Checking the stdin buffer if it’s empty

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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.

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