Menu
I'm new to arduino and I have stumbled upon a problem. I want to send data via my esp8266 to my php page. But I don't know how to join my data with this GET request.
Here is my code:
String card = "2-3d-fg-d6-12-68-32-3f-35-45-42-53-2a-3";
char *hello = "GET /insert.php?card="+card+"&error=1 HTTP/1.1\r\nHost: testsite.com\r\n\r\n";
wifi.send((const uint8_t*)hello, strlen(hello));
And this is what I get in arduino console:
error: cannot convert 'StringSumHelper' to 'char*' in initialization cannot convert 'StringSumHelper' to 'char*' in initialization
921
What does char * mean in Arduino? A variable of type "char *" is a pointer. That means it holds the memory address of some other variable or array element or parameter or whatever, and in that address is a char.21-Apr-2020.
String concatenation in Arduino is quite straightforward and also robust. You simply use the + operator to join strings.
The += operator can also be used to concatenate the String expression on its right to the String variable or property on its left, and assign the result to the variable or property on its left. When you use the += operator, you might not be able to determine whether addition or string concatenation will occur.
You can strip off the first character with . substring(1) and you can convert the resulting String from digit characters to a number with . toInt() (or . toFloat() if there is a decimal point).
You can use std::string::c_str() function, which returns a pointer to a const char buffer:
String card = "2-3d-fg-d6-12-68-32-3f-35-45-42-53-2a-3";
char *prefix = "GET /insert.php?card=";
char *postfix ="&error=1 HTTP/1.1\r\nHost: testsite.com\r\n\r\n";
String url = prefix +card+ postfix;
const char *url_complete = url.c_str();
//...
See also the related post: How concatenate a string and a const char?
87
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
