Re: malloc questions in C
- From: Michael Mair <Michael.Mair@xxxxxxxxxxxxxxx>
- Date: Sun, 22 Jan 2006 09:31:11 +0100
questions? wrote:
Keith Thompson wrote:
"questions?" <universal_used@xxxxxxxxxxx> writes:
Ico wrote:
questions? <universal_used@xxxxxxxxxxx> wrote:
if I use malloc() in a function to allocate a space for my array of structures. I didn't free() them anywhere in the program. I found that I can still use that space after I come back to main().
That is how it is supposed to work. When you allocate memory with malloc() or one of the similar functions, it will be available until you call free(). A lot of C implementations will also free this memory for you when your program exits, but don't count on that.
So, with malloc() function, it is static regardless in the sense that whether you are in main or in a function, it is permanant,right?
"Static" isn't the right word for it.
The lifetime of a memory block allocated with malloc() doesn't depend on where you call it.
There are three storage durations: "static" (for static or global objects; these exist for the entire lifetime of the program), "automatic" (for non-static objects declared locally to a function or block; these cease to exist when the function or block finishes), and "allocated" (for objects allocated by malloc(); these exist until they're free()d).
Note that malloc() and free() aren't the only relevant functions, but I'm too lazy to go into the details of calloc() and realloc().
And main() *is* a function. An object declared inside main() isn't global; it's local to the function.
Thank you all for the answer!!! It is really really helpful.
1) what is really a memory leak?
Ask wikipedia or the jargon file http://en.wikipedia.org/wiki/Memory_leak http://www.catb.org/jargon/html/M/memory-leak.html
2) if I forgot to free a memory from malloc(), after termination of my program. It will be returned back to system,right?
Yes, if your system does this. And as long as the usage of your code does not change. Imagine that you wrote a module and a "test" programme for it. #include "myhappyfunmodule.h"
void test (void)
{
/* use myhappyfunmodule stuff */
/* a) myhappyfunmodule has an internal memory leak */
/* b) myhappyfunmodule returns allocated storage and
** test() does not free() it */
}
int main (void)
{test();
return 0; } Now, one year after having written it, you want to use myhappyfunmodule for a programme which runs for hours and days in a loop. You thoughtlessly copy "test", throw out main(), rename test() to runhappyfun(), change a couple of things and try it out. runhappyfun() is called millions of times and leaks a couple of bytes every time; this effectively becomes a problem over time as the programme eats your resources. You may have to terminate it early/it dies by itself/it takes down your system and valuable data. Now, you have a look into runhappyfun(), find b), remove it. Seems to work. a) still leaks a few bytes at a time but maybe not always. Then comes the one time it really counts and, sadly, a) eats your memory once again and does Bad Things. So, you have to debug your old code, find a) and fix it. Maybe you even knew about the leaks when writing myhappyfunmodule but thought "test() is only a test function, I do not want to bother with deallocation" and "well, in a very special situation, we have a leak (a)) but preventing this costs ten lines extra which I always can do later", respectively... If you do it right from the beginning, you can save yourself trouble further down the road.
Cheers Michael -- E-Mail: Mine is an /at/ gmx /dot/ de address. .
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- Re: malloc questions in C
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- Re: malloc questions in C
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- Re: malloc questions in C
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