在C printf中无效使用void表达式

Question

I am studying c. I have this struc:

typedef struct mystuff
{
    char* name;
    int      val;
}MyStuff;

In main I call:

MyStuff fruit1 ={"watermellon", 1};

I pass data to insertLifo:

insertLifo(myQueue, &fruit1);

This is the function

bool insertLifo(LifoQueue queue, void* data)

Inside the function I want to print the data in order to debug:

printf("insertLifo()  %s \n", *data);

I get the following error:

MemAlloc.c: In function ‘insertLifo’:
MemAlloc.c:42:32: warning: dereferencing ‘void *’ pointer [enabled by default]
  printf("insertLifo()  %s \n", *data);
                                ^
MemAlloc.c:42:2: error: invalid use of void expression
  printf("insertLifo()  %s \n", *data);
  ^

I have tried:

printf("insertLifo()  %p \n", (void*)*data);

and other ways.

I want to understand fully how to use pointers

Answer 1

A void * is a generic pointer. The type of the data it points to is unknown, therefore you can't dereference a void * .

You should change your function to accept a MyStuff * and modify the printf call to print each field.

bool insertLifo(LifoQueue queue, MyStuff* data)
{
    ...
    printf("insertLifo()  name=%s val=%d \n", data->name, data->val);
    ...

Answer 2

Your mistake is in how you declare insertLifo() , as you declare incorrectly the second parameter. It should be:

bool insertLifo(LifoQueue queue, MyStuff* data)

so you can pass the variable by reference. Then, you have to do:

printf("bla bla %s", data->name);

as the %s format specifier requires to pass a parameter of type char * , as name is.

But, as I told you in a comment to your question, we need a complete and verifiable example to be able to make a diagnostic. You didn't do this, so you get incomplete responses. Please read How to create a Minimal, Complete, and Verifiable example

Another mistake is that probably you need to pass also the first parameter of insertLifo() by reference, as you'll need probably to modify that structure, and if you pass it by value, it will be copied into the function, and any modification you do to it will be lost at function return.

Finally, you say

I want to understand fully how to use pointers.

Well, that's completely out of the scope of this application. StackOverflow is only to help you to correct a small problem in your code, but not to teach you how to program. I can say that many people pass all they lifes trying to understand how to use pointers and never succeed. That depends on many aspects, but a good thing you can do is to buy a good book on C.

Answer 3

In order to print pointer address you shell use %p printf format instead of %s which is for the string.

If you need a to string printf you can use following:

MyStuff *stuff = (MyStuff*)data;
printf("insertLifo() data=%p = {name=%s, val=%d} \n", data, stuff->name, stuff->val);