Char* and string conversion from an ESP8266 web server

Question

I have the following Arduino code for configuring a wifi connection through a simple webpage:

I declare char* ssid;. Later I read a value from a SPIFFS config file where q_ssid is declared as a string and successfully populated from the file:

strncpy(ssid, q_ssid.c_str(), 10);

In the line above, I am limiting it to exactly 10 as that is the length of my wifi AP name that this connects to. If I change the initial char* to char ssid[10];, all works well, but SSIDs can have varying names, and if I do not use 10 chars, then the trailing spaces prevent connection in the statement WiFi.begin(ssid, password);.

On the web client handler that reads in values from the user, I use this:

String q_ssid = server.arg("ssid");
strncpy(ssid, q_ssid.c_str(), 10 );    

How can I dynamically assign the size of char* ssid; so that it can be used with the correct number of characters for the AP name that the user has stored from the webpage?

Any help is greatly appreciated.

Thanks.

Answer 1

What you need to do is creating a fixed string, this can be done in two ways:

static:

char ssid[100];

Where 100 is the maximum length ever possible. Note that this will always use up 100 bytes, even if less are used (e.g. your string length is 5. dynamic:

char* p_ssid = malloc(100);

This way a pointer is created to a dynamically created string that can hold 100 characters. Disadvantage is that using malloc in a microcontroller can cause empty memory 'holes' when freeing/mallocing a lot.

With either way you can than use a normal strncpy or strcpy (in first case use &ssid to get a pointer to the ssid). Also don't forget to add 1 to the length because of the trailing \0 to denote the string is ended. Thus for storing 10 characters, you need an 11 char sized array.