chg: Use static memory buffer for MAC address conversion

This patch uses several buffers in a round-robin fashion to avoid overwriting a just-returned result.
The number of buffers internally used is choosen at 16 to allow for enough spare buffers even
when more complicated situations need to be processed.

ddhcp.c

@@ -193,11 +193,7 @@ void ddhcp_dhcp_process(uint8_t* buffer, ssize_t len, struct sockaddr_in6 sender
 void ddhcp_dhcp_renewlease(struct ddhcp_mcast_packet* packet, ddhcp_config* config) {
   DEBUG("ddhcp_dhcp_renewlease(request,config)\n");
 
-#if LOG_LEVEL_LIMIT >= LOG_DEBUG
-  char* hwaddr = hwaddr2c(packet->renew_payload->chaddr);
-  DEBUG("ddhcp_dhcp_renewlease(...): Request for xid: %u chaddr: %s\n", packet->renew_payload->xid, hwaddr);
-  free(hwaddr);
-#endif
+  DEBUG("ddhcp_dhcp_renewlease(...): Request for xid: %u chaddr: %s\n", packet->renew_payload->xid, hwaddr2c(packet->renew_payload->chaddr));
 
   int ret = dhcp_rhdl_request(&(packet->renew_payload->address), config);
 
@@ -232,11 +228,7 @@ void ddhcp_dhcp_leaseack(struct ddhcp_mcast_packet* request, ddhcp_config* confi
   // Stub functions
   DEBUG("ddhcp_dhcp_leaseack(request,config)\n");
 
-#if LOG_LEVEL_LIMIT >= LOG_DEBUG
-  char* hwaddr = hwaddr2c(request->renew_payload->chaddr);
-  DEBUG("ddhcp_dhcp_leaseack(...): ACK for xid: %u chaddr: %s\n", request->renew_payload->xid, hwaddr);
-  free(hwaddr);
-#endif
+  DEBUG("ddhcp_dhcp_leaseack(...): ACK for xid: %u chaddr: %s\n", request->renew_payload->xid, hwaddr2c(request->renew_payload->chaddr));
 
   dhcp_packet* packet = dhcp_packet_list_find(&config->dhcp_packet_cache, request->renew_payload->xid, request->renew_payload->chaddr);
 
@@ -257,11 +249,7 @@ void ddhcp_dhcp_leasenak(struct ddhcp_mcast_packet* request, ddhcp_config* confi
   // Stub functions
   DEBUG("ddhcp_dhcp_leasenak(request,config)\n");
 
-#if LOG_LEVEL_LIMIT >= LOG_DEBUG
-  char* hwaddr = hwaddr2c(request->renew_payload->chaddr);
-  DEBUG("ddhcp_dhcp_leaseack(...): NAK for xid: %u chaddr: %s\n", request->renew_payload->xid, hwaddr);
-  free(hwaddr);
-#endif
+  DEBUG("ddhcp_dhcp_leaseack(...): NAK for xid: %u chaddr: %s\n", request->renew_payload->xid, hwaddr2c(request->renew_payload->chaddr));
 
   dhcp_packet* packet = dhcp_packet_list_find(&config->dhcp_packet_cache, request->renew_payload->xid, request->renew_payload->chaddr);
 

dhcp.c

@@ -330,11 +330,7 @@ int dhcp_hdl_request(int socket, struct dhcp_packet* request, ddhcp_config* conf
         payload.xid = request->xid;
         payload.lease_seconds = 0;
 
-#if LOG_LEVEL_LIMIT >= LOG_DEBUG
-        char* hwaddr = hwaddr2c(payload.chaddr);
-        DEBUG("dhcp_hdl_request(...): Save request for xid: %u chaddr: %s\n", payload.xid, hwaddr);
-        free(hwaddr);
-#endif
+        DEBUG("dhcp_hdl_request(...): Save request for xid: %u chaddr: %s\n", payload.xid, hwaddr2c(payload.chaddr));
 
         // Send packet
         ddhcp_mcast_packet* packet = new_ddhcp_packet(DDHCP_MSG_RENEWLEASE, config);

hook.c

@@ -7,11 +7,7 @@
 #include <sys/wait.h>
 
 void hook(uint8_t type, struct in_addr* address, uint8_t* chaddr, ddhcp_config* config) {
-#if LOG_LEVEL_LIMIT >= LOG_DEBUG
-  char* hwaddr = hwaddr2c(chaddr);
-  DEBUG("hook(type:%i,addr:%s,chaddr:%s,config)\n", type, inet_ntoa(*address), hwaddr);
-  free(hwaddr);
-#endif
+  DEBUG("hook(type:%i,addr:%s,chaddr:%s,config)\n", type, inet_ntoa(*address), hwaddr2c(chaddr));
 
   if (!config->hook_command) {
     DEBUG("hook(...): No hook command set\n");

tools.c

@@ -79,15 +79,26 @@ dhcp_option* parse_option() {
   return option;
 }
 
+//! Number of pre-allocated string buffers
+#define hwaddr_strcount 2u
+//! Maximum string length required to format an MAC address
+#define hwaddr_strlen 18u
+static char hwaddr_strbuf[hwaddr_strcount][hwaddr_strlen] = { 0 };
+static uint8_t hwaddr_stridx = 0;
+
+//! The function hwaddr2c formats a hwaddr (AKA MAC address) passed in the first argument
+//! into one (of several) static buffers. The buffer is chosen in a round-robin-like fashion
+//! by cycling through a list of hwaddr_strcount buffers, each of hwaddr_strlen bytes in size.
+//! If the last of the pre-allocated buffers has been reached filling up new buffers restarts
+//! with the first on.
+//!
+//! \note The memory returned SHOULD NOT be referenced by any long-living pointers as it
+//! can be overwritten at any time when the next hwaddr is converted by calling this function.
 char* hwaddr2c(uint8_t* hwaddr) {
-  char* str = calloc(18, sizeof(char));
+  char* str = hwaddr_strbuf[hwaddr_stridx];
+  hwaddr_stridx = (hwaddr_stridx + 1u) % hwaddr_strcount;
 
-  if (!str) {
-    FATAL("hwaddr2c(...): Failed to allocate buffer.\n");
-    return NULL;
-  }
-
-  snprintf(str, 18, "%02X:%02X:%02X:%02X:%02X:%02X",
+  snprintf(str, hwaddr_strlen, "%02X:%02X:%02X:%02X:%02X:%02X",
     hwaddr[0], hwaddr[1], hwaddr[2], hwaddr[3], hwaddr[4], hwaddr[5]);
 
   return str;