Using Realloc to resize a char **

Question

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 I run my code(before realloc()):

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

All good so far.
Now status after realloc(Before the assign *s = tmp):

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

Still ok, right ? Now what I get after *s = tmp:

• *w = "" with memory adress: 0x605050
• *s = "I Like Coffe" with memory adress: 0x605160 CHANGED
• l->corrente->str = "" with memory adress: 0x605050

What I need:
1) Change l->corrente->str value in my_func();
2) Or somehow, change *s value to the new value after strcat. And keep l->corrente->str the same.

Answer 1

If I understand you correctly and you want to create a concatenated value while keeping *s or l->corrente->str the same, then it would make more sense to have my_func return a pointer to the new concatenated string while keeping both input strings unchanged. If I don't understand what your are attempting to do, please leave a comment.

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

char *my_func(char *s, char*s2);

int main (void) {

    char *a = strdup ("I like coffee.");
    char *b = strdup ("I like 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: I like coffee.
 b: I like tea.
 c: I like coffee.I like tea.

---

void - preserving *s, concatenate in *s2

Instead of returning a pointer, this implementation of my_func remains void and takes s and s2, keeping s unchanged, but concatenating "ss2" in s2. If I 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 like coffee.");
    char *b = strdup ("I like 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 to 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: I like coffee.
 b: I like coffee.I like tea.