Man, this stuff is kinda hard when the standards are ill-specified

src/kiss.c

@@ -6,13 +6,13 @@ enum kiss_flags {
     KISS_ESCAPE = 0x02
 };
 
-int patty_kiss_read(int fd, void *buf, size_t *len, int *channel) {
+ssize_t patty_kiss_read(int fd, void *buf, size_t len, int *port) {
     int flags = KISS_NONE;
     size_t i, b = 0;
 
-    *channel = 0;
+    *port = 0;
 
-    for (i=0; i<PATTY_KISS_BUFLEN; i++) {
+    for (i=0; i<len; i++) {
         unsigned char c = ((char *)buf)[i];
 
         if (flags & KISS_FRAME == 0) {
@@ -48,13 +48,23 @@ int patty_kiss_read(int fd, void *buf, size_t *len, int *channel) {
                 break;
             }
 
+            /*
+             * Not all KISS TNCs will emit a type byte at the start of a frame
+             * to the host; if the low nybble has a value of zero, then presume
+             * the high nybble to be the radio port number from which the
+             * packet originated.
+             */
+            if (c & 0x0f == 0) {
+                *port = (c & 0xf0) >> 4;
+
+                continue;
+            }
+
             ((char *)buf)[b++] = c;
         }
     }
 
-    *len = b + 1;
-
-    return 0;
+    return b + 1;
 
 error_io:
     return -1;