Thread: passing structs & pointers to structs as arguments

I'm writing a whole set of functions that needs to edit a huge data structure, and I'm getting a little confused over exactly how some things work:

1) I know things like arrays are passed to functions as pointers, does a struct work the same way? eg if I pass a struct to a function & edit it, do those changes remain after the function terminates?

2)if I want to be able to edit a pointer to a struct (eg free & malloc it to somewhere else) inside a function, then by my understanding I have to pass the function a pointer to a pointer to a struct. this leads to the clunky syntax:

(*struct)->member;

is there any way to reduce the number of pointers to pointers involved here?


thanks
-mark

1) Yes.

Code:

struct foo {
    int bar;
};

void function( struct foo *ptr )
{
    ptr->bar = 5;
}

int main( void )
{
    struct foo instance;

    function( &instance );
    printf("instance->bar = %d\n", instance->bar );
    return 0;
}

2) I'm not sure what you mean exactly. Perhaps...

Code:

struct foo {
    int bar;
};

void function( struct foo **ptrptr )
{
    struct foo *ptr;

    ptr = *ptrptr;
    ...do all you need using ptr...

    ....update ptrptr when finished...
}

Something like that should suffice.

Quzah.

1)not quite what I meant, I know that the code you've given will work fine, I was wondering about something slightly different. Anyway I just tested it myself & it don't work, so nevermind.

2)thanks quzah. a temp pointer (*ptrptr) does clean things up a fair bit..... I've used temp pointers all over the rest of the program, for some reason the idea simply didn't hit me to use it here, till now.


thanks again
-mark

1) I know things like arrays are passed to functions as pointers, does a struct work the same way? eg if I pass a struct to a function & edit it, do those changes remain after the function terminates?

Originally posted by quzah
1) Yes.

Wrong. structs are NOT passed to functions as pointers, they are copied. Any changes made to the structure only change the copy and they will be lost once the function terminates.

Code:

#include <stdio.h>

struct foo {
    int bar;
};

void function( struct foo instance )
{
    instance.bar = 5;
}

int main( void )
{
    struct foo instance;
    instance.bar = 1;
    printf("instance.bar = %d\n", instance.bar); /* 1 */
    function( instance );
    printf("instance.bar = %d\n", instance.bar ); /* still 1 */
    return 0;
}

If you want to edit a structure in a function either pass in a pointer to a structure (explicitly) or return the structure.

Originally posted by major_blagger
Wrong. structs are NOT passed to functions as pointers, they are copied. Any changes made to the structure only change the copy and they will be lost once the function terminates.

You took a literal translation. I provided a way to do what they wanted to.

Quzah.

Originally posted by major_blagger
I took the question he asked. You answered what he didn't ask and already knew.

I misread. I thought he was saying "Is there a way to make it so you can update a structure when passed to a function?".

Quzah.