 seconds time limit.
while(time(0) < end)
{
scanf("%s", &st);
if(st != NULL)
{
printf ("Thank you, you entered >%s<\n", st);
exit(0);
}
}
main();
}

Answer 1

Here is an example program that shows how you can use O_NONBLOCK flag on a stdin file descriptor.
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#define INPUT_LEN 10
int main()
{
printf ("Please enter a line of text : ");
fflush(stdout);
time_t end = time(0) + 5; //5 seconds time limit.
int flags = fcntl(STDIN_FILENO, F_GETFL, 0);
fcntl(STDIN_FILENO, F_SETFL, flags | O_NONBLOCK);
char answer[INPUT_LEN];
int pos = 0;
while(time(0) < end)
{
int c = getchar();
/* 10 is new line */
if (c != EOF && c != 10 && pos < INPUT_LEN - 1)
answer[pos++] = c;
/* if new line entered we are ready */
if (c == 10)
break;
}
answer[pos] = '\0';
if(pos > 0)
printf("%s\n", answer);
else
puts("\nSorry, I got tired waiting for your input. Good bye!");
}

Answer 2

Since you have fcntl.h try setting stdin to non-blocking. It's not pretty (active waiting), but if you do not have select then this is the easyest way to go:
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
int main()
{
// get stdin flags
int flags = fcntl(0, F_GETFL, 0);
if (flags == -1) {
// fcntl unsupported
perror("fcntl");
return -1;
}
// set stdin to non-blocking
flags |= O_NONBLOCK;
if(fcntl(0, F_SETFL, flags) == -1) {
// fcntl unsupported
perror("fcntl");
return -1;
}
char st[1024] = {0}; // initialize the first character in the buffer, this is generally good practice
printf ("Please enter a line of text : ");
time_t end = time(0) + 5; //5 seconds time limit.
// while
while(time(0) < end // not timed out
&& scanf("%s", st) < 1 // not read a word
&& errno == EAGAIN); // no error, but would block
if (st[0]) // if the buffer contains something
printf ("Thank you, you entered >%s<\n", st);
return 0;
}
A remark to your code: if (st != NULL) will always be satisfied since st is a stack pointer.