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.