Create string from char[] in c returns wrong value

Question

I have the following code, that declares a char data[] with simple JSON data. ({"x":123,"y":137}) 17 chars long.

On my main function I then try to copy the char data[] char by char and concatenate to a new string using strcat function.

When I print the char that I want to concatenate it appears to be the valid char, but when I print the final string, it is wrong.

Code:

#include <stdio.h>
#include <string.h>

int getDataLength(){
    return 17;
}
static char data[] = {'{','"','x','"',':','1','2','5',',','"','y','"',':','1','3','7','}'};
char getData(int i){
    return data[i];
}

int main() {
    int dataLength = getDataLength();
    char data[dataLength + 1];
    for(int i=0 ; i < dataLength ; i++){
        char chr = getData(i);
        // --- looking good ---
        printf("%c",chr);
        strcat(data,&chr);
    }
    data[dataLength] = '\0';
    // --- broken string ---
    printf("\n%s",data);
}

Output:

{"x":125,"y":137}
(���{"x":1% 

What am I missing?

Answer 1

You should look at your compiler warnings.

strcat(data,&chr);

strcat expects two strings as parameters. Your second parameter is no string but only a single character. The nul-termination is missing and you copy random garbage data.

Additionally you don't initialize data before you start adding to it.

Both errors cause undefined behaviour in your program.