Esp32 string length limit

Question

I found that the length of the line in app that runs on esp32 is limited. Below is an application that generates a string:

main :
  loop ::= 2459
  text  := ""
  stamp := "1234567890"
  loop.repeat :
    text += stamp
  print ("$text.size")

If loop ::= 2458 everything is fine:

micrcx@micrcx-desktop:~/toit_apps/Hsm3/tests$ toit run test_str.toit
2021-06-02T17:03:43.237309Z: <process initiated>
24580
2021-06-02T17:03:44.575860Z: <process terminated - exit code: 0>

If loop ::= 2459, a memory allocation error appears:

micrcx@micrcx-desktop:~/toit_apps/Hsm3/tests$ toit run test_str.toit
2021-06-02T17:04:03.507773Z: <process initiated>
2021-06-02T17:04:04.907154Z: <stack trace>
EXCEPTION error. 
ALLOCATION_FAILED
  0: string.+                  <sdk>/core/string.toit:274:5
  1: main.<block>              /home/micrcx/toit_apps/Hsm3/tests/test_str.toit:6:10
  2: SmallInteger_.repeat      <sdk>/core/numbers.toit:209:3
  3: main                      /home/micrcx/toit_apps/Hsm3/tests/test_str.toit:5:8
  4: __entry__.<lambda>        <sdk>/core/entry.toit:46:20
2021-06-02T17:04:04.954257Z: <process terminated - exit code: 1>

All these errors are of course not fatal, it's just nice to know the real numbers, for example, the maximum length of a string, list, hash maps & etc, in order to write an app that at one point will not fall because the size of the json file has exceeded the allowable number.

Florian Loitsch · Accepted Answer · 2021-06-03 08:57:14Z

1

There aren't any real size limitations on strings, lists, maps, etc.

The system just tries to allocate space for these objects, and reports an error when there isn't enough memory left. This, obviously, imposes fundamental limits on all objects: The ESP32 has at most 520K RAM, so no data structures can every exceed that size.

However, there are many more restrictions that come into play. IRAM memory, for example, has different properties than DRAM, and isn't well suited for normal data. Similarly, there is some memory that is used by the system.

For some data structures, like strings, it's also important that the memory is in a continuous region. Fragmentation can lead to cases where a bigger string just can't be allocated anymore because all free memory is in smaller holes where the string wouldn't fit.

Note, that using + to build up a string also means that there is always a copy of the previous string around, which doubles the space needed for the program.

Here is an example, to show that strings aren't fundamentally limited to ~25K:

main :
  loop ::= 4500
  stamp := "1234567890"
  text := stamp * loop
  print "$text.size"

The * operator on strings preallocates the required size, and thus doesn't need any other copy (as in the loop).

answered Jun 3, 2021 at 8:57

Florian Loitsch

8,1421 gold badge29 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Michael Kanzieper Over a year ago

I actually have nothing against restrictions. They always exist. It's just that in this case, a situation is possible when the application works on the console, and the developer is sure that everything is ok, and then he transfers it to the chip and sees with horror that everything is falling. :-)

Pietro · Accepted Answer · 2021-11-16 16:38:19Z

Using Arduino ESP32 webserver library I realized that when I try to print the HTML string that makes the webpage I get the following error, even if 520K RAM is quite a lot of RAM and I have 173K of free heap:

    String WEBPAGE = "lots of chars"
    Serial.println(WEBPAGE);
    Serial.printf("Free heap: %u\n", ESP.getFreeHeap());

I have on console

    E (49837) uart: uart_write_bytes(1159): buffer null
    Free heap: 173831

I found out the cause to be the string WEBPAGE that is too large (around 50k characters), with smaller strings the error doesn't happen. So also the following function doesn't work and send and empty string instead and I see a white blank page on browser.

server.send(200, "text/html", WEBPAGE);

I beleive this is quite a big limitation using strings on ESP32

More details here.

Ameen · Accepted Answer · 2022-12-29 15:25:32Z

0

In my view the understanding of the device purpose is key, or design system that would not challenge memory limits as these are small i would use it to read and control IO, and leave heavy lifting like webserver to serial or cloud connected infra. if we start hitting limits we should re look at the design

answered Dec 29, 2022 at 15:25

Ameen

373 bronze badges

Collectives™ on Stack Overflow

Esp32 string length limit

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related