Pointer String Leaks

This is a discussion on Pointer String Leaks within the C Programming forums, part of the General Programming Boards category; I'm curious how code such as Code: const char * szSomeString = "Hello World"; will fair against memory leaks? This ...

  1. #1
    Registered User valaris's Avatar
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    Pointer String Leaks

    I'm curious how code such as

    Code:
    const char * szSomeString = "Hello World";
    will fair against memory leaks? This syntax allocates some memory on the heap and returns a pointer to it correct? However I cannot free() it when I am done with it or it will error. What is the proper way to dispose of elements like this, or are they not a leak?

    Cheers

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    Registered User carrotcake1029's Avatar
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    IIRC, that is allocated from the stack. You need to free things allocated with malloc() and any of its derivatives.

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    You don't free it, as CC mentioned. It will be returned to free memory, when the program stops.

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    Registered User valaris's Avatar
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    Hmmm
    The pointer is allocated on the stack for sure, but the characters too? I had read those are written to static memory(the heap?) and then the pointer is returned. If the characters were copied onto the stack then how would this work?
    Code:
    char * Test()
    {
    	char * test = "Hi Friend";
    	return test;
    }
    
    ...
    
    printf ("%s\n", Test());

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    cph
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    Quote Originally Posted by valaris View Post
    Hmmm
    The pointer is allocated on the stack for sure, but the characters too? I had read those are written to static memory(the heap?) and then the pointer is returned. If the characters were copied onto the stack then how would this work?
    Code:
    char * Test()
    {
    	char * test = "Hi Friend";
    	return test;
    }
    
    ...
    
    printf ("%s\n", Test());
    you'll probably get an undefined behavior if you do that.
    I suggest you do this
    Code:
    char *Test ()
    {
        char *test = "Hi Friend";
    
        return(strcpy(calloc(strlen(test) + 1, sizeof(char)), test));
    }

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    Registered User valaris's Avatar
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    Hmm well on my compiler it works...msvc...and i'm n ot sure why if the above descriptions are indeed correct.

  7. #7
    Algorithm Dissector iMalc's Avatar
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    String literals are neither dynamically allocated, nor created on the stack. They are typically just part of the applications data segment that has been loaded into RAM (hence the reason they are really const).

    For all intents and purposes though you can ignore where they exist. They just exist somewhere for the lifetime of the program. Your code doesn't play any part in the string literal coming into or going out of existence.
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    Registered User valaris's Avatar
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    Quote Originally Posted by iMalc View Post
    String literals are neither dynamically allocated, nor created on the stack. They are typically just part of the applications data segment that has been loaded into RAM (hence the reason they are really const).

    For all intents and purposes though you can ignore where they exist. They just exist somewhere for the lifetime of the program. Your code doesn't play any part in the string literal coming into or going out of existence.
    Awesome! And thankyou. That makes much more sense...And would explain why I can return pointers to them that I create in a function and don't need to worry about their memory management.

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    Recently we had some similar discussion, and yes, string literals (anything "<stuff here>") are guaranteed to exist for the duration of the execution [1]. So there is no need to copy strings like cph suggests, for example.


    [1] Ok, not entirely true. In various modern operating systems, such as Linux or Windows, sometimes have "initializing" sections of code. These are removed at the beginning of execution. Obviously, strings stored in such initializing sections will be removed without the compilers knowledge - and as such, are undefined behaviour if they are used after the removal [most likely causing a crash, as we are talking about kernel memory that is removed from the address map, so it would be "not mapped into memory"]. The purpose of this is to allow the OS to remove code & data that is only used during startup. Since many kernel drivers and may have fairly large amounts of "initialization code", it is worth going through this extra step of removing that when the driver has been initialized.

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    Registered User valaris's Avatar
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    http://msdn.microsoft.com/en-us/magazine/cc301805.aspx

    So string literal data is usually placed in the .rdata section? (Read - Only Data). And basically global to the entire process?

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    Quote Originally Posted by valaris View Post
    http://msdn.microsoft.com/en-us/magazine/cc301805.aspx

    So string literal data is usually placed in the .rdata section? (Read - Only Data). And basically global to the entire process?
    Correct. And because the section is read-only, any attempt o modify the data will fail.

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