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Topic: Interesting discovery - Can you figure out what's wrong? (Read 1 time) previous topic - next topic



Yes.. I noticed that  a lot of people was doing it like that.

If I declare a Char Temp[100] and I insert 50 characters followed by a null terminator. Does that reserve the full 100 in memory or only the bit up to the null terminator?

thanks again for all the help.


As an aside, the String class you use in your example can also be problematic since it exposes a heap corruption problem in versions up to 1.0.4 and introduces the risk of heap fragmentation in all versions. In a system with such limited heap space and such limited ability to manage the heap, using dynamic (heap) memory to store dynamic data is not a smart approach. I recommend that you use c-strings (null-terminated char arrays) instead. There are no particular advantages to using the String class and IMO you would be better off forgetting that it exists.

Peter, String has a huge advantages... :) It prevents a huge amount of frustration.

The function I am working on will generate GCode commands.. I just want to build simple string but I am finding it hard to even just add a char to char *.  :smiley-eek-blue:

Well, adding a char variable to a char string can be trivial if you know exactly where in the string the char variable goes by index. (Either by having an index variable, or knowing a rigid character position structure.) That's because it's just an array that can be accessed just like any other array. (You as the programmer just need to make sure there is a null (0x00, '\0', 0, 00, etc.) at the index position right after the last character of interest before trying to handle the char string as a singular entity.) If you want to put 'r' at the 10th position of char string outputBuffer, it's as simple as outputBuffer[9] = 'r';. If you are continually adding a new character, simply keep track of where you are in the char string with an index variable. Assign the new char at the current index location, increment the index, and insert a null at the new current index location. Wash, rinse, repeat for each new char to be appended to the char string. (Ok, yes, you also need to watch to make sure you don't try to go past the bounds of your char string...)

The C string libraries just make this easier for you as long as you can remember the abbreviation construction of the function names, or have http://www.cplusplus.com/reference/cstring/ bookmarked. (It's the latter for me...)

Nick Gammon

If I declare a Char Temp[100] and I insert 50 characters followed by a null terminator. Does that reserve the full 100 in memory or only the bit up to the null terminator?

It reserves the amount you declare.


Typically it could be done something like this

I'm not sure that it is a good idea to give an example that makes any assumptions about what
I am finding it hard to even just add a char to char *.


I find that people that make statements like this assume that pointers and arrays are the same thing. While there are strong ties between pointers and arrays, that is ONLY true when the pointer points to allocated memory. Far too often, newbies create a pointer that points to nothing, and then assume that they can use it as an array that holds as much as they want.

Rather than suggesting how they might do what they suggest in the question, I think it's better to ask them to post some code, so that it can be determined that the pointer in question does indeed point to allocated memory.


Code: [Select]

  struct position {
    int x;
    int y;
    int z;
    position(){ 0; 0; 0; }
    position(int X,int Y, int Z){ x = X; y = Y; z = Z; }
  } CurrentPosition, DefaultPosition;

  void GRBLCommand()
    int XChange = CurrentPosition.x - DefaultPosition.x;
    int YChange = CurrentPosition.y - DefaultPosition.y;
    int ZChange = CurrentPosition.z - DefaultPosition.z;
        (XChange > 10 || XChange < -10) ||
        (YChange > 10 || YChange < -10) ||
        (ZChange > 10 || ZChange < -10)
      Serial.print("G1 ");
      // This can be shortened  to a method that takes the Change,Axis letter and returned the string.
      if(XChange < -10) Serial.print("X-1000");
      if(XChange > 10)  Serial.print("X1000");
      if(YChange < -10) Serial.print("Y-1000");
      if(YChange > 10)  Serial.print("Y1000");
      if(ZChange < -10) Serial.print("Z-1000");
      if(ZChange > 10)  Serial.print("Z1000");

This is my working code. My original idea was to create method that returned a string that gets printed with Serial.print();
I could not get that going because String kept on locking up my Arduino. Because I can't get strings to work properly I ended up just doing the Serial.print inside the method. (It works but I don't like it cause I am sure the code can look much simpler.)

Any ideas on how I can optimize this method and removing the dependency on Serial?

Thanks for everyone's help up to now.

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