Allocate space for struct pointer in subfunction - c

Question

How can I allocate memory for a struct pointer and assign value to it's member in a subfunction?
The following code will compile but not execute:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct _struct {char *str;};
void allocate_and_initialize(struct _struct *s)
{
s = calloc(sizeof(struct _struct), 1);
s->str = calloc(sizeof(char), 12);
strcpy(s->str, "hello world");
}
int main(void)
{
struct _struct *s;
allocate_and_initialize(s);
printf("%s\n", s->str);
return 0;
}

Answer 1

You are passing s by value. The value of s is unchanged in main after the call to allocate_and_initialize
To fix this you must somehow ensure that the s in main points to the memory chunk allocated by the function. This can be done by passing the address of s to the function:
// s is now pointer to a pointer to struct.
void allocate_and_initialize(struct _struct **s)
{
*s = calloc(sizeof(struct _struct), 1);
(*s)->str = calloc(sizeof(char), 12);
strcpy((*s)->str, "hello world");
}
int main(void)
{
struct _struct *s = NULL; // good practice to make it null ptr.
allocate_and_initialize(&s); // pass address of s.
printf("%s\n", s->str);
return 0;
}
Alternatively you can return the address of the chunk allocated in the function back and assign it to s in main as suggested in other answer.

Answer 2

In your example:
void allocate_and_initialize(struct _struct *s)
{
s = calloc(sizeof(struct _struct), 1);
s->str = calloc(sizeof(char), 12);
strcpy(s->str, "hello world");
}
Assigning to s here doesn't change s in the caller. Why not return it instead?
struct _struct *allocate_and_initialize(void) {
struct _struct *s;
s = calloc(sizeof *s, 1);
s->str = calloc(1, 12); /* sizeof(char) is always 1 */
strcpy(s->str, "hello world");
return s;
}
and use it thus:
struct _struct *s;
s = allocate_and_initialize();
/* use s... */
free(s); /* don't forget to free the memory when you're done */

Answer 3

you must change your code like that:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct _struct {char *str;};
void allocate_and_initialize(struct _struct **s)
{
*s = (_struct*)calloc(sizeof(struct _struct), 1);
(*s)->str = (char*)calloc(sizeof(char), 12);
strcpy((*s)->str, "hello world");
}
int main(void)
{
struct _struct *s;
allocate_and_initialize(&s);
printf("%s\n", s->str);
return 0;
}
The reason is, that you change the adress of the pointer, but not the "content" of the pointer. So, if you code in c, you have to use a "double" pointer. If you code in c++ you can use a reference.

Answer 4

You can create struct object then pass its address to subfunction, then assign values in subfunction by creating pointer. The exact code is,
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct _struct {char *str;};
void allocate_and_initialize(struct _struct *s)
{
s -> str = malloc(12);
strcpy(s->str, "hello world");
}
void main(void)
{
struct _struct _struct;
allocate_and_initialize(&_struct);
printf("%s\n", _struct.str);
}