auto allocation

8 posts / 0 new
Log in or register to post comments
Last post
Mon, 2012-01-23 20:27
#1
JosDuchIt
JosDuchIt's picture
Offline
Last seen: 8 years 1 week ago
Joined: 2012-01-08 20:57
auto allocation

Since in the code below struct fulist gets allocated its memory automatically,
(144 bytes) as the Printf confirms, why do i have a crash on the 2d line of main ?

  1. struct fulist {    
  2.  char	curdir[140];		
  3.  LONG	dirtime;		
  4.  
  5. }*fls;
  6.  
  7. int main(void)  ///char *dirname)
  8. {
  9. 	Printf("size fulist %ld\n", sizeof(struct fulist));
  10. 	fls->curdir[0] = 0; //crash
  11. }
Top
Mon, 2012-01-23 21:03
billyfish
billyfish's picture
Offline
Last seen: 3 years 10 months ago
Joined: 2012-01-23 20:45
The Printf doesn't confirm

The Printf doesn't confirm you've allocate the space, it simply calculates the size of the structure. You'll need to use malloc, AllocMem, etc. to get it to point at accessible memory e.g.

  1.   if (fls = (struct fulist *) AllocVec (sizeof (struct fulist)))
  2.     {
  3.       fls->curdir[0] = 0; 
  4.  
  5.       FreeVec (fls);
  6.     }
Top
Mon, 2012-01-23 21:18
(Reply to #2)
JosDuchIt
JosDuchIt's picture
Offline
Last seen: 8 years 1 week ago
Joined: 2012-01-08 20:57
@billyfish Must i understand

@billyfish
Must i understand that structures can never be seean as variables that are auro allocated when entering a bloc or declared as extern? That was what i was trying to test.

Top
Mon, 2012-01-23 21:24
(Reply to #3)
billyfish
billyfish's picture
Offline
Last seen: 3 years 10 months ago
Joined: 2012-01-23 20:45
If you want to allocate it

If you want to allocate it from the stack then you'd have something like the code below. Notice that fls is no longer a pointer and you'd access the elements using . rather than ->

  1. struct fulist 
  2. {
  3. ...
  4. }; 
  5.  
  6. int main (void)
  7. {
  8.  struct fulist fls;
  9. }
Top
Tue, 2012-01-24 21:48
(Reply to #4)
JosDuchIt
JosDuchIt's picture
Offline
Last seen: 8 years 1 week ago
Joined: 2012-01-08 20:57
@billyfish thanks i go things

@billyfish
thanks i go things mixed up.
But why do i get a crash here

  1. #include <exec/types.h>
  2. #include <proto/exec.h>
  3. #include <exec/exec.h>  
  4. #include <dos/dos.h>
  5. #include <dos/datetime.h>
  6. #include <proto/dos.h>
  7.  
  8. struct GCmain    ///def/glob_GCmain.h
  9. {	
  10. 	//UBYTE dateformat;					// default = 3 (dd-mm-yy)
  11. 	uint8 dateformat;
  12. 	char  something[2000];
  13. };
  14.  
  15. int main(void)
  16. {
  17. 	struct GCmain  *gd;
  18. 	gd->dateformat = FORMAT_CDN; //3; crashes
  19. 	IDOS->Printf("OK\n");
  20. 	return 1 ;
  21. }

I do see the OK being printed though

Top
Tue, 2012-01-24 22:25
(Reply to #5)
Belxjander
Belxjander's picture
Offline
Last seen: 8 years 11 months ago
Joined: 2011-02-25 11:53
is FORMAT_CDN #define'd as a

is FORMAT_CDN #define'd as a direct object itself or the pointer to a static instance of the object?

when you put "struct name {};" you are only declaring a class "class"

change "struct GCMain {} gd;" and make reference to the "gd" global structure
using dynamic constructs inside it.

otherwise using "struct GCMain *gd" anywhere in code is asking for a pointer which
does not point to anything initially
(you trigger the access violation as you don't say *what* to point to that I can see)

so either fill the pointer in by Allocation of a Memory block to contain the structure,
or use the above change I suggested to have it allocated when the program is loaded into memory
as part of the .data or .bss sections of your program (I don't know specifically where it is).

Top
Wed, 2012-01-25 17:18
(Reply to #6)
OldFart
OldFart's picture
Offline
Last seen: 3 months 2 weeks ago
Joined: 2010-11-30 14:09
@JosDuchIt This

@JosDuchIt

This line:

  1. 	struct GCmain  *gd;

tells the whole story: it is a pointer to an area which, when existed, would be treated as a structure of type GCmain. The point being that that area does not exist anywhere, so the pointer is probably pointing to NULL, which will lead to a crash. QED.

Allocating the struct on the stack:

  1.         struct GCmain GD; // Now the structure is allocated some space
  2. 	struct GCmain  *gd = &GD; // And now the pointer points to the address of that space

You might also write it like this:

  1.   struct GCmain *gd = (struct GCmain *)IExec->AllocVecTags(sizeof(struct GCmain), AVT_ClearWithValue, 0, TAG_END);
  2.  
  3.   if (gd)
  4.    {
  5.     gd->dateformat = FORMAT_CDN; //3; NO LONGER crashes
  6.     IDOS->Printf("OK\n");
  7.  
  8.      IExec->FreeVec(gd);
  9.    }
  10.   else
  11.    {
  12.      IDOS->Printf("ERROR: Failed to Allocate Memory (struct GCmain)\n");
  13.     }

N.b.: a macro that I use for allocating structures is this one:

  1. #define ALLOCATE_STRUCT(StrucType) (StrucType *)IExec->AllocVecTags(sizeof(StrucType), AVT_ClearWithValue, 0, TAG_END)

I think it makes code a bit more readable.

The example above is then rewritten like this:

  1.   struct GCmain *gd = ALLOCATE_STRUCT(struct GCmain);
  2.  
  3.   if (gd)
  4.    {
  5.      gd->dateformat = FORMAT_CDN; //3; NO LONGER crashes
  6.      IDOS->Printf("OK\n");
  7.  
  8.      IExec->FreeVec(gd);
  9.    }
  10.   else
  11.    {
  12.      IDOS->Printf("ERROR: Failed to Allocate Memory (struct GCmain)\n");
  13.     }

OldFart

Top
Wed, 2012-01-25 20:15
(Reply to #7)
JosDuchIt
JosDuchIt's picture
Offline
Last seen: 8 years 1 week ago
Joined: 2012-01-08 20:57
@Belxjander thanks for

@Belxjander thanks for expanations and @OldFart for the very helpfull examples.

I do see now that i did copy wrongly the example given by BillyFish.w


AllocVecTags(sizeof(StrucType), AVT_ClearWithValue, 0, TAG_END)

I am glad to see this form. Till now i was convinced you had to use the AVT_Type tag too.

Top
Log in or register to post comments