Concatenate/append using <string.h>

hello,

i doing preparations new project. involves basic c knowledge.....

i have following test code:

code: [select]


#include <string.h>
#include <stdlib.h>

//constants
const byte iputmaxlength = 30 + 1 + 1 + 1;//30 plus header plus footer plus terminating zero

const char header = '~';
const char footer = '#';

const char btnu[] = "buttonup";
const char btnd[] = "buttondown";
const char btnl[] = "buttonlefr";
const char btnr[] = "buttonright";

char stringin[iputmaxlength] = {
 '\0'};

int ledpin = 13;     // led connected digital pin 13

void setup(){
 pinmode(ledpin, output);      // sets digital pin output
 serial.begin(9600);
}

void loop(){  
 serialread();
}

void serialread(){
 // see if there's incoming serial data:
 if(serial.available() > 0) {
   getincomingchars();
 }
 serial.println(stringin);
}

void getincomingchars() {
 // read incoming data char:
 char charin = serial.read();
 if ((strlen(stringin)) < iputmaxlength) {
   strcat(stringin, charin)
 }
}


i getting error message:

code: [select]


in function 'void getincomingchars()':
error: invalid conversion 'char' 'const char*'


what doing wrong?

thanks in advance!

there 2 problems in code.  one problem minor , fixed.  the other problem serious.


first minor problem...

strcat expects both parameters strings (arrays of characters terminated null).  you passing char second parameter.  something past problem...

code: [select]

 char charin[2];
 charin[0] = serial.read();
 charin[1] = 0;
 ...
   strcat(stringin, charin);  // note: forgot semicolon


the second (serious) problem line of code...

code: [select]

 if ((strlen(stringin)) < iputmaxlength) {

you not leaving space null terminator.  i'll use example illustrate problem...

assume iputmaxlength = 4 , stringin has letters 'a' , 'b' stored.  this memory contents...

 [a]

• 
  
  the character 'c' arrives.  strlen(stringin) = 2 less iputmaxlength strcat executed leaving this...
  
   [a] [c]
  • 
    
    no problem.  the character 'd' arrives.  strlen(stringin) = 3 less iputmaxlength strcat executed leaving this...
    
     [a] [c] [d]
    • 
      
      oops!  strcat wrote null-terminator past end of stringin corrupting whatever next in memory.
      

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