Convert String in to MAC address

Question

Inside the file in SPIFFS, I'm saving information about the mac address in the form "XX:XX:XX:XX:XX:XX". When I read the file, I need to switch it from STRING to a array of hexadecimal values.

uint8_t* str2mac(char* mac){
  uint8_t bytes[6];
  int values[6];
  int i;
  if( 6 == sscanf( mac, "%x:%x:%x:%x:%x:%x%*c",&values[0], &values[1], &values[2],&values[3], &values[4], &values[5] ) ){
      /* convert to uint8_t */
      for( i = 0; i < 6; ++i )bytes[i] = (uint8_t) values[i];
  }else{
      /* invalid mac */
  } 
  return bytes;
}

wifi_set_macaddr(STATION_IF, str2mac((char*)readFileSPIFFS("/mac.txt").c_str()));

But I'm wrong in the code somewhere

When i put AA:00:00:00:00:01 in file, my ESP8266 set 29:D5:23:40:00:00

I need help, thank you

Stephan Lechner · Accepted Answer · 2018-02-12T20:06:21.543

7

You are returning a pointer to a "local" variable, i.e. one which's lifetime ends when the function is finished. Using such a pointer then is UB, which may be, for example, the behaviour you are seeing.

To overcome this, you could pass the array as parameter; then the caller is responsible for memory management. BTW: you could use format %hhx to read in directly into an 8 bit unsigned data type:

int str2mac(const char* mac, uint8_t* values){
    if( 6 == sscanf( mac, "%hhx:%hhx:%hhx:%hhx:%hhx:%hhx",&values[0], &values[1], &values[2],&values[3], &values[4], &values[5] ) ){
        return 1;
    }else{
        return 0;
    }
}

int main() {

    uint8_t values[6] = { 0 };
    int success = str2mac("AA:00:00:00:00:01", values);
    if (success) {
        for (int i=0; i<6; i++) {
            printf("%02X:",values[i]);
        }
    }
}

edited Feb 12 '18 at 20:06

answered Feb 12 '18 at 19:59

Stephan Lechner

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Sorry, but I always get 0 for the variable "success" – Dusan Krstic Feb 12 '18 at 20:23
can you `printf("'%s'",mac)` inside `str2mac`? – Stephan Lechner Feb 12 '18 at 20:25
printf("'%s'",mac) does not return anything – Dusan Krstic Feb 12 '18 at 20:41
then it is clear that the function returns `0`. Please check the input, probably by using a debugger. – Stephan Lechner Feb 12 '18 at 20:42
Sorry, this is a microcontroller problem https://forum.arduino.cc/index.php?topic=437982.0 Thanks for the effort – Dusan Krstic Feb 12 '18 at 21:32

Killzone Kid · Answer 2 · 2018-02-12T22:36:16.160

Your code doesn't seem to be compatible with wifi_set_macaddr (I looked up API documentation). It expects a uint8 pointer to mac address, which means the way you wrote it is not going to work (returning pointer to local variable etc). Here is an example which you should be able to adapt to your purpouse:

#include <iostream>
#include <fstream>

// mock up/print result
bool wifi_set_macaddr(uint8_t index, uint8_t *mac) 
{
    std::cout << "index: " << (int)index << " mac: "; 
    for (int i = 0; i < 6; ++i)
        std::cout << std::hex << (int)mac[i] << " ";
    std::cout << std::endl;

    return true;
}


// test file
void writeTestFile()
{   
    std::ofstream ofs("mac.txt");
    if (!(ofs << "AA:00:00:00:00:01" << std::endl))
    {
        std::cout << "File error" << std::endl;
    }
    ofs.close();
}

int main() 
{
    writeTestFile();

    uint8_t mac[6];
    int i = 0, x;

    std::ifstream ifs("mac.txt");
    for (; i < 6 && ifs >> std::hex >> x; ++i)
    {
        mac[i] = static_cast<uint8_t>(x);
        ifs.ignore();
    }

    if (i < 6)
    {
        std::cout << "File error or invalid MAC address" << std::endl;
        return 1;
    }

    wifi_set_macaddr(0x00, mac);

    return 0;
}

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Convert String in to MAC address

2 Answers2