Question about passing a variable created in a function

Question

Suppose there exists a function which returns a message say of the following format:

struct message
{
void* data;
}msgG;

Which would be the best way to extract the data (i.e. Get the message accessible to fun1 in the code): 1- using a global variable 2- Using double pointers(pointer to a pointer)

//Note: msgG is the global variable

void fun2(struct message **ptr)
{
  **ptr = msgCreate(); // msgCreate returns a type struct message;
  msgG = msgCreate();

}

void fun1()
{
....
.....

struct message *ptr;
ptr = malloc(sizeof(struct message));

fun2(&ptr);
...
}

Now we have the message stored in msgG and ptr ? Which is the better one? Using global variable or accessing the pointer since one is allocated in the heap and the other in the bss(not sure of this)?? Is there any other way to deal with this kind of situation?

Could you give a little more context? I'm not sure I understand enough of the situation to give an opinion. — Juan, Dec 14 '09 at 04:26
@juan: The msgCreate() is usally like a recv() from a netwrking program. I am talkin abt handling a received message . — tomkaith13, Dec 14 '09 at 04:32

score 3 · Accepted Answer · answered Dec 14 '09 at 05:06

Don't use a global variable. What you're trying to do can be done this way:

void fun2(struct message *ptr)
{
    *ptr = msgCreate();
}

void fun1()
{
    struct message *m = malloc(sizeof *m);
    if (m == NULL) {
        /* error handling */
    }
    fun2(m);
}

If struct message is big, consider not having a function returning such a struct. In most of the cases, it is more efficient to return a pointer to some memory than to return a big automatic variable from a function.

score 1 · Answer 2 · answered Dec 14 '09 at 04:27

1

It's good practise to avoid globals.

Note: if you are trying to code object-oriented in C, have a look to this documentation ooc.pdf

answered Dec 14 '09 at 04:27

nico

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Eli Bendersky · Answer 3 · 2009-12-14T15:47:52.310

0

It can be as simple as this:

struct message
{
  void* data;
} msgG;


void fun2(struct message the_msg)
{
  /* access the_msg.data */
}

void fun1()
{
  struct message *ptr;
  ptr = malloc(sizeof(struct message));
  ptr->data = ... /* initialize it to something */

  fun2(*ptr);
}

But this way, fun2 won't be able to manipulate the_msg, because it's passed a copy of the structure by-value. It will be able to manipulate the stuff pointed to by the data pointer inside the_msg, because that's a pointer.

If you want to manipulate the contents of the_msg itself, such as retarget the data pointer, fun2 should accept a pointer to message (a double pointer is unnecessary for this).

And a global variable is almost always a bad solution. Don't use it.

edited Dec 14 '09 at 15:47

answered Dec 14 '09 at 04:25

Eli Bendersky

231,995
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@eliben: So when do we exactly use double pointers. I used to think it is used in these kind of situations – tomkaith13 Dec 14 '09 at 04:35
@tomkaith13: double pointers are used when *the pointer* has to be modified, not its contents. One case is when you want a function to create a new pointer to something, and you pass your own pointer as an argument to it. – Eli Bendersky Dec 14 '09 at 04:52
@tomkaith13: for more on double pointers: http://stackoverflow.com/questions/758673/uses-for-multiple-levels-of-pointer-dereferences, and also http://stackoverflow.com/questions/897366/how-do-pointer-to-pointers-work-in-c – Eli Bendersky Dec 14 '09 at 04:55
@eliben: I'm a bit confused, shouldn't fun2() have parameters that look like `fun2(struct message* the_msg)`. As in it should use the tag name 'message' and not 'msgG' and then it should be a pointer to that type since that's what you passed it with fun2(ptr). -1 until corrected – SiegeX Dec 14 '09 at 05:49

Question about passing a variable created in a function

3 Answers3