CODEWITHSUNDEEP
c
Alice
Alice

Reputation: 11

Passing a struct to void* param then copying it to a void*, reconstructing gives garbage value

I am really confused with passing my struct to void pointers, I'm not sure which one can be assigned directly and which one should be memcpyed, I've tried a lot of combinations but it does not seem to work. Any help would be very appreciated!

This is my C code

struct SomeStruct {
  int a;
  char name[10];
};

void *randoms[10];

void transferFunction(void* data, int index) {
    // This function copies data to randoms[index]
    // I would like to have the whole struct's data in randoms[index]
    memcpy(&randoms[index], data, sizeof(struct SomeStruct));
}        

struct SomeStruct *ss = malloc(sizeof(struct SomeStruct)); 
ss->a = 1;
strcpy(ss->name, "abc");
transferFunction(ss, 0);

My goal is to have the randoms[index] having the struct's data as another function is going to read from it, as shown below, but I am unable to retrieve the struct data correctly, it gives me some garbage value

void readFunction() {
     struct *SomeStruct ss = malloc(sizeof(struct SomeStruct)); 
     memcpy(ss, &randoms[index], sizeof(struct SomeStruct));
     printf(ss->name);
}

Does anyone knows how to solve this problem? Thank you very much!!!

Upvotes: 0

Views: 63

Answers (2)

Zakk
Zakk

Reputation: 2063

Your code has some problems:

  • struct *SomeStruct ss = ... should be struct SomeStruct *ss =.
  • You are not cheking the return value of malloc() (which may fail).
  • You are not freeing ss allocated with malloc(). You should call free() on ss.

My goal is to have the randoms[index] having the struct's data

Lev M.'s answer already answers this part.

as another function is going to read from it

Simply assign your void pointer to a SomeStruct pointer:

void readFunction(int index)
{
    if (index >= 10) // Index out of range
        return;
    
    struct SomeStruct *ss = randoms[index];
    printf("%s\n", ss->name);
}

Upvotes: 0

Lev M.
Lev M.

Reputation: 6269

You can not "copy in to a void".

A void * can contain a memory address, but does not contain any information about the size of the data at that address.

Also, it can not contain any data, only an address!

In this line:

void *randoms[10];

You create an array that can hold 10 addresses.

You never initialize this array, so it will start out all zeroes (this only works for global variables in C).

You can put the address of your structure in to the array, like so:

random[0] = (void*)ss;

However, this does not transfer any data, so if you free the original structure (ss) your data is gone, and the address in random[0] is illegal.

If you want to transfer data you need to create array of struct SomeStruct or you need to allocate another SomeStruct, store its address in random[0] then memcpy to that address.

void transferFunction(void* data, int size, int index)
{
    randoms[index] = malloc(size);
    if (randoms[index] != NULL) {
        memcpy(randoms[index], data, size);
    }
}

Upvotes: 2

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