Pass modified string/char to function wanting const char? WiFi.softAP(ssid)

Projects Programming
1

The WiFi.softAP class appears to need a cont char - how can I feed it one?

bool softAP(const char* ssid, const char* passphrase = NULL, int channel = 1, int ssid_hidden = 0);

Background and code:
Platform is ESP8266 currently, possibly going to an ESP-WROOM-32
What I want to do.... On device start up:

  • look at a hex rotary switch (0-f) on four input pins
  • convert the encoder input to a character
  • append that character to a default name for the access point
  • start the access point
  • serve an HTML page no matter the domain requested

I have been able to get the serving of pages going, with a static AP name:
WiFi.softAP("ThisisaStaticName"); The access point SSID appears as expected.
When I try to pass a string that is modified, anything I get to compile only shows the first character of the AP name I want. I have a feeling that there will need to be pointers and referencing/dereferencing done - but I do not have a good handle on the matter.
I have had an error "error: invalid conversion from 'const char*' to 'char' [-fpermissive]" on the WiFi.softAP() line before.

// WiFi libraries
#include <ESP8266WiFi.h>
#include <DNSServer.h>
#include <ESP8266WebServer.h>

const byte DNS_PORT = 53;
IPAddress apIP(192,168,10,1);
DNSServer dnsServer;
ESP8266WebServer webServer(80);

String responseHTML = ""
  "<!DOCTYPE html><html><body><p>Some text</p></body></html>";

void setup() {
  Wire.begin();
  /*Look at address lines, use the 4 to create a letter to 
   * append to the SSID of the access point (0-F)*/
   volatile char UnitAddr;
   UnitAddr = 'a';  // ' denotes a char - single character variable like 'C' or '

.
                    // " denotes a String - a string of characters.
  String APname = "ThisIsADynamicName" + UnitAddr;
 
  /* Start the AP with appended SSID
  *  Soft Access Point Class — ESP8266 Arduino Core 3.1.2-4-gd3c102e7 documentation */
 
  WiFi.mode(WIFI_AP);
  WiFi.softAPConfig(apIP, apIP, IPAddress(255, 255, 255, 0));
  WiFi.softAP(APname.c_str()); // <<<<<< PROBLEM HERE, SSID of access point is"T"

2

The const declaration is the function's guarantee to you that it won't change the value that you pass it. You can pass it a non-const array.

Post the EXACT error message. The message you posted implies that softAP() expects a character, not a string. That does not seem to be the case based on the fact that you can send the function a static (also const) string.

3

Thanks for clarification on the const char* - it did seem rather odd that we wouldn't be allowed to modify the AP name in our code (By requiring a constant to be fed to the function)

I don't have hardware in front of me, so can't test the code right now. Does "Note 3" code look like it will do what I want to do?

void setup() {
    char UnitAddr;   /* This variable will be written to based on the values seen 
                              * from the rotary switch. 0b0000 -> 0b1111 = '0' -> 'f' */

    UnitAddr = 'a';  // ' denotes a char - single character variable like 'C' or '

.
                          // " denotes a String - a string of characters.
    String APname = "accesspoint" + UnitAddr;

/* NOTE 3: Untested on hardware - but does compile when not commented out
        char ssid[13] = "accesspoint_";
        APname.toCharArray(ssid,12);
    */
   
    WiFi.mode(WIFI_AP);
    WiFi.softAPConfig(apIP, apIP, IPAddress(255, 255, 255, 0));

//WiFi.softAP(APname.c_str()); /* NOTE 1: Compiles error free, but AP gets only the first character */

//WiFi.softAP(APname); /* NOTE 2: COMPILE ERROR: no matching function for call to 'ESP8266WiFiClass::softAP(String&)'*/

WiFi.softAP(ssid); /* NOTE 3: Compiles - but untested on hardware for behaviour */

}

4

There is no point in assigning an initial value to ssid, and then immediately overwriting it.

There is no point in making a copy of the data in APname. There is no need to make APname a String. A string will work fine.

   char APname[40];
   sprintf(APname, "accesspoint%c", UnitAddr);

Of course, if you ARE going to use Strings, you can pass the data wrapped by the String instance (using the c_str() method) to the softAP() method. Exactly what the cast needs to look like depends on the EXACT error message that the original code generated.

5

Thank you!

Arduino documentation was not getting me anywhere near this. The code above became a case of throwing stuff at the wall to see what sticks. Just trying to figure out the gaps in the documentation.

I had read an article quite a while ago (The Evils of Arduino Strings | Majenko's Hardware Hacking Blog) and didn't want to use String objects.

To help others out looking at this thread with a similar issue .... this behaves how I want it to.

/*Look at address lines, use the 4 to create a letter to 
   * append to the SSID of the access point (0-F)*/
   byte UnitAddr;
   UnitAddr = 0b00001111; 
   
   /* sprintf http://www.cplusplus.com/reference/cstdio/sprintf/
    *  Formatting (%X) http://www.cplusplus.com/reference/cstdio/printf/ 
    * char array (APname) must be at least ONE larger than the assigned string to allow
    * for NULL termination 
    * %X is replaced by contents of UnitAddr, with the X denoting the format
    *    X = Unsigned hexadecimal integer (uppercase)*/

   char APname[11]; 
   sprintf(APname, "MyAccess_%X", UnitAddr);
     
  /* Start the AP with appended SSID
   *  http://arduino-esp8266.readthedocs.io/en/latest/esp8266wifi/soft-access-point-class.html#set-up-network */
  
  WiFi.mode(WIFI_AP);
  WiFi.softAPConfig(apIP, apIP, IPAddress(255, 255, 255, 0));
  WiFi.softAP(APname);