They pass in either a valid IPv4 or IPv6 address and then rely on this function to determine which they have (assuming a return value of 0).

After reading the spec more carefully I realise that -1 should be returned only when the address family is not AF_INET or AF_INET6.

By changing the return value in the IPv6 code to 0 instead of -1, we could get the correct behaviour without any extra code.

Here is a patch to try and save you a bit of work:

--- a/musl/src/network/inet_pton.c
+++ b/musl/src/network/inet_pton.c
@@ -46,24 +46,24 @@
                        if (!s[1]) break;
                        continue;
                }
-               if (hexval(s[0])<0) return -1;
+               if (hexval(s[0])<0) return 0;
                while (s[0]=='0' && s[1]=='0') s++;
                for (v=j=0; j<5 && (d=hexval(s[j]))>=0; j++)
                        v=16*v+d;
-               if (v > 65535) return -1;
+               if (v > 65535) return 0;
                ip[i] = v;
                if (!s[j]) {
-                       if (brk<0 && i!=7) return -1;
+                       if (brk<0 && i!=7) return 0;
                        break;
                }
                if (i<7) {
                        if (s[j]==':') continue;
-                       if (s[j]!='.') return -1;
+                       if (s[j]!='.') return 0;
                        need_v4=1;
                        i++;
                        break;
                }
-               return -1;
+               return 0;
        }
        if (brk>=0) {
                memmove(ip+brk+7-i, ip+brk, 2*(i+1-brk));
@@ -73,6 +73,6 @@
                *a++ = ip[j]>>8;
                *a++ = ip[j];
        }
-       if (need_v4 &&inet_pton(AF_INET, (void *)s, a-4) <= 0) return -1;
+       if (need_v4 &&inet_pton(AF_INET, (void *)s, a-4) <= 0) return 0;
        return 1;
 }