Handle strings in the sketch - heap problem.

[robertgregor]

Hello, it seems, that all the strings used in the sketch are going to the heap. I have a sketch which is using ESP8266WebServer. In each handler, I am using the Strings to output. Now I've reached the situation, that if I put any more Strings in the sketch, the heap is full. It is very strange, thus based on the hint I am now doing this:
static const char XSD_SW_1[] = "blablba-bigstring";

Then in the handler method:
server.send(200, "text/xml", "");
WiFiClient client = server.client();
client.write(&XSD_SW_1[0], sizeof(XSD_SW_1)-1);

But looks like that that const is a part of the heap, because if I will do XSD_SW_1[] = ""; then everything is OK. If I leave big - aroung 1400 characters - maximum buffer of the Webserver write method, my sketch doesn't work at all - seems, it will even not start correctly.
Is there any way, how to tell to compiler, that the strings should stay on flash and should be read, when it is needed? In arduino, there is for this purpose the F() macro or PROGMEM directive. But I belive, this is avr specific.

[Makuna]

I did a little investigation on this. While it is possible to move strings off the heap, the attribute to do this is not compatible with the use like F() nor PSTR(). It requires that target be global in scope and a variable.

will work

PROGMEM const char titleString[] = "Title of my app";

setup() {
...
serial.print(titleString);
}

will not

setup() {
...
serial.print(F("Title of my app"));
}

[Makuna]

I now have a grasp on how to fix it. Its going to take a bit as a lot of original Arduino abstraction for it was removed from the code base and I have to add it all back to get it to work.

[igrr]

@Makuna could you please drop into our chat at https://gitter.im/esp8266/Arduino to discuss the changes required?

[robertgregor]

Hello,
for me it doesn't really work. Everything seems is going to the stack:
I have:
PROGMEM const char WSDL[] = "abcd\n";
void setup() {
Serial.write(&WSDL[0], sizeof(WSDL)-1);
}
Output is:
abcd
Free heap:32568

Then I will change:

PROGMEM const char WSDL[] = "/webservice/SimpleSwitch?xsd=1">/xsd:import/xsd:schema<message name="switchOnFor3Minutes"><part name="parameters" element="tns:switchOnFor3Minutes">.........................Long string";
void setup() {
Serial.write(&WSDL[0], sizeof(WSDL)-1);
}

Output:
/webservice/SimpleSwitch?xsd=1
Free heap:31704

So there should be some way, how to store the bytes to the flash and read it directly from flash. Especially for the Webserver.

[Makuna]

I have solution. It is in progress and a pull will be posted with the fixes. The current PROGMEM just mapped to ram.

It will work just like Arduino does today, storing strings into the instruction ".irom.text" section.

It will support F(), PSTR(), PROGMEM, Print extensions, String extensions, and str*_P methods.

[Makuna]

#236 pending with fixes

[Makuna]

this was merged in

[tontito]

Work:
client.write(F("
Password<input type="password" name="pass" placeholder="Example: ">"));

Not Work:
client.write(F("
Password<input type="password" name="pass" placeholder="Example: ">") + F("12345678"));

In file included from C:\Program Files (x86)\Arduino\hardware\esp8266com\esp8266\cores\esp8266/Arduino.h:212:0,
from C:\Program Files (x86)\Arduino\hardware\esp8266com\esp8266\libraries\ESP8266WiFi\src/WiFiClient.h:24,
from C:\Program Files (x86)\Arduino\hardware\esp8266com\esp8266\libraries\ESP8266WiFi\src/ESP8266WiFi.h:32,
from sketch_may14a.ino:1:
sketch_may14a.ino: In function 'void setup()':
C:\Program Files (x86)\Arduino\hardware\esp8266com\esp8266\cores\esp8266/WString.h:38:95: error: invalid operands of types 'const _FlashStringHelper' and 'const _FlashStringHelper' to binary 'operator+'
#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))
^
sketch_may14a.ino:10:128: note: in expansion of macro 'F'
Derleme sırasında hata oluştu.

[Makuna]

This is invalid syntax and use of F(). It will not compile when targeting AVR either.

F is not a string class that does string manipulation, use String for that. It is simply a way to annotate string literals so they are stored in rom rather than ram.

The simple fix to your example is

client.write(F("Password12345678"));

But I suspect there is more to your case than your example? The following will work and is supported.

client.write( String(F("Password")) + F("12345678") );

[tontito]

Ok thanks. I fixed it

i have define to FLASH ROM
#define webcss F("body{font-family:Trebuchet MS;font-size:16px;-webkit-font-smoothing:antialiased}h1{text-align:center;color:#00f}")

error: invalid conversion from 'const __FlashStringHelper*' to 'uint8_t {aka unsigned char}' [-fpermissive]

How do I do that

[Makuna]

how are you using webcss? What are you passing it to? Provide the code for the call.

please read up on the differences between F() and PSTR() in the Arduino documentation.

[tontito]

webss is my css file send client.

ok now I'm looking at. thank you so much.

[robertgregor]

Hello, great work!!! Working well - PROGMEM and also F() macro. I have now a lot of stack free:))))
Thanks for fixing that!

[robertgregor]

Just a question: I know, that AVR is doing this: If I have two same strings in the sketch will it occupy only one space in the program flash? I.e.:
................
F("abcd");
............
............
F("abcd");

Will this occupy 5 bytes or 10 bytes? In case of AVR and arduino, it will take only 5 bytes (4+1 null terminator.)