How can I assign the address of a pointer to an already-existing variable? (C)-CodePudding

In short: I am trying to assign the address of pt (pointer to struct) to &text (declaration of struct). Only errors: error: lvalue required as left operand of assignment.

I am trying to store a struct in dynamic memory. I succeeded in a previous mini-project, but I used pointer-to-struct, and passed this pointer to all my functions. Now I am burning to know if I could simply omit passing the pointer and pass the variable struct itself into the function.

My current example

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

typedef struct s_text t_text;

typedef struct s_text
{
    int     letters;
// some more stuff
} t_text;


int main(void)
{
    t_text text;
    t_text *tp;

    tp = malloc(sizeof(t_text));
    //&text = tp; <-- this here I tried
    return (0);
}

In the above code I allocate memory on the heap for the tp. This is the memory I'd like to use. Now, on the stack, memory was reserved for (t_text) text. I would like to discard this and only use the heap.

t_text &text = malloc(sizeof(t_text)); <-- this may work in C , i don't know, but in C definitely not.

In another post's discussion on NULL pointers, someone claimed in C that the address of a variable could point to NULL with the following code int &x = *(int*)0; but this definitely is not appreciated by my compiler. In fact, I tried several things with the address of a variable, but each time I try to set eg &text = (some address) this error pops up:
error: lvalue required as left operand of assignment.

(link to the post I refered to: ttps://stackoverflow.com/questions/57483/what-are-the-differences-between-a-pointer-variable-and-a-reference-variable/57492#57492 )

Below what I tried earlier and what worked:

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

typedef struct s_text t_text;

void    fn_promt_user(t_text *tp);
void    fn_calc_letters(t_text *tp);

typedef struct s_text
{
    int     letters;
// some more stuff
} t_text;


int main(void)
{
    t_text *tp;

    tp = malloc(sizeof(t_text));
    fn_promt_user(tp);
    fn_calc_letters(tp);
    return (0);
}

To conclude this post with my question: Is there a way I can pass a struct variable as an argument to a function, or should I just accept passing pointer-to-struct is the one and only way to go?

Thanks!

CodePudding user response：

You can pass a struct to a function, like ....

int myfunc(t_text mytext) {....}
then ...
t_text thistext;
...
myfunc(thistext);

and this puts the entire struct onto the stack for the subroutine to use. but the C language has no 'ref' feature like C .

You can ...

tp = (t_text *)malloc(sizeof(t_text));
myfunc(*tp);

Your second example, passing pointers to objects, is a very conventional means of using structs in C. It has the advantage of not needlessly copying structs to the stack, merely pointers. It has the disadvantage of allowing functions to modify the objects that are pointed to. The latter problem can be remedied by declaring that the argument points to a const struct. Like:

void fn_promt_user(const t_text *tp) {...}