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I have quite a big Arduino project (in eclipse) going on with a lot of debug messages using the Serial.print("some string text") commands so that I can debug along the way.

One thing that I have noticed is that I reach a limit for how many of these I can have in the project. If i put too many, the program halts at very strange places. Ie: often long before my newest addition of a print command is supposed to execute.

My project .hex file is around 20k at the moment. The Arduino Uno limits around 30kb right? So it should not be too big.

So I feel that the actual problem is probably that maybe these serial commands are filling up my sram. Which is just 2kb. I am using a lot of libraries.

Is the command Serial.print("some string text") occupying SRAM? Surely gcc puts these string cnstants are in program space? but maybe they are not?

Or is it something else? I have an alternative theory that there is a serial.print buffer somewhere, and I am probably just filling it up with too many messages.

SpiRail
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    +1. I just had the same problem on AVR. But be aware that reading from flash is slower than from RAM, so maybe you find a good balance to, maybe, keep the most needed strings in RAM or so. – glglgl Oct 10 '12 at 07:11
  • Thanks, that's good to know also. Explains why its not kept out of the SRAM by default. – SpiRail Oct 10 '12 at 09:02
  • This is not the only reason. Accessing RAM is much easier; accessing Flash often requires extra work. – glglgl Oct 10 '12 at 09:46

4 Answers4

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Yup, string are stored in RAM by default. Although they're in the Flash memory too, but they're loaded into RAM when the Arduino boots.

However, if you use The Arduino IDE version 1.0 or later you can tell the compiler to read strings directly from Flash and not to bother loading them into RAM with the F() macro:

Serial.Println(F("This string is read from Flash!"));

This will save RAM which is a good thing as there's much less RAM than Flash. See here for more details: * http://www.arduino.cc/playground/Main/Printf

Marty
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  • Excellent. I will try this as soon as possible. I'm using eclipse (rather than the raw arduino ide), I assume that the fact that I'm adding arduino.h will be enough to let me use this function? – SpiRail Oct 10 '12 at 08:57
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    I have a related question: lets say I've defined a debug print function to be "#define debug(M) Serial.print("Debug: "); Serial.println((M))" -- will each instance of the string "Debug: " use up more memory? Should I set this string to a global variable instead? – vitiral Dec 06 '13 at 18:39
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    I answered my own question, it does: http://linux.dd.com.au/wiki/Arduino_Static_Strings That's fun! – vitiral Dec 06 '13 at 18:44
  • Bless you kind sir for telling us about this magical `F()` macro! – binar Mar 18 '14 at 22:26
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This is not my code, but I find that the solution at: http://www.utopiamechanicus.com/article/low-memory-serial-print/ is very good for debugging. A decent combination of printf, flash memory usage, and macros so conversion is often as easy as removing the '.' from Serial.print().

I am a total noob to C++ and arduino though, hope someone finds it useful.

Alluvian
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Please try and mark the strings as PROGMEM, which should place them in the flash. Arduino does not seem to have Serial.write implemented for PROGMEM, so a mem-copy is required. See http://arduino.cc/en/Reference/PROGMEM (String arrays) for details.

EDIT: http://deans-avr-tutorials.googlecode.com/svn/trunk/Progmem/Output/Progmem.pdf explains the PROGMEM argument nicely.

Rudolf Mühlbauer
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Yes it get's stored in RAM by default. You can use the solution by @Marty.

Alternatively you can also use MemoryFree library to keep track of your memory.

Sudar
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