The cause of the error is that atoi() requires a char * argument, and rdBuffer[x] is a char. Assuming that x is where the string that you wish to convert to an int starts within rdBuffer, something like:
However, given that x is also used as an index in the access to map, I'm guessing what you really are looking for is the value of a digit character at position x within the input buffer, in which case you're really looking for something more like:
map[y][x] = atoi(&rdBuffer[x]);
Note that this does no error/range checking, so non-ASCII digit characters will end up giving you bogus values in your map. You should check for these.
map[y][x] = rdBuffer[x] - '0';