NULL-pointers

This is a discussion on NULL-pointers within the C Programming forums, part of the General Programming Boards category; Hey! I have just malloc'd a chunk of memory for a struct with various members. Some of those members is ...

  1. #1
    Registered User Drogin's Avatar
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    NULL-pointers

    Hey!
    I have just malloc'd a chunk of memory for a struct with various members.
    Some of those members is pointers.

    I want to check if the pointer isnt given a value by doing so:
    Code:
    if(structPtr->ptr == NULL) ...;
    Is it enough to zero-fill the memory, and assume that a zerofilling means the ptr will be NULL?

    In other words, is it safe to assume that NULL == 0(or something similar, like (void *) 0)?
    Or can NULL be implemented to be -1 or something like that?
    Last edited by Drogin; 10-11-2009 at 09:49 AM.

  2. #2
    spurious conceit MK27's Avatar
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    Quote Originally Posted by Drogin View Post
    Is it enough to zero-fill the memory, and assume that a zerofilling means the ptr will be NULL?
    Yes. As you know, you cannot do this:
    Code:
    typedef struct {
    	char *ptr = NULL;
    } this;
    But this will set eg.ptr to NULL:
    Code:
    this eg = {0};

    Beware King Mir's caveat tho.
    Last edited by MK27; 10-11-2009 at 10:33 AM.
    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
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  3. #3
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    Quote Originally Posted by Drogin View Post
    Hey!
    I have just malloc'd a chunk of memory for a struct with various members.
    Some of those members is pointers.

    I want to check if the pointer isnt given a value by doing so:
    Code:
    if(structPtr->ptr == NULL) ...;
    Is it enough to zero-fill the memory, and assume that a zerofilling means the ptr will be NULL?

    In other words, is it safe to assume that NULL == 0(or something similar, like (void *) 0)?
    Or can NULL be implemented to be -1 or something like that?
    NULL can theoretically be implemented as something like -1, so using memset or similar functions will technically not guarantee null initialization. However, if you explicitly set the pointers to 0, then it is guaranteed to work, because a 0 in a pointer context will always be the same as NULL.

    Code:
    So initializing a struct with {0} will always work.
    Practically I've never heard of a compiler that does not implement NULL as 0. Checking for 0 is such a common operation even not counting pointers, that architectures tend to make a separate optimized instruction. Checking for -1 is not so common.

    The same rule applies to floating point numbers.
    Last edited by King Mir; 10-11-2009 at 10:26 AM.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

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