malloc doesn't do anything?

This is a discussion on malloc doesn't do anything? within the C Programming forums, part of the General Programming Boards category; I just recently started programming in C. I'm running into problems with Malloc. It seems like malloc doesn't do anything ...

  1. #1
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    Question malloc doesn't do anything?

    I just recently started programming in C.

    I'm running into problems with Malloc. It seems like malloc doesn't do anything with my string?

    I have the following code:

    Code:
      char *s;
      s = malloc( 5 * sizeof(char));
      strcpy(s, "123451234567890876432123456789678");
     or   s = "123456678";
      printf("String is: %s", s);
    Why can I put a string in that is larger dan 5 bytes?
    I've tried many different ways and everytime I reserve a few bytes I can make the string larger.

    Maybe i'm using things wrong but i would appreciate it if someone can explain to me why this works...

  2. #2
    Nub SWE
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    Malloc guarantees you, in this case, 5 bytes of contiguous memory. There might be more available after your desired block, or there might be garbage which will crash your program when you try to access it. Malloc just returns a pointer to the start of a block of memory that will fit what you want it to. It might be more, but it will never be less.

  3. #3
    C++ Witch laserlight's Avatar
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    Why can I put a string in that is larger dan 5 bytes?
    Luck.

    Incidentally, s = "12345678" changes what s points to.
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    Ok, so it's just plain luck now...
    Can I determine the size of the newly allocated block (after malloc)?

    e.g. sizeof(s) or strlen(s) ? Will it return the allocated space?
    When I use these functions now they always return 4 bytes even after mallocing 50

  5. #5
    Nub SWE
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    You've defined it, so you already know it. It's 5 bytes.

    Doing anything with the data that you "luckily" have access to is dangerous. Don't do it.

    If you need more memory than five bytes, allocate more than five bytes.

  6. #6
    C++ Witch laserlight's Avatar
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    When I use these functions now they always return 4 bytes even after mallocing 50
    You're probably taking sizeof(s) which returns the size of the pointer s, not the size of what was allocated.

    You've defined it, so you already know it. It's 5 bytes.
    Yep. Also, note that sizeof(char) is guaranteed to be 1.
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  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  8. #8
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    Thanks that makes it a lot less confusing...

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