Unable to concatenate two char * pointer value?

Question

All I want to ask a question that if we have two string but these should be in the given code

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

char *getln()
{
    char *line = NULL, *tmp = NULL;
    size_t size = 0, index = 0;
    int ch = EOF;

    while (ch) {
        ch = getc(stdin);
        if (ch == EOF || ch == '\n')
            ch = 0;
        if (size <= index) {
            size += index;
            tmp = realloc(line, size);
            if (!tmp) {
                free(line);
                line = NULL;
                break;
            }
            line = tmp;
        }
        line[index++] = ch;
    }

    return line;
}
char * combine(char *a,char *buffer){
   int stop_var;
   int newSize = strlen(a)  + strlen(buffer) + 1; 
   char * newBuffer = (char *)malloc(newSize);
   strcpy(newBuffer,a);
   strcat(newBuffer,buffer); // or strncat
   //printf("Final String %s",newBuffer);
   free(a);
   a = newBuffer;
   return newBuffer;
}
char * fun(char *str)
{
    char *str1;
    str1=getln();
    str=combine(str,str1);
    return str;
}
int main(void)
{
    char *str;
    str= getln();
    printf("Initial String %s \n",str);
    str=fun(str);
    printf("Final String %s \n",str);
}

this code is working fine but string char *str="Fixed value" is fixed is gives runtime error

int main(void)
{
    char *str;
    str= "Fixed Value";
    printf("Initial String %s \n",str);
    str=fun(str);
    printf("Final String %s \n",str);
} 

So, I want to know is it any other way to run the above case in which the string is fixed. I know that "How can we achieve the final value in the same character pointer".

Please Read Note: There are many Questions related to this question I have tried all the solutions. I looked the below solutions

1. concatenate-two-char-arrays-into-single-char-array-using-pointers

2. concatenate-two-char-arrays

3. how-to-concatenate-pointer-arrays

4. concatenate-char-array-in-c

5. concatenate-two-arrays-using-void-pointer-c

I have searched many more solutions over the internet, but no solutions are fulfilled my condition. These all the solution is using the third variable and showing its value. I want value in the same variable. Even if we are creating any extra variable, its value finally should be assign to char *str.

Please provide me any suggestion to do this in c language only.

The given question is different from my question because in that question they checking the behaviour of the string literals bu for my case I am looking to assigns concatenate value to variable str. And its solution changing pointer to an array but I cannot change because its value is using many functions for my work.

Answer 1

Your function combine() calls free() on its first argument. In your fun() call, which is passed from fun() which points to a string literal in your code.

You cannot call free() on a string literal, that's undefined behaviour. I suggest you to restructure your code so that combine() no longer calls free() on its first argument. Freeing memory should be the callers responsibility.

Answer 2

I would suggest pairs of allocation/deallocation functions:

char * combine_allocate(const char *a, const char *b) {
    char* result = malloc(...)
    combine ...
    return result;
}

void combine_deallocate(char* p) { free(p); }

Having that:

char* a = initial_allocate();
char* b = initial_allocate();
char* c = combine_allocate(a, b);
initial_deallocate(a);
initial_deallocate(b);
// ... use c
combine_deallocate(c)

You may omit (and should) initial_allocate/initial_deallocate for string literals. It might be a bit too verbose, but object oriented C does nothing else (with nicer function names).