data to buffer

Projects Programming
1

Hi, i have a question about a code i wrote:

//print data to buffer

#define buf1Max 256
char buf1[buf1Max];
char data[30];

int bufIndex = 0;

void dataToBuffer(){
    for (int x = 0; x < strlen(data); x++){ //data to buffer
      if (bufIndex < buf1Max){
        Serial.println(bufIndex);
        buf1 [bufIndex] = (data[x]);
        bufIndex++;
      }//end if bufIndex
  }//end for dataToBuffer
  
} //end void dataToBuffer

void setup() {
  
  Serial.begin(9600);

  sprintf(data,"%d",12000); 

  for (int x = 0; x < 200; x++){ //save data to buffer, more than buffer1 length.
    dataToBuffer();
  }
  
  Serial.println(buf1);
  Serial.println(strlen(buf1));



}

void loop() {
  // put your main code here, to run repeatedly:

}

I created a buffer with 256 chars (buf1[0 to 255] and since i typed "bufIndex < buf1Max" i would expect its gonna to fill the buffer from 0 to 255. But.. it the serial outputs this (I deleted first 251 values):

252
253
254
255
120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000112000
261

I got 261 chars, but i told the array is max 256 chars long.. Why is the array now 261 chars long, why are the last values are a mess and where the # are the chars from 255 to 261 stored?

When i change "char buf1[buf1Max]" to "char buf1[buf1Max+1]" it seems to work; i got 256 chars stored. but why do i need to make the char array one char larger? i dont want to make the buf longer then 255chars and fill it completely up with values..

Its not really about the solution.., but about what i dont understand well.

3

Here is a good resource about null-terminated strings.

4

That is a dangerous way of writing code, you should avoid to write code like that. Try to solve the problem in a different way.

This is what you do:

index = 0;
for ( int i = 0; i < 200; i++) 
{
  for ( int j = 0; j < 5; j++)
  {
    if ( index < 256)
    {
      buffer[index] = data[x];
      index++;
    }
  }
}

The buffer is written until the very last character. That means there is no zero-terminator. The value of "261" is not an index, that is the strlen. You can easily see that, when you change it into:

  Serial.print( "buf1 = ");
  Serial.println( buf1);
  Serial.print( "strlen(buf1) = ");
  Serial.println( strlen(buf1));

The result is:

252
253
254
255
buf1 = 120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000120001200012000112000
strlen(buf1) = 261
5

If i understand correctly, never try to fill up a char array completely, and use the /0? I was thinking about this:

char buf1[bufMax];
int data =0;

sprintf(buf1, "%s%d,", buf1, data);

in this case, will the already written buf1 data re-written? Is this method fast and reliable?

7

it combines strings? I need to add a int to a string.

first put it in a separate char array and then combine arrays? or..?

9

hmm.

Does itoa always start filling the array at index 0, or can i also point to another index?

11

Thanks, helps. I saw itoa gives a return value, a pointer to "/0". So i tried to make it thing visible:

  sprintf(buff,"%s","11111111111111111111111111");
  
  *buff = 'a';   // puts an a in slot 0
  *(buff + 2) = 'b';   // puts an b in slot 3

  startIx = buff;

  Serial.print("Start index position: ");
  Serial.println(startIx);
  
  retVal = itoa (anInt, buff + 15, DEC);  // puts a '5' in slot 5.

  Serial.print("in Array stored: ");
  for(int x = 0; x < 30;x++){
    Serial.write(buff[x]);
  }
  Serial.println("");
  Serial.print("New index position: ");
  Serial.println(retVal-startIx);

but.. it return the "\o" before the new value is added. I can easily find the new end with strlen ofcourse, but why does it return a value what you already know since you have to fill it in in the itoa statement?

12

So this is what i got:

  itoa (anInt, buff + strlen(buff), DEC);

code before:

sprintf(buff, "%s%d,", buff, anInt);

I cant see why, but the new one should be a better way?

13

Well, if Delta_G claims that it is not fast and bloats code, then he might have to prove how much slower it is :grin:

The sprintf() function is not small, but it processes the formatted string pretty fast.

15

i would like to know if my Sprintf version not the completely rewrites into the buffer. when i say:

sprintf(buff, "%s%d,", buff, anInt);

Does it overwrite buff[0],buff[1], etc with the same values, because %s is the same as the buffer? or is it that smart and only rewrites %d?

16

sprintf() has some input arguments, and a pointer to where to write to. It does not care what is already in the memory location to be overwritten. You could use

sprintf(&buff[strlen(buff)], "%d", anInt);

to NOT overwrite the first n characters of buff. But there is little advantage in doing that.

18

Thanks. Im new to pointers, this is a big help, tabks.

I need to seperate values with "," or " ". So i think i can best use sprintf then, like paul suggest with pointer to the "/0" position.