#define ROUTE_FILE_NAME "/proc/net/route"

/*

Iface Destination Gateway Flags RefCnt Use Metric Mask MTU Window IRTT 

wan1    00000000        5692550A        0003    0       0       0       00000000        0       0       0 

wan1    0000000A        00000000        0001    0       0       0       000000FF        0       0       0

br0     0022A8C0        00000000        0001    0       0       0       00FFFFFF        0       0       0

×/

int getGatewayByDevName(const char *devName, char *outText){
    
    outText[0] = ‘\0‘;
    
    char buffer[BIG_MAX_FIELD_LEN];
    char destination[NORMAL_FIELD_LEN];
    unsigned long gateway;
    
    FILE* f = fopen(ROUTE_FILE_NAME, "r");
    
    if(!f){
        return 1;
    }
    int devNamelen = strlen(devName);
    while(fgets(buffer, sizeof(buffer)-1, f)){
        if(!strncmp(buffer, devName, devNamelen)){
            sscanf(buffer, "%*s%s%lX", destination, &gateway);
            if(!strncmp(destination, "00000000", 8)){
                if (gateway != 0) {
                    sprintf(outText, "%s", inet_ntoa(*(struct in_addr*)&gateway));
                    break;
                }
            }
        }
    }
    
    fclose(f);
    
    return 0;
}