struct manipulation

[Darek Adamkiewicz]

Hello folks
I hope code below will explain what I'm trying to achive.
It seems trivial/common but not for me at the moment
Help very apreciated.
Regards
Darek.
-----------------
#include <windows.h>
#include <stdio.h>
#include <tchar.h>

typedef struct Test {
char name[128];
int count;
};

int test(Test *t);

int test(Test *t)
{
Test *loct = t;
t = (Test *)HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof(Test));
strcpy(t->name,"TestEntry");
t->count = 256;
//? so address of t has been changed, is there a way to assign struct
values to old one?

return 0;
}

int main()
{
Test *t;
char out[128];
test(t);
// Is there a way to get values passed to t?
// now of course printf statement doesnt print proper string value
printf("%s",t->name);
return 0;
}

 

[Richard Bos]

#include <windows.h>

This is not an ISO C header, so it's off-topic here.

#include <stdio.h>
#include <tchar.h>
Ditto.

typedef struct Test {
char name[128];
int count;
};

int test(Test *t);

int test(Test *t)
{
Test *loct = t;
t = (Test *)HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof(Test));

Neither HeapAlloc() not GetProcessHeap is an ISO C function, so they're
off-topic here.
Even so, you probably want

t = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof *t);

since casts are rarely necessary for properly declared allocation
functions in C, and sizeof *pointer_i'm_allocating_for means less
maintenance than sizeof (some_type) in case the type of the pointer
changes.

strcpy(t->name,"TestEntry");
t->count = 256;
//? so address of t has been changed, is there a way to assign struct
values to old one?

Sure. You've taken the precaution of saving the old value of t in loct;
since you didn't change it when you changed t, it still points at the
old address.

Note, however, that you never pass the new value of t outside of this
function, and don't free the allocated memory, either; this means that
you have, in all probability, created a memory leak. (I say in all
probability, because, HeapAlloc() not being ISO C, there is no saying
what it might be doing behind the screens - including, for example,
keeping a global record of HeapAlloc()ated pointers. Had you use
malloc(), I'd have been sure that this _is_ a memory leak.)
Note that a pointer is just an object, not something magical: passing
one to a function and then changing its value inside that function will
not change the value in the caller any more than doing the same thing to

int main()
{
Test *t;
char out[128];
test(t);
// Is there a way to get values passed to t?

t is an object; one doesn't "pass" values to objects. One passes values
to functions, and assigns them to objects. But see the FAQ entry above.

Richard

 

[Dan Pop]

I hope code below will explain what I'm trying to achive.

I'm afraid you're overoptimistic. The code below doesn't make any sense.

#include <windows.h>

Undefined behaviour.

#include <stdio.h>
#include <tchar.h>

Undefined behaviour.

typedef struct Test {
char name[128];
int count;
};

int test(Test *t);

Pointless declaration.

int test(Test *t)

Why make test() return int, if it doesn't return anything useful to its
caller?

{
Test *loct = t;
t = (Test *)HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof(Test));

There is no such function in the standard C library. When posting to this

strcpy(t->name,"TestEntry");

Undefined behaviour: strcpy is not a function returning int and you have

t->count = 256;
//? so address of t has been changed, is there a way to assign struct
values to old one?

Nope, the address of t has not changed and it cannot change. If you're
talking about the *value* of t, you have saved the original value in
loct and you can use it.

return 0;
}

Note that the new value assigned to t in test() gets lost at this point.
The object allocated in test() becomes a memory leak, because its address
is lost forever.

int main()
{
Test *t;
char out[128];
test(t);

You have NOT initialised t. What's the point of passing an indeterminate
value to the test() function. If you don't understand what I mean, read
the FAQ asap.

// Is there a way to get values passed to t?

Yes, if t is initialised, test() can store something into the object it
points to. But your test() doesn't even try...

// now of course printf statement doesnt print proper string value

Obviously, because:

1. t was passed to test() uninitialised.

2. test() saved the original value of t, but didn't attempt to do anything
else with it.

printf("%s",t->name);
return 0;
}

You really need to read the chapter dealing with functions in your
favourite C book. You don't seem to understand how function argument
passing works in C. And the way you handle t in main() denotes a lack of
basic understanding about pointer usage, too.

At your level, it makes *absolutely* NO sense to mess with Windows
programming. Restrict yourself to the basic features of the C language.

Dan

 

[Darek Adamkiewicz]

See the FAQ: <http://www.eskimo.com/~scs/C-faq/q4.8.html>.

So while I can do the following (is it correct?)
--------------------------
#include <stdio.h>

void f(int *ip);

void f(int *ip)
{
static int dummy = 5;
*ip = dummy;
}

int main()
{
int *ip;
f(ip);
printf("%d\n", *ip);
}
--------------------
I can't do similar with struct in C? - pass struct pointer to function,
declare local struct, allocate memory, set values to and finally make
that pointer address contain pointer to that struct?

Regards
Darek

 

[Darek Adamkiewicz]

You'll be surprised but I read K&R book and wrote some working till
today C ANSI programs about 3 years ago - It's seems I forgot almost
everything I learned or overrode by perl data manipulation techniques
which allow programmer to do (almost) everything he/she wants. Thanks
anyway - I must do "my homework" again

Regards
Darek

 

[Richard Bos]

So while I can do the following (is it correct?)
--------------------------
#include <stdio.h>

void f(int *ip);

void f(int *ip)
{
static int dummy = 5;
*ip = dummy;
}

int main()
{
int *ip;
f(ip);
printf("%d\n", *ip);
}

That's almost correct. Point ip at a valid int before you pass it to
f(), then it's correct.

I can't do similar with struct in C?

Wrong. This

#include <stdio.h>

struct st {
int i;
};

void f(struct st *stp)
{
struct st dummy={5}
*stp=dummy;
}

int main()
{
struct st temp;
f(&temp);
printf("%d\n", temp.i);
}

is perfectly legal.

However, it isn't what you were doing in the original post. There, the
object you modified in f() was a pointer. Not the struct the pointer
pointed to - the pointer itself.
As the FAQ explains, if you wish to change _anything_ in a function, you
must pass a pointer to it. So if the object you want to change is a
pointer, you must pass a pointer _to_ that pointer. Like this:

#include <stdio.h>

struct st {
int i;
};

void f(struct st **stp)
{
struct st dummy={5}

*stp=malloc(sizeof **stp);
if (*stp)
**stp=dummy;
}

int main()
{
struct st *stp_pointer;
f(&stp_pointer);

if (stp_pointer)
printf("%d\n", stp_pointer->i);
else
puts("Null pointer - allocation failure.");

free(stp_pointer);
}

Richard