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Dynamically Allocated Static Variable

Posted on 2008-06-09
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Last Modified: 2011-10-03
if i have a function with a static local variable, that's dynamically allocated once:
int funct(void)
{
     static char* buffer = NULL;

     if (!buffer)
          buffer = malloc(SIZESLOW);
}

If there's no explicit free call, and if the process is killed; does this constitute as a leak? When the process is killed, is the heap essentially freed? I guess I'm after an in depth explanation on what happens on a windows platform (i imagine unix would be in the same boat)
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Question by:spiffles
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10 Comments
 
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Expert Comment

by:ozo
ID: 21748163
When the process is killed, memory is freed
0
 

Author Comment

by:spiffles
ID: 21748290
And from an efficiency and/or design perspective would you use this kind of allocation?
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Accepted Solution

by:
ozo earned 20 total points
ID: 21748319
If SIZESLOW is unknown until runtime I might use it.
What is the purpose of buffer in your function?
What is the purpose of funct in your program?
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LVL 15

Assisted Solution

by:efn
efn earned 20 total points
ID: 21748337
It's platform-dependent, not guaranteed by the language.  I believe ozo's answer is correct for most, if not all, current operating systems, but I believe it is not true for some old versions of Windows.

From an efficiency perspective, it's fine.  If you have a good reason for allocating the memory dynamically (e.g., SIZESLOW is not known until run time, as ozo said), it's fine.  If you could just use a statically allocated array, that would be simpler and save you from worrying about whether the memory leaks, so I'd consider that preferable, but not by much.
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Expert Comment

by:ozo
ID: 21748385
> not guaranteed by the language
This is true.  The language specs say nothing about what happens after the program exits.
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Author Comment

by:spiffles
ID: 21748554
Yeah I'm doing some code reviews and was worried about a potential leak since there's no explicit free call. By the sounds of it, it's probably safe for the platform it's deployed on.

Closer inspection reveals SIZESLOW has elements which could be updated after a product restart. There's also a comment about dynamic allocation being used to avoid stack overflow. Anyways thanks ozo, efn for the feedback.

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Expert Comment

by:aphillips
ID: 21756236
This is not specified by the language.  However, I have never heard of any compiler that does not release the memory when the program terminates.

Note that the behaviour is implementation specific but depends on the compiler run-time routines (in cooperation with the operating system in some implementations).

However, one important thing to note is that some run-time libraries may report this as a problem.  For example, Microsoft compilers have for some time been reporting this sort of allocation as a "memory leak" in debug builds, although a one-off allocation is not a "leak" more a "splash".  But if you rely on this reporting to find real memory leaks you may want to free the memory when the program terminates.  In C you can do this by registering a function pointer with atexit().
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Author Comment

by:spiffles
ID: 21766960
Yes atexit would be a good alternative, but since there's no precedence in the existing code based, I probably won't use it.
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