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Reason for Segmentation fault


 
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# 15  
Old 12-06-2007
What's wrong with this then... (rhetorical)

Code:
char *get_string(void)
{
char c='a';
char *p=&c;

      return p;
}

# 16  
Old 12-06-2007
Quote:
Originally Posted by porter
What's wrong with this then... (rhetorical)

Code:
char *get_string(void)
{
char c='a';
char *p=&c;

      return p;
}

Forgive me, I may be stating the obvious here

p (as in the pointer by itself and not the value being pointed to) is not assigned
the address of c, when in fact it should be.

Ok, so is there something your trying to teach me here ?

Last edited by JamesGoh; 12-07-2007 at 11:24 PM..
# 17  
Old 12-07-2007
Porter

Before my last post, I didnt make an effort to actually test your code segment out for myself and see what was going on. so my bad on that part

However, I did run it just then and it turned out ok. I even ran the debugger and found out that the memory addressess of c and *p were idential after char *p=&c;

Heres the code I typed in (inc. line numbers)

Code:
18      char c= 'a';
19      char *ptr = &c;
20      return ptr;

Heres the gdb output at line 20

Code:
20      return ptr;

(gdb) print &c
$3 = 0xbffff6b7 "aq\203\004\b\2008\001@\024Wl\003@\001"

(gdb) print ptr
$4 = 0xbffff6b7 "aq\203\004\b\2008\001@\024Wl\003@\001"

So if your code suggestion works fine, then what is wrong with it ?? (Maybe something I missed ?)
# 18  
Old 12-09-2007
Quote:
Originally Posted by JamesGoh
Ok, so is there something your trying to teach me here ?
Yes, although the code compiles the pointer is to a local variable that will evaporate when the function returns, hence you are returning a pointer to bogus memory. This is where you need to use static or heap memory.

The lesson is you need to be careful how you pass pointers to local variables.
# 19  
Old 12-09-2007
I will look into it myself further

Thanks heaps so far for your insights. Im really learning something

Last edited by JamesGoh; 12-11-2007 at 07:48 AM..
# 20  
Old 12-13-2007
After some testing, Ive reached a conclusion that it comes down to a matter of where you are creating the pointers from

You can get away with

Code:
char c='a';
char *ptr=&c;

in main() because both c and ptr will exist as long as the program is being run

If you create pointers elsewhere and wish to return them to main(), the safest and most appropriate way to create them is thru the use of the heap (even though for some odd reason, the code segment above created no dramas when placed in a local func that returned a char ptr)

Thoughts, comments ??
# 21  
Old 12-13-2007
Quote:
Originally Posted by JamesGoh
(even though for some odd reason, the code segment above created no dramas when placed in a local func that returned a char ptr)
The pointer may be pointing to some memory that has not been trashed yet, but is not safe.

Memory in global data is permenant, it is forever for the life of the program.

Memory on the stack is only for the life of the function in which it is declared.

Memory on the heap is dynamic, long last and can be allocated when needed and free, main issue is time it takes to allocate and free, a performance thing, and also fragmentation.

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