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Questions about releasing memory...

This is a discussion on Questions about releasing memory... within the C Programming forums, part of the General Programming Boards category; Hi there, I have three questions. 1. What is the difference between using: Code: const char varchar[] = "this is ...

  1. #1
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    Questions about releasing memory...

    Hi there,

    I have three questions.

    1. What is the difference between using:

    Code:
    const char varchar[] = "this is an example";
    and

    Code:
    const char * varchar = "this is an example";
    In both cases do I need to use the free function to release memory? I mean,

    Code:
    free(varchar);
    2. Do I need to release memory if I define this line?

    Code:
    int array[4]={1,2,3,4};
    then,

    Code:
    free(array);
    3. What if I wanted to release a variable of type int from the memory? For instance,

    Code:
    int varint = 4;
    Thank you!

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    You don't need to, and should not free memory in any of these cases.

  3. #3
    - - - - - - - - oogabooga's Avatar
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    This
    Code:
    const char varchar[] = "this is an example";
    allocates space for the length of the string literal (plus one) to the varchar array and then copies the string literal into that space.
    This
    Code:
    const char * varchar = "this is an example";
    only allocates space for a pointer-to-char and sets that pointer to the address of the string literal.

    You only call free() to free memory that you've dynamically allocated (with malloc or calloc, for example). If you define the variables you've shown inside a function, then they are automatically allocated in the stack frame for that function and automatically deallocated when the function ends. If you define them globally (or statically in a function) then they are automatically allocated when the program starts and automatically deallocated when the program ends. (Actually, all memory that a program uses should be automatically deallocated when the program ends, even if it was dynamically allocated.)
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  4. #4
    ATH0 quzah's Avatar
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    Quote Originally Posted by oogabooga View Post
    This
    Code:
    const char * varchar = "this is an example";
    only allocates space for a pointer-to-char and sets that pointer to the address of the string literal.
    It should be noted here that the string literal is actually allocated as well. The thing about string literals though is that this:
    Code:
    char *p = "hello";
    char *q = "hello";
    These may or may not actually be pointing to the same spot in memory.


    Quzah.
    Last edited by quzah; 04-07-2012 at 10:14 AM.
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    Dear Subsonics,


    So this means that I only need to release memory when it was allocated dinamically?


    Thank you for reply.

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    Dear oogabooga,

    Thank you very much for reply. Everything is clear now!

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    Dear quzah,

    Thanks for the clarification.

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    Quote Originally Posted by adarpodracir View Post
    Dear Subsonics,


    So this means that I only need to release memory when it was allocated dinamically?


    Thank you for reply.
    Yes, only when you explicitly ask for a chunk of memory (malloc() etc.) or are using a function that does (strdup() for example).

  9. #9
    Algorithm Dissector iMalc's Avatar
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    You only use free for something that you allocated using malloc, calloc, or realloc.
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  10. #10
    spurious conceit MK27's Avatar
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    Quote Originally Posted by iMalc View Post
    You only use free for something that you allocated using malloc, calloc, or realloc.
    No, as Subsonics mentions WRT strdup, it is not unusual for library functions to return pointers which also need to be freed (which is why you should pay attention to API documentation).
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
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  11. #11
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    Quote Originally Posted by MK27 View Post
    No, as Subsonics mentions WRT strdup, it is not unusual for library functions to return pointers which also need to be freed (which is why you should pay attention to API documentation).
    That is because such library functions, directly or indirectly, allocate memory using malloc(), calloc(), or realloc().

    So iMalc is correct. As is Subsonics, with the caveat that strdup() is also not standard C. Despite the fact that, on systems that support it, it is often placed in the <string.h> header.
    Right 98% of the time, and don't care about the other 3%.

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