is_numeric Function In C

[señorbadger]

Hello, I am trying to write a function where by you pass a character array to
my function and it will check to see if it is a valid number.

1) What is wrong with this code (if anything) ? and how do i use my function
2) Is their already such a function

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