Minimal C code to read an int from a stream. This doesn't comply with the standard atoi at all, but it's just for simplicity.

atoi.c

int i = 0;
	
//FILE* in = stdin;

char c = getc(in);
while( c >= '0' && c <= '9' ) {
	//i = i*10 + (c - '0');  //Equivalent to the line below 
	i = (i<<3)+(i<<1)+c-'0';
	c = getchar();
}
ungetc(c, in);