is_numeric Function In C

[twanvl]

The comparison input[c] < '0' && input[c] > '9' is is not right, this expression is never true, you should use >='0' && <='9' if you mean "between 0 and 9".
While it is not an error, the ok variable is not needed here. You could simply use return statements:

int is_numeric(char input[256])
{
	int c;
	for(c = 0; input[c] != "\0";c++)
	{
		if(input[c] < '0' || input[c] > '9')
		{
			return 0;
		}
	} 
	return 1;
}