Question

I was trying to implement a simple function that can concatenate any number of strings passed to it. My call to realloc fails. Is it something to do with the fact that the string arguments that i pass to the function are stored in data segment where as realloc looks to allocate memory from the heap ? This is just an idea i have. I’m a beginner so please excuse if it seems stupid. How can i make this function run ?

 //Program to implement a function that can concatenate any number of argumnets 

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

char *mstrcat(char *first, ...);
int main(int argc, int **argv){
    char *s;
    s=mstrcat("I ","Love ","Stack","Overflow");
    printf("%s\n",s);
}
char *mstrcat(char *first, ...){
       char *s=first,*p;
       int len=0;  // stores the length of the string as it grows
       len=strlen(s);
       va_list aptr; // creates a pointer to the unnamed argument list
       va_start(aptr,first); // initialise aptr to the first unnamed argument
       if(aptr==NULL){
        return s;
      }
      while((p=va_arg(aptr,char *))!=NULL){ // till there are no more arguments to process
         len+=strlen(p);
         if((s=(char *)realloc(s,len+1))!=NULL){
            strcat(s,p);
         }
         else{
            printf("Failed to concatenate\n");    
            return first; 
         }
      }
      return s;
}

Answer 1

Your code has an Undefined Behavior. The standard mandates that the pointer being passed to realloc should exactly match the pointer which was allocated dynamic memory using a memory management function. Memory management functions specified by the standard are:
aligned_alloc, calloc, malloc, and realloc.

The pointer you are passing to realloc() was not returned by any of these and hence the Undefined Behavior.

Reference:
c99 standard: 7.22.3.5 The realloc function

Synopsis: #1

#include <stdlib.h>
void *realloc(void *ptr, size_t size);

#3

If ptr is a null pointer, the realloc function behaves like the malloc function for the
specified size. Otherwise, if ptr does not match a pointer earlier returned by a memory
management function, or if the space has been deallocated by a call to the free or
realloc function, the behavior is undefined. If memory for the new object cannot be
allocated, the old object is not deallocated and its value is unchanged.