Question

Let’s say I have a struct:

typedef struct {
    int number1;  /* dummy */
    int number2;  /* dummy */
    int number3;  /* dummy */
    char *name1;
    char name2[];
} Klass;

and the rest of the code is:

int main(int argc, char const *argv[])
{
    char *name1 = "this is a name";            /* 1st case */
    char name2[] = "this is also a name";      /* 2nd case */

    Klass k;
    k.number1 = 10;
    k.number2 = 20;
    k.number3 = 30;
    k.name1 = "this is my first name";         /* 3rd case */

    /* error: invalid use of flexible array member */
    k.name2 = "this is my second name";

    Klass *kp = (Klass*)malloc(sizeof(Klass));
    kp->number1 = 100;
    kp->number2 = 200;
    kp->number3 = 300;
    kp->name1 = "this is also my first name";  /* 4th case */

    /* error: invalid use of flexible array member */
    kp->name2 = "this is my second name";

    return 0;
}

1. Can anyone clarify for me how the memory is allocated (heap vs stack) in the marked cases?
   
2. How should I free memory at the end of main block?
   
   
3. What’s the reason compiler is giving the error: invalid use of flexible array member?

EDIT
If you say k.name = "this is my name"; and kp->name = "this is also my name"; is on the stack, can you explain how I can reach "this is my name" like this:

Klass *kp;

int foo() {
    Klass k;
    k.number1 = 10;
    k.number2 = 20;
    k.number3 = 30;
    k.name = "this is my name";
    kp = &k;
} // k is destroyed now

int main(int argc, char const *argv[])
{
    kp = (Klass*)malloc(sizeof(Klass));
    foo();
    printf("%d\n", kp->number1); /* segfault */
    printf("%d\n", kp->number2); /* segfault */
    printf("%d\n", kp->number3); /* segfault */
    printf("%s\n", kp->name);    /* prints "this is my name" */
    return 0;
}

Answer 1

char *name1 = "this is a name";            /* 1st case */

This just allocates a pointer, setting it to point to the string (which is kept as static data).

char name2[] = "this is also a name";      /* 2nd case */

This is short for char name2[sizeof(init_string)] = "this is also a name"; and allocates enough space to store the characters in the string.

Your third case

char name2[];

allocates no space at all! There is nowhere to store a string (which would have had to be copied using strcpy anyway).

The 4th case

kp->name1 = "this is also my first name";  /* 4th case */ 

is similar to case 1 – name1 is a pointer that is set to point to a static text.