C struct to void* pointer

Question

I have a struct defined as:

typedef struct {
   int type;
   void* info;
} Data;

and then i have several other structs that i want to assign to the void* using the following function:

Data* insert_data(int t, void* s)
{
    Data * d = (Data*)malloc(sizeof(Data));
    d->type = t;
    d->info = s;

    return d;
}

struct {
   ...
} Struct#;

then i just call

insert_data(1, variable_of_type_Struct#);

When i compile this it gives a warning

warning: assignment from incompatible pointer type

i tried to cast the variable in the insert to (void*) but didn't work

insert_data(1, (void *) variable_of_type_Struct#);

How can i get rid of this warning?

Thanks

Casting the result of `malloc` [is unnecessary](http://stackoverflow.com/q/605845/512904). `Data * d = malloc(sizeof(Data));` would suffice. — Matheus Moreira, Apr 25 '12 at 15:58
I have feeling these sharp '#' characters are in your code as well. They are connected with macros, not sure what are they doing in your code. — Tomas Pruzina, Apr 25 '12 at 16:05
Why was this struct {...} Struct#; in your code? I'm just curious. — octopusgrabbus, Apr 25 '12 at 17:22

score 15 · Accepted Answer · answered Apr 25 '12 at 15:49

15

Pass in the address of the struct, not a copy of it (i.e. not passed by value):

insert_data(1, &variable_of_type_Struct);

answered Apr 25 '12 at 15:49

hmjd

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tried that. insert_data(1, (void *) &variable_of_type_Struct#); didn't work either – Nitrate Apr 25 '12 at 15:50
what type is your variable? btw no need to cast here. Provede relevant types, we are no magicians to see whats hidden behind your typedefs. Answer is valid though. – Tomas Pruzina Apr 25 '12 at 15:56
1

@Nitrate where you screwed up? – Tomas Pruzina Apr 25 '12 at 16:05
@hmjd don't trust Ideone. I have a very similar problem on gcc but my code actually compiles without warnings on Ideone. – mfloris Mar 08 '17 at 12:58

score 5 · Answer 2 · answered Apr 25 '12 at 15:49

5

Pass a pointer to the struct object:

struct your_struct_type bla;

insert_data(1, &bla);

answered Apr 25 '12 at 15:49

ouah

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Sangeeth Saravanaraj · Answer 3 · 2012-04-25T16:28:48.957

4

Hope this program helps!

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

typedef struct {
   int type;
   void* info;
} Data;

typedef struct {
    int i;
    char a;
    float f;
    double d;  
}info;

Data* insert_data(int t, void* s)
{
    Data * d = (Data*)malloc(sizeof(Data));
    d->type = t;
    d->info = s;

    return d;
}

int main()
{
    info in; 
    Data * d;
    d = insert_data(10, &in);

    return 0;
}

edited Apr 25 '12 at 16:28

answered Apr 25 '12 at 15:53

Sangeeth Saravanaraj

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that is what i have. struct { ... } Struct#; then i just call insert_data(1, variable_of_type_Struct#); which is a Struct# *variable – Nitrate Apr 25 '12 at 15:55
Say you are having `struct { ... }`, then you have to call `insert_data(int, struct *)` and not `insert_data(int, struct)`. Pass the address of the structure. – Sangeeth Saravanaraj Apr 25 '12 at 15:57
2

this is nice but you work with uninitialised data here. also ignoring any kind of checks (malloc, frees). – Tomas Pruzina Apr 25 '12 at 15:58
@AoeAoe Agree. I was trying to demonstrate how to send a address of a struct instead of struct. – Sangeeth Saravanaraj Apr 25 '12 at 16:02
@SangeethSaravanaraj you sent a copy of a pointer to struct imo. to send adress of struct, he'd need _&struct_ . – Tomas Pruzina Apr 25 '12 at 16:24
@SangeethSaravanaraj how to chk that the d have info struct in the program ? – Man Man Yu Feb 07 '22 at 05:04

score 2 · Answer 4 · answered Apr 25 '12 at 16:04

I'm not quite sure what this was:

struct {
   ...
} Struct#;

So, I cleaned up your program a little bit and got no warnings, after putting the address of the struct into the call, insert_data(1, &variable_of_type_Struct);

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

typedef struct {
    int type;
    void* info;
} Data;

Data* insert_data(int t, void* s);

Data variable_of_type_Struct;

Data* insert_data(int t, void* s)
{
    Data * d = (Data*)malloc(sizeof(Data));
    d->type = t;
    d->info = s;

    return d;
}

void test()
{
    insert_data(1, &variable_of_type_Struct);
}

score 0 · Answer 5 · answered Apr 25 '12 at 16:07

0

insert_data waits for a void*, you put a Data.

insert_data(1, &variable_of_type_Struct#);

It miss a level of indirection.

answered Apr 25 '12 at 16:07

md5

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C struct to void* pointer

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