Understanding memory fragmentation due to char arrays in functions

Question

This is similar to: Is this function subject to memory fragmentation?

I am trying to get my head around Memory fragmentation in Arduino's and embedded systems in general.

Currently I have a function in a __lcdAux__ Class which is responsible for doing basic operation on the lcd. One of the members of the class is a char array double2StrBuff1 which is used with __dtostrf()__ to create a double which is then passed to sprintf. So the following function (__lcdAux::UpdatePosition__) is responsible for updating the position on the lcd.

void lcdAux::UpdatePosition(double MtXPosLengthUnits) {
    lcd->setCursor(0, 2);
    dtostrf(MtXPosLengthUnits, 0,3, double2StrBuff1);
    sprintf(lcdBuff, "X: %s", double2StrBuff1);
    lcd->print(lcdBuff);
}

Currently, I am under the impression that by defining once the class as a global variable and then using it throughout the program, then the char array is allocated only once memory space. Therefore by using it afterwards no memory fragmentation occurs.

However, I read that functions allocate space on the stack which is immediately recoved when the function is exited. Therefore I am curious if the following declaration is any different in terms of memory fragmentation.

void lcdAux::UpdatePosition(double MtXPosLengthUnits) {
    lcd->setCursor(0, 2);
    char double2StrBuff1[10];
    dtostrf(MtXPosLengthUnits, 0,3, double2StrBuff1);
    sprintf(lcdBuff, "X: %s", double2StrBuff1);
    lcd->print(lcdBuff);
}

To sum up:

• by putting the declaration for variable __char double2StrBuff1[10];__ inside the function, is the memory allocated from the stack and therefore recovered upon exiting the function (therefore there is no need to declarate a member variable on the class level),

OR

• is the fact that __double2StrBuff1__ is a pointer means that the pointer is allocated in the stack but the values are located in the heap (therefore what I am doing makes sense)?

PS: Because some might point out that __"X: %s"__ is worse in terms of memory usage and fragmentation, I'll state the following. Instead of the format string __"X:%s"__ I use a _PROGMEM const char_ array and __strcpy_P()_, but to simplify the question I replaced it with the literal string , since I feel that is another problem.

Answer 1

by putting the declaration for variable char double2StrBuff1[10]; inside the function, is the memory allocated from the stack and therefore recovered upon exiting the function

This is correct.

is the fact that double2StrBuff1 is a pointer means that the pointer is allocated in the stack but the values are located in the heap (therefore what I am doing makes sense)?

No, double2StrBuff1 is not a pointer, it's an array. In C and C++, when you use an array identifier, it decays to a pointer in many situations. However, for the purpose of memory allocation, an array is very different from a pointer.

Heap allocation only happens when using malloc() and new.

Answer 2

The examples in the question above use 1) global and 2) local aka stack allocation and do not give heap fragmentation. The heap is not involved.

Below is a version that may give heap fragmentation (though a very low risk):

void lcdAux::UpdatePosition(double MtXPosLengthUnits) {
    lcd->setCursor(0, 2);
    char* double2StrBuff1 = (char*) malloc(10);
    dtostrf(MtXPosLengthUnits, 0,3, double2StrBuff1);
    sprintf(lcdBuff, "X: %s", double2StrBuff1);
    lcd->print(lcdBuff);
    free(double2StrBuff1);
}

Please not that heap allocation is very slow compared to stack allocation.

C/C++ does not have any implicit heap allocations (in the language). A local array in a function is allocated on the stack and removed when the function returns. The only library functions that can generate stack fragmentation are setjmp and longjmp if used to create a spaghetti stack.