How to deallocate space with free.

This is a discussion on How to deallocate space with free. within the C Programming forums, part of the General Programming Boards category; Why my last printf does not appear when i run the program? Code: #include <stdio.h> #include <stdlib.h> int main(void){ char ...

  1. #1
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    How to deallocate space with free.

    Why my last printf does not appear when i run the program?

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    int main(void){
    	char *buf;
    		buf=(char *)malloc(50*sizeof(char));
    		if(buf == NULL){
    			printf("Not enought memory to allocate buffer.\n");
    			exit(1);
    		}
    			printf("String was successfully allocated.\n");
    			
    		free(buf);
    		if(buf == NULL){
    			printf("Successfull free.\n");
    		}
    		
    return 0;
    }

  2. #2
    Woof, woof! zacs7's Avatar
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    free() does not set the pointer to NULL (which it couldn't do anyway) -- you pass the pointer by value to free().

    There is not standard way to check if free "succeeded". Nor is there any reason to, if it fails... not your problem :-)

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    Because free is not supposed to set the pointer to null (actually, since you're passing the value of the pointer to the function, it's actually impossible for it to set your copy to null, but that's another issue). All it does is deallocate the memory. When you allocated it, it set it aside so that only you could use it - no one else could allocate the same block of memory. When you free it, it's no longer set aside. Your pointer hasn't changed, but you no longer have any guarantee that the memory isn't already in use.

  4. #4
    Registered User Char*Pntr's Avatar
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    Smile

    Once you free the buff, whatever it was pointing to is gone forever....

    So I think after free(buff) now it points to nothing.

    Nothing will be printed in the last statement, unless by coincidence buff is
    pointing at a NULL. Then you'll get the last printf() statement.

    I'm sure you'll get a lot more expert details soon. :-)

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    thank you all. from now on i' ll just free the space and don' t care further...about what exactly is happening

  6. #6
    Registered User Char*Pntr's Avatar
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    Cool

    Let's just imagine, for a second, that free() did return a numeric value if pass/fail. Let's say NULL
    is a fail.

    Then I would write the code this way:

    Code:
    int test;
    
    test = free(buf);
    
    if(test != NULL) {
    
    printf("Successfull free.\n");
    
    return (whatever)
    }
    Last edited by Char*Pntr; 08-24-2010 at 10:10 PM.

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    Smile

    Or this one...
    Code:
    if(free(buf) != NULL) {
    
    printf("Successfull free.\n");
    
    return (whatever)
    }
    i hope i am not wrong! thank you...

  8. #8
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    i hope i am not wrong!
    I don't know why Char*Pntr posted the code he did, because it's based on pretending that free() returns a value. free() does not return a value, and provides no way for you to check if the call was successful or failed. In most situations, you don't really care if free failed or succeeded. If free() is failing often enough for you to care, you should really consider getting a new operating system.

  9. #9
    Registered User Char*Pntr's Avatar
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    Question

    Code:
    Quote: Let's just imagine, for a second, that free() did return a numeric value if pass/fail. Let's say NULL is a fail.
    I was trying to explain the fact that the last printout statement would most likely
    never be printed because of the construct of the if() statement even if free() did return a value.
    I thought I made that perfectly clear... I think most people (brack) understood my point.

    So next time if I want to describe a hypothetical argument, I will use "Let's pretend..." instead
    of "Let's imagine..." so that everyone here will understand. :-)
    Last edited by Char*Pntr; 08-25-2010 at 02:53 PM.

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    Quote Originally Posted by brack View Post
    thank you all. from now on i' ll just free the space and don' t care further...about what exactly is happening
    Just set the pointer to NULL after you free to save yourself a lot of headaches.

  11. #11
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    Quote Originally Posted by brack View Post
    Why my last printf does not appear when i run the program?
    Code:
    free(buf);
    if(buf == NULL){
    printf("Successfull free.\n");
    As the others have pointed out free doesn't return a value, you have to trust it.
    If you are working on complex code and want a confirmation that free was called you can do this...

    Code:
    // release memory
    int Release(void* ptr)
      { free(ptr);
         ptr = NULL;
         return 1; }
    
    // main code
    if (Release(MyMem))
      printf(....
    Last edited by CommonTater; 08-26-2010 at 10:58 AM. Reason: sloppy proof reading

  12. #12
    and the hat of wrongness Salem's Avatar
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    > If you are working on complex code and want a confirmation that free was called you can do this...
    Did you test it?
    All you managed was to make your local variable (ptr) NULL, not the value of MyMem in main()
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  13. #13
    Registered User Char*Pntr's Avatar
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    Thumbs up

    Brack, I think your post was excellent and I've learned a lot from the replies. You have intellectual curiosity, and like me, dare to experiment with the code we learn.

    A few weeks ago when the book that I was using covered malloc() and the free() functions. I tried to do something similar: I allocated some memory to store and print out "this is a test message." It worked fine. Then I wondered, what if I don't use the free(). That string must still exist somewhere in memory... would I be able to go back to verify this? So I created a pointer and initialized it to the string right after malloc().

    However, later in main(), I was unsuccessful printing out the original string. I did not invoke free(), so I would think that it still exists somewhere. I'm 3/4 of the way through my C book, so when I finish it, I will revisit this question.... and if it still wont work, I'll post a question.

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    Quote Originally Posted by Char*Pntr View Post
    However, later in main(), I was unsuccessful printing out the original string. I did not invoke free(), so I would think that it still exists somewhere. I'm 3/4 of the way through my C book, so when I finish it, I will revisit this question.... and if it still wont work, I'll post a question.
    Could you post what you've tried? Because of course it still exists in memory until you free it. But how did you try to find it back? The normal way is to keep a pointer to the allocated memory (in fact, that's the only way except maybe in the case of exploits that sometimes use "hacky" methods to find an address).

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    The point is that the data may still exist in memory, even after you free it - but that memory is (hopefully), listed back in the pool of available memory. The operating system may use it in a micro-second, or it may not use it for years on end, if it's just idling away.
    It's entirely up to the OS.

    There is nothing preventing a smart compiler from freeing memory that you have not, if it determines that the memory in question, is never accessed again by your program.
    Last edited by Adak; 08-26-2010 at 03:30 PM.

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