<stdlib.h> Can it parse communication and obtain a single letter?

why not use readBytesUntil() and sscanf()


void
loop (void)
{
    if (Serial.available ())  {
        char buf [90];
        int n = Serial.readBytesUntil ('\n', buf, sizeof(buf)-1);
        buf [n] = '\0';

        int  val0;
        int  val1;
        char c0;
        sscanf (buf, "%d, %c, %d", &val0, &c0, &val1);

        Serial.println (val0);
        Serial.println (val1);
        Serial.println (c0);
        Serial.println ();
    }
}

void
setup (void)
{
    Serial.begin (9600);
}