Resolving the malloc() Exception Issue when Working with PROGMEM on ESP8266

Описание: Learn how to dynamically allocate memory for character arrays stored in flash memory using `PROGMEM` on ESP8266 while avoiding common pitfalls.
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Tackling the malloc() Exception Issue with PROGMEM on ESP8266

When working with limited resources on microcontrollers like the ESP8266, developers often face challenges related to memory management, especially when dynamically allocating memory with malloc() and using PROGMEM. If you're encountering an exception and a subsequent reboot loop while trying to read a file from flash memory, you're not alone. This issue arises commonly due to a misunderstanding of how PROGMEM interacts with dynamic memory allocation. In this guide, we will explore the problem and present an effective solution.

Understanding the Problem

The core issue you're encountering stems from trying to combine malloc() with PROGMEM. Here’s a summary of the situation:

Objective: You need to read a file stored in flash memory using LittleFS and allocate memory for a character array that needs to persist and be modifiable at runtime.

Challenge: Adding the PROGMEM attribute to the character pointer leads to exceptions and infinite loops during execution.

Essential Requirement: Since the ESP8266 has limited RAM, using RAM for storage is impractical; thus, the character array needs to reside in flash memory.

The Nature of PROGMEM and malloc()

To understand the conflict, let’s break down the behavior of PROGMEM and malloc():

PROGMEM:

Purpose: The PROGMEM directive tells the compiler to place the specified variable in flash memory instead of RAM.

Limitation: Only constants can utilize PROGMEM, as it is resolved at build time by the linker.

malloc():

Purpose: The malloc() function dynamically allocates memory from the heap (RAM) at runtime.

Conflict: Attempting to create a character array as a pointer stored in PROGMEM and then allocating memory for it using malloc() is contradictory. This is because PROGMEM expects a fixed location in flash memory, while malloc() operates within the dynamic RAM.

Solution: Streamlined Data Handling

Process the Data on-the-go

Instead of trying to store dynamically allocated memory in PROGMEM, a better approach is to read and process the file simultaneously. Here’s how we can achieve that:

Read from LittleFS: Instead of reading the entire file into memory first, read it in smaller chunks and process it as you go.

Use Appropriate Data Structures: Rather than working with character pointers directly stored in PROGMEM, use classes like CertStoreBearSSL where you can handle certificates directly from the filesystem.

Suggested Code Changes

Here’s an improved version of your code that avoids the pitfalls associated with PROGMEM and malloc():

[[See Video to Reveal this Text or Code Snippet]]

Using CertStoreBearSSL

Utilize CertStoreBearSSL for managing your certificates seamlessly. This way, you can load certificates directly from the filesystem without needing to manually allocate memory for them.

Conclusion

In conclusion, when working with the ESP8266 and trying to manage memory with malloc() in conjunction with PROGMEM, it's crucial to understand that they serve different purposes. Instead of forcing them together, process your data directly from the flash using LittleFS. This approach not only simplifies your code but also ensures reliability without running into memory exceptions.

By following the outlined strategies, you can efficiently manage your memory and dynamically handle files on ESP8266 without the headache of crashes and reboots.