Unable to concatenate two char * pointer value? - c

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
concatenate-two-char-arrays-into-single-char-array-using-pointers
concatenate-two-char-arrays
how-to-concatenate-pointer-arrays
concatenate-char-array-in-c
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).