Question

I have a few months that i started programming in C, but I now find myself with a doubt, for example, let see the next example code:

    typedef struct
    {
        char *var1;
    }myFooStruct;

    myFooStruct struct1 [ 200 ];

my doubt is what would I get for **struct1, &struct1, *struct1, struct1, as I passed the struct to a function that takes a two-dimenssion pointer ( **myFooStruct ), I have basic knowledge about pointers 1-but I find myself confused with pointers to structs and 2-how can I modify the struct if I passed it as at parameter to a function

If there is another similar question post it here please, I could not find anything alike, if you know some lecture I could read is welcome too, thank you very much!!

Answer 1

Way 1 Using dynamic memory allocation. Generally used in linked list and all..

If you want to modify the struct in another function. first declare a pointer to a struct.

myFooStruct* struct1;

Allot memory for the struct

struct1 = malloc(sizeof(myFooStruct));

Send the address to the function

func1(struct1);

Receive it and access it to modify in the function.

void func(myFooStruct* struct1)
{
    (*struct1).member1 = ...; // whatever you wanna do
    ...

Way 2

Declare a struct.

myFooStruct struct1;

Send the address of the struct to the function

func1(&struct1);

Receive it and access it to modify in the function.

void func(myFooStruct* struct1)
{
    (*struct1).member1 = ...; // whatever you wanna do
    ...

Answer 2

* is a dereference operator - think of it as meaning "the value contained at location xyz".

& is a reference operator - think of it as meaning "the location in memory of variable xyz".

Accordingly:

myFooStruct struct1 is a physical structure - this is the actual object.

&struct1 is equivalent to the location in memory of struct1 - this is usually an address (like 0xf0004782). You'll usually see this used when passing by reference (see Wikipedia for more info) or when assigning to a pointer (which literally points to a location in memory - get it?).

*struct1 dereferences struct1 - that is, it returns the value contained at location struct1. In the example you give, this is invalid, as struct1 is not a pointer to a location in memory.

**struct1 is tricky - it returns the value contained at the location that is contained within struct1. In other words: struct1 points to a certain location in memory. At that location is the address of another location in memory! Think of it as a scavenger hunt - you go to a location, find a clue, and follow that to another location.

As to how to access structs: think of a struct as a box. When you have the box in front of you, you simply need to open it up and look at what's inside. In C, we do this using the . operator:

char *my_var = struct1.var1

When you don't have the box in front of you - that is, you have a pointer to the struct - you need to access the location the box is at before you can look at what's inside. In C, we have a shortcut for this - the -> operator:

myFooStruct *pointer_to_struct1 = &struct1
char *my_var = pointer_to_struct1->var1
//NOTE: the previous line is equivalent to:
//  char *my_var = (*pointer_to_struct1).var1

Answer 3

If you need to access myFooStruct from function, you can define single pointer: fn( myFooStruct * st ). The you call the function with fn( struct1 ) and change values st[N].var1 = .... Double pointer may be necessary if your object is pointer with allocated memory, not static array as yours.

Answer 4

struct1 is just a table and to be speciffic it's just pointer to a place in the memory.

*struct1 would be thing, that is pointed by struct1, so it's a first struct in a table of structs.

But **struct1 won't be any string. First of all you do not allocate memory for string and second string is member of this struct not struct itself. **struct is undefined behavior, nothing more.

&struct is a pointer to the table, so it's a pointer to the pointer, that points first struct in a table.

You have to decide on your own, what you want. If you want to pass table of your structs then the cleanest way would be:

void function(myFooStruct structTab[]);

Answer 5

1. You should pass a struct pointer to function to access struct inside it .

Declare a struct pointer -

myFooStruct *struct1;

Allocate memory for struct

And pass it to function which is declared as -

 type fn(myFooStruct *struct1){
     .....
     }

Call this function like this -

fn(struct1);

Access struct member like this -struct->member1

2. You can also pass what you have declared right now.

myFooStruct struct1[ 200 ];

define function as -

type fn(myFooStruct struct1[]){
.....
}

Access struct members like this - struct[i].member1.

Struct points as parameter to function C

5 个答案: