c - Using Realloc to resize a char ** -

typedef struct element element; struct element{     dado_t str;     elemento* preview;     elemento* next; };  typedef struct lista2 lista2; struct lista2{     elemento* primeiro;     elemento* ultimo;     elemento* corrente; };   void caller(lista2* l){     char *s = l->corrente->str;     char *s2 = l->corrente->next->str;     my_func(&s, &s2); }  void my_func(char **s, char**s2){     size_t len = strlen(*s);     size_t len2 = strlen(*s2);     char *w = *s;     char *tmp = realloc(w, len + len2 + 1); //problem here     if(tmp != null)        *s = tmp;     else         *s = null;     strcat(*s, *s2); }   

when run code(before realloc()):

• *w = "i coffe" memory adress: 0x605050
• *s = "i coffe" memory adress: 0x605050
• l->corrente->str = "i coffe" memory adress: 0x605050

all far.
status after realloc(before assign *s = tmp):

• *w = "" memory adress: 0x605050
• *s = "" memory adress: 0x605050
• l->corrente->str = "" memory adress: 0x605050

still ok, right ? after *s = tmp:

• *w = "" memory adress: 0x605050
• *s = "i coffe" memory adress: 0x605160 changed
• l->corrente->str = "" memory adress: 0x605050

what need:
1) change l->corrente->str value in my_func();
2) or somehow, change *s value new value after strcat. , keep l->corrente->str same.

if understand correctly , want create concatenated value while keeping *s or l->corrente->str same, make more sense have my_func return pointer new concatenated string while keeping both input strings unchanged. if don't understand attempting do, please leave comment.

#include <stdio.h> #include <stdlib.h> #include <string.h>  char *my_func(char *s, char*s2);  int main (void) {      char *a = strdup ("i coffee.");     char *b = strdup ("i tea.");      char *c = my_func (a, b);      printf ("\n a: %s\n b: %s\n c: %s\n\n", a, b, c);      return 0; }  char *my_func(char *s, char*s2) {     size_t len = strlen(s);     size_t len2 = strlen(s2);     char *w = strdup (s);      char *tmp = realloc(w, len + len2 + 1); //problem here      if(!tmp) {         fprintf (stderr, "%s() error: realloc failed.\n", __func__);         return null;     }      w = tmp;     strcat(w, s2);      return w; } 

output

$ ./bin/realloc_post   a: coffee.  b: tea.  c: coffee.i tea. 

---

void - preserving *s, concatenate in *s2

instead of returning pointer, implementation of my_func remains void , takes s , s2, keeping s unchanged, concatenating "ss2" in s2. if misunderstood again, let me know.

#include <stdio.h> #include <stdlib.h> #include <string.h>  void my_func(char **s, char **s2);  int main (void) {      char *a = strdup ("i coffee.");     char *b = strdup ("i tea.");      my_func (&a, &b);      printf ("\n a: %s\n b: %s\n\n", a, b);      free (a);     free (b);      return 0; }  void my_func(char **s, char **s2) {     size_t len = strlen(*s);     size_t len2 = strlen(*s2);     char *w = strdup (*s);     char *p = *s2;          /* save start address free */      char *tmp = realloc(w, len + len2 + 1);      if(!tmp) {         fprintf (stderr, "%s() error: realloc failed.\n", __func__);         return;     }      strcat(tmp, *s2);     *s2 = tmp;     free (p); } 

output

$ ./bin/realloc_post   a: coffee.  b: coffee.i tea.