How do we check if a pointer is NULL pointer

Question

I always think simply if p    NULL      will do the job  But after reading this Stack Overflow question  it seems not   So what s the canonical way to check for NULL pointers after absorbing all discussion in that question which says NULL pointers can have non-zero value

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First  to be 100  clear  there is no difference between C and C   here  And second  the Stack Overflow question you cite doesn t talk about null pointers  it introduces invalid pointers  pointers which  at least as far as the standard is concerned  cause undefined behavior just by trying to compare them  There is no way to test in general whether a pointer is valid   In the end  there are three widespread ways to check for a null pointer   if   p    NULL        if   p    0        if   p         All work  regardless of the representation of a null pointer on the machine  And all  in some way or another  are misleading  which one you choose is a question of choosing the least bad  Formally  the first two are indentical for the compiler  the constant NULL or 0 is converted to a null pointer of the type of p  and the results of the conversion are compared to p  Regardless of the representation of a null pointer   The third is slightly different  p is implicitly converted to bool  But the implicit conversion is defined as the results of p    0  so you end up with the same thing   Which means that there s really no valid argument for using the third style mdash it obfuscates with an implicit conversion  without any offsetting benefit    Which one of the first two you prefer is largely a matter of style  perhaps partially dictated by your programming style elsewhere  depending on the idiom involved  one of the lies will be more bothersome than the other  If it were only a question of comparison  I think most people would favor NULL  but in something like f  NULL    the overload which will be chosen is f  int    and not an overload with a pointer  Similarly  if f is a function template  f  NULL   will instantiate the template on int   Of course  some compilers  like g    will generate a warning if NULL is used in a non-pointer context  if you use g    you really should use NULL    In C  11  of course  the preferred idiom is   if   p    nullptr           which avoids most of the problems with the other solutions   But it is not C-compatible -

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Apparently the thread you refer is about C     In C your snippet will always work  I like the simpler if  p

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The compiler must provide a consistent type system  and provide a set of standard conversions  Neither the integer value 0 nor the NULL pointer need to be represented by all-zero bits  but the compiler must take care of converting the  0  token in the input file to the correct representation for integer zero  and the cast to pointer type must convert from integer to pointer representation   The implication of this is that  void  p  memset  amp p  0  sizeof p   if p            is not guaranteed to behave the same on all target systems  as you are making an assumption about the bit pattern here   As an example  I have an embedded platform that has no memory protection  and keeps the interrupt vectors at address 0  so by convention  integers and pointers are XORed with 0x2000000 when converted  which leaves  void   0 pointing at an address that generates a bus error when dereferenced  however testing the pointer with an if statement will return it to integer representation first  which is then all-zeros

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I always think simply if p      NULL      will do the job    It will

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The actual representation of a null pointer is irrelevant here  An integer literal with value zero  including 0 and any valid definition of NULL  can be converted to any pointer type  giving a null pointer  whatever the actual representation  So p    NULL  p    0 and p are all valid tests for a non-null pointer   You might run into problems with non-zero representations of the null pointer if you wrote something twisted like p    reinterpret cast lt void  gt  0   so don t do that   Although I ve just noticed that your question is tagged C as well as C    My answer refers to C    and other languages may be different  Which language are you using

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Well  this question was asked and answered way back in 2011  but there is nullptrin C  11  That s all I m using currently   You can read more from Stack Overflow and also from this article

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Do this  int IS NULL PTR char  k   memset  amp k  k  sizeof k   if k    return 71      return 72    int PRINT PTR SAFE char  KR       char  E KR       if  IS NULL PTR KR   71           printf   s r n  KR         else           printf   s    null  r n            int main int argc char  argv         int i 0      char  A malloc sizeof char  9        strcpy A  hello world        for  i   int  A  -10 i lt 1e 40 i             PRINT PTR SAFE i              Then watch the show    Edit as you wish  Just credit me if you really want more of this

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The representation of pointers is irrelevant to comparing them  since all comparisons in C take place as values not representations  The only way to compare the representation would be something hideous like   static const char ptr rep sizeof ptr      0    if   memcmp  amp ptr  ptr rep  sizeof ptr

[c] How do we check if a pointer is NULL pointer?

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