Why is my code failing to copy one string to another?

I want to write a C function called append() the target of which would be to append a character at the end of a given string. But, I am having trouble to get my code work because of some pointer-related issue.

#include <stdio.h>

void append(char ** string, int length, char ch)
{
    printf("%s", string);

    char * newString = (char *) malloc(sizeof(char)*(length+2));

    strcpy(newString, string);

    newString[length ] = ch;
    newString[length + 1] = '\0';

    strcpy(string, newString);

    free(newString);
}

int main()
{
    char * string = "my1name234is56";
    append(string, strlen(string), 'x');

    printf("%s", string);
}

Inside the function append(): the second strcpy() before free() is giving me an exception, and the program is showing:

Unhandled exception at 0x5E8D40D9 (msvcr120d.dll) in mips_2_1.exe: 
0xC0000005: Access violation writing location 0x00D1585C.

I have also tried

strcpy(*string, newString);

and

strcpy(**string, newString);

and, nothing seems to be working.

.

strcpy(&string, newString);

seems to be working, but, the end result is not what I would expect.

Edit: I made the edit according to alk.

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

void append(char ** string, int length, char ch)
{
    printf("%s\n\n", *string);

    char * newString = (char *)malloc(sizeof(char)*(length + 2));

    strcpy_s(newString, length+1, *string);

    newString[length] = ch;
    newString[length + 1] = '\0';

    strcpy_s(*string, length + 2, newString);

    *string = newString;
}

int main(void)
{
    char * string = "my1name234is56";
    append(&string, strlen(string), 'x');

    printf("%s", string);
}

its still not working.

Edit2: I made the following edit according to "Weather Vane":

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

char * append(const char * string, int length, char ch)
{
    char * newString = (char *)malloc(sizeof(char)*(length + 5));

    printf("%s\n\n", string);

    strcpy_s(newString, length+5, string);

    newString[length] = ch;
    newString[length + 1] = '\0';

    return newString;
}

int main(void)
{
    char * string = "my1name234is56";

    printf("%s\n\n", string);

    char * string2 = append(string, strlen(string), 'x');

    printf("%s", string2);
}

Now, it is working.

Edit3: the following modification, according to alk, worked for me:

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

void append(char ** string, int length, char ch)
{
    char * newString = (char *)malloc(sizeof(char)*(length + 2));

    strcpy_s(newString, length + 1, *string);

    newString[length] = ch;
    newString[length + 1] = '\0';

    *string = newString;
}

int main(void)
{
    char * string = "my1name234is56";

    printf("%s\n\n", string);

    append(&string, strlen(string), 'x');

    printf("%s\n\n", string);
}

char * string = "my1name234is56"; is a string literal. There are two reasons why you can't write the extended string back to it:

• There isn't enough memory for the extra character.
• It is read-only.

In any case, the function definition has too many stars, it should be

void append(char * string, int length, char ch)

Or even better

void append(const char * string, int length, char ch)

which indicates it should not be written to, and the whole idea is off-limits. You have to create a new string and return a pointer to that one

char *append(const char * string, int length, char ch) {
    // ...
    return newString;
}

int main() {
    //...
    string = append(string, strlen(string), 'x');
}