Checking the stdin buffer if it's empty

Question

I am trying to read a number character with character, but I don't know if the stdin buffer is empty or not.

My first solution whas to look for \n character in stdin buffer, but this is no good if I what to enter multiple numbers separated by " ".

How can I know if in stdin buffer I have characters or not?

I need to do it in C and to be portable.

Answer 1

There are several soutions:

poll or select with timeout of 0 - these would return immediately and result is either -1 with errno EAGAIN if no data available or number of descriptors with data (one, since you're checking only stdin).

ioctl is a swiss army knife of using descriptors. The request you need is I_NREAD:

if (ioctl(0, I_NREAD, &n) == 0 && n > 0)
    // we have exactly n bytes to read

However the correct solution is to read everything you got (using scanf) as a line, then process the result - and this works good enough with sscanf:

char buf[80]; // large enough
scanf("%79s", buf); // read everything we have in stdin
if (sscanf(buf, "%d", &number) == 1)
    // we have a number

... as long as you properly handle re-reading, strings that are longer than your buffer, and other real-life complications.

Answer 2

For anyone who comes here from google – easy select solution to check stdin emptyness:

fd_set readfds;
FD_ZERO(&readfds);
FD_SET(STDIN_FILENO, &readfds);
fd_set savefds = readfds;

struct timeval timeout;
timeout.tv_sec = 0;
timeout.tv_usec = 0;

int chr;

int sel_rv = select(1, &readfds, NULL, NULL, &timeout);
if (sel_rv > 0) {
  puts("Input:");
  while ((chr = getchar()) != EOF) putchar(chr);
} else if (sel_rv == -1) {
  perror("select failed");
}

readfds = savefds;

Needs unistd.h, stdlib.h and stdio.h.

Explanation can be found here.

UPD: Thanks DrBeco for noticing that select returns -1 on error -- error handling was added.

Actually, select returns:

• the number of ready descriptors that are contained in the descriptor sets
• 0 if the time limit expires
• -1 if an error occurred (errno would be set)

Answer 3

I inspired from this as referenced by @stek29's post on this page and prepared a simple example as follows:

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

int main(void)
{
    fd_set readfds;
    FD_ZERO(&readfds);

    struct timeval timeout;
    timeout.tv_sec = 0;
    timeout.tv_usec = 0;

    char message[50];

    while(1)
    {
        FD_SET(STDIN_FILENO, &readfds);

        if (select(1, &readfds, NULL, NULL, &timeout))
        {
            scanf("%s", message); 
            printf("Message: %s\n", message);
        }

        printf("...\n");
        sleep(1);
    }

    return(0);
}