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Copy file name to clipboardExpand all lines: Language/Variables/Utilities/PROGMEM.adoc
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@@ -21,29 +21,30 @@ Store data in flash (program) memory instead of SRAM. There's a description of t
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The `PROGMEM` keyword is a variable modifier, it should be used only with the datatypes defined in pgmspace.h. It tells the compiler "put this information into flash memory", instead of into SRAM, where it would normally go.
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PROGMEM is part of the http://www.nongnu.org/avr-libc/user-manual/group\__avr__pgmspace.html[pgmspace.h] library. It is included automatically in modern versions of the IDE, however if you are using an IDE version below 1.0 (2011), you'll first need to include the library at the top your sketch, like this:
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PROGMEM is part of the link:http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html[pgmspace.h] library. It is included automatically in modern versions of the IDE, however if you are using an IDE version below 1.0 (2011), you'll first need to include the library at the top your sketch, like this:
Note that because PROGMEM is a variable modifier, there is no hard and fast rule about where it should go, so the Arduino compiler accepts all of the definitions below, which are also synonymous. However experiments have indicated that, in various versions of Arduino (having to do with GCC version), PROGMEM may work in one location and not in another. The "string table" example below has been tested to work with Arduino 13. Earlier versions of the IDE may work better if PROGMEM is included after the variable name.
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`const dataType variableName[] PROGMEM = {}; // use this form` +
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`const PROGMEM dataType variableName[] = {}; // or this one`+
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`const dataType PROGMEM variableName[] = {}; // not this one
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`const PROGMEM dataType variableName[] = {}; // or this one`+
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`const dataType PROGMEM variableName[] = {}; // not this one`
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While `PROGMEM` could be used on a single variable, it is really only worth the fuss if you have a larger block of data that needs to be stored, which is usually easiest in an array, (or another C data structure beyond our present discussion).
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Using `PROGMEM` is also a two-step procedure. After getting the data into Flash memory, it requires special methods (functions), also defined in the http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html[pgmspace.h] library, to read the data from program memory back into SRAM, so we can do something useful with it.
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Using `PROGMEM` is also a two-step procedure. After getting the data into Flash memory, it requires special methods (functions), also defined in the link:http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html[pgmspace.h] library, to read the data from program memory back into SRAM, so we can do something useful with it.
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@@ -185,6 +186,7 @@ The following code WILL work, even if locally defined within a function:
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const static char long_str[] PROGMEM = "Hi, I would like to tell you a bit about myself.\n"
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