Rename check_carrier() to carrier_isup() and use bool return.

src/arp.c

@@ -180,7 +180,7 @@ static int arp_send(struct client_state_t cs[static 1],
         return ret;
     }
 
-    if (check_carrier()) {
+    if (!carrier_isup()) {
         log_error("%s: (%s) carrier down; sendto would fail",
                   client_config.interface, __func__);
         ret = -99;

src/dhcp.c

@@ -108,7 +108,7 @@ static ssize_t send_dhcp_unicast(struct client_state_t cs[static 1],
     }
     size_t payload_len =
         sizeof *payload - (sizeof payload->options - 1 - endloc);
-    if (check_carrier()) {
+    if (!carrier_isup()) {
         log_error("%s: (%s) carrier down; write would fail",
                   client_config.interface, __func__);
         ret = -99;
@@ -286,7 +286,7 @@ static ssize_t send_dhcp_raw(struct dhcpmsg payload[static 1])
         .sll_halen = 6,
     };
     memcpy(da.sll_addr, "\xff\xff\xff\xff\xff\xff", 6);
-    if (check_carrier()) {
+    if (!carrier_isup()) {
         log_error("%s: (%s) carrier down; sendto would fail",
                   client_config.interface, __func__);
         ret = -99;

src/ifchange.c

@@ -224,12 +224,11 @@ static int ifchwrite(const char buf[static 1], size_t count)
     return -1;
 }
 
-// Returns 0 if there is a carrier, -1 if not.
-int check_carrier(void)
+bool carrier_isup(void)
 {
     char buf[256];
     snprintf(buf, sizeof buf, "carrier:;");
-    return ifchwrite(buf, strlen(buf));
+    return ifchwrite(buf, strlen(buf)) == 0;
 }
 
 int ifchange_deconfig(struct client_state_t cs[static 1])

src/ifchange.h

@@ -29,7 +29,9 @@
 #ifndef IFCHANGE_H_
 #define IFCHANGE_H_
 
-int check_carrier(void);
+#include <stdbool.h>
+
+bool carrier_isup(void);
 int ifchange_bind(struct client_state_t cs[static 1],
                   struct dhcpmsg packet[static 1]);
 int ifchange_deconfig(struct client_state_t cs[static 1]);