Weird issue with char

Hello guys,
can someone point me out the issue ?
I try to display a single char from a char array :

char myarray[ ] = "abcdef";
int len = sizeof(myarray);
char temp= "0 ";
for (int i = 0;i < len-1 ; i++){
strcpy(temp, myarray[i]);
showOledMsg( 8,3,temp );

showoledmsg definition:

void showOledMsg(int startCol, int startLine, char* msg)

Serial debug show a b c d e f
But nothing is shown on oled display.
If i replace showOledMsg( 8,3,temp ); by showOledMsg( 8,3, "somechar"); it works.

Someone have an idea ?
Thanks you all !

Post the whole code.

Post the code in code tags so the forum doesn't mess up the array indexes.

Read the forum guidelines to see how to properly post code.
Use the IDE autoformat tool (ctrl-t or Tools, Auto format) before posting code in code tags.


Did this code compile?
char myarray = "abcdef" seems incorrect
strcpy expects char* but you are giving it char.

Try this code:

const char* myarray = "abcdef";
int len = strlen(myarray);
char temp = '\0';

for(int i = 0; i < len; i++)
    temp = myarray[i];


    showOledMsg(8, 3, temp);


I think your code should be more like:

char myarray[] = "abcdef";
int len = sizeof(myarray);
char temp[] = "0 ";

void setup()

  for (int i = 0; i < len - 1 ; i++)
    temp[0] = myarray[i];
    showOledMsg(8, 3, temp);


void loop() {}

I don't have your OLED display and you didn't how how it was initialized but when I comment out the one OLED line, the rest of the sketch compiles without error or warning and produces the expected output on Serial:


I believe it should be strlen() instead of sizeof(), although in this case it probably doesn't matter since its char.

It matters since strlen() does not count the null-terminator whereas sizeof() does.

But in the for loop of your code the test is for i < len - 1. And in my sample the test is for i < len. The end result is the same.

Yes, one may just be more confusing than the other for beginners.

If you change it to strlen(), be sure to change the 'len - 1' in the loop.

Yes. For strings it should be < len - 1 when sizeof() is used (or strlen and < len) but for any other data representation in char form (for example binary data like audio samples) it should strictly be sizeof() and reading till < len. Using strlen() in this case will fail since binary data streams can contain the null terminating character mid-stream as well.

Typically native or OS APIs (in Windows, atleast) never return char buffers (not strings) just as an array and expect the calling code to infer its length. It is something like this:

// Function that returns a char buffer
void get_buffer(char** buf, int& szLen)
// OR
//void get_buffer(char** buf, size_t& szLen)
    (*buf) = new char[10]; // allocate 10 bytes for example

    //Fill data in the buffer

    szLen = 10;

// Usage

char* myBuf = ""; //Initialisation may or may not be done.

int bufLen;

/// Call get_buffer()
get_buffer(&myBuf, &bufLen);

for(int i = 0; i < bufLen; i++)
    // Use the buffer data
    char elem = myBuf[i];
    Serial.write(elem); // Just an example

delete[] myBuf; // Clean up

That is because OP did not use code tags :frowning: So [] becomes [].

Yes, got it.

I have never seen the Windows API behave like that. Normally you would pass a null-buffer to a function to get the required buffer size and use "new" or "malloc" to reserve that buffer before recalling the function with a valid buffer. Your example is also leaking memory, btw.

Have a look at the DirectSound8 or WaveIn/WaveOut or the setUpDiXXX (these relate to HID) APIs. They return an interface by expecting the address of a pointer.

I may have mixed it up with this, but the method I outlined is when the called function allocates memory versus the caller allocating it.

As for the memory leak, that is something from memory and may not be perfect - the caller has to clean up - updated the code for this. Basically I had to do it this way to send audio data (char*) via platform-invoke to .NET code.

Yes the way you suggested is also done -call it twice, once for the expected size, then allocate memory and then call with the actual buffer and sz parameter as zero.

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