Pointer to 2D arrays in C

Question

I know there is several questions about that which gives good  and working  solutions  but none IMHO which says clearly what is the best way to achieve this  So  suppose we have some 2D array    int tab1 100  280     We want to make a pointer that points to this 2D array   To achieve this  we can do    int   pointer  280      pointer creation pointer   tab1    assignation pointer 5  12    517     use int myint   pointer 5  12      use   or  alternatively     int   pointer  100  280      pointer creation pointer    amp tab1    assignation   pointer  5  12    517     use int myint     pointer  5  12      use    OK  both seems to work well  Now I would like to know     what is the best way  the 1st or the 2nd   are both equals for the compiler    speed  perf     is one of these solutions eating more memory than the other   what is the more frequently used by developers

User · Answer

int  pointer 280     Creates 280 pointers of type int   In 32 bit os  4 bytes for each pointer  so 4   280   1120 bytes   int   pointer  100  280      Creates only one pointer which is used to point an array of  100  280  ints   Here only 4 bytes   Coming to your question  int   pointer  280   and int   pointer  100  280   are different though it points to same 2D array of  100  280    Because if int   pointer  280   is incremented  then it will points to next 1D array  but where as int   pointer  100  280   crosses the whole 2D array and points to next byte  Accessing that byte may cause problem if that memory doen t belongs to your process

User · Answer

Both your examples are equivalent   However  the first one is less obvious and more  hacky   while the second one clearly states your intention   int   pointer  280   pointer   tab1    pointer points to an 1D array of 280 integers   In your assignment  you actually assign the first row of tab1   This works since you can implicitly cast arrays to pointers  to the first element    When you are using pointer 5  12   C treats pointer as an array of arrays  pointer 5  is of type int 280    so there is another implicit cast here  at least semantically    In your second example  you explicitly create a pointer to a 2D array   int   pointer  100  280   pointer    amp tab1    The semantics are clearer here   pointer is a 2D array  so you need to access it using   pointer  i  j    Both solutions use the same amount of memory  1 pointer  and will most likely run equally fast   Under the hood  both pointers will even point to the same memory location  the first element of the tab1 array   and it is possible that your compiler will even generate the same code   The first solution is  more advanced  since one needs quite a deep understanding on how arrays and pointers work in C to understand what is going on   The second one is more explicit

User · Answer

Ok  this is actually four different question  I ll address them one by one      are both equals for the compiler   speed  perf       Yes  The pointer dereferenciation and decay from type int     100  280  to int     280  is always a noop to your CPU  I wouldn t put it past a bad compiler to generate bogus code anyways  but a good optimizing compiler should compile both examples to the exact same code      is one of these solutions eating more memory than the other    As a corollary to my first answer  no      what is the more frequently used by developers    Definitely the variant without the extra   pointer  dereferenciation  For C programmers it is second nature to assume that any pointer may actually be a pointer to the first element of an array      what is the best way  the 1st or the 2nd    That depends on what you optimize for    Idiomatic code uses variant 1  The declaration is missing the outer dimension  but all uses are exactly as a C programmer expects them to be  If you want to make it explicit that you are pointing to an array  you can use variant 2  However  many seasoned C programmers will think that there s a third dimension hidden behind the innermost    Having no array dimension there will feel weird to most programmers

User · Answer

defines an array of 280 pointers  1120 or 2240 bytes  int   pointer1  280      defines a pointer  4 or 8 bytes depending on 32 64 bits platform  int   pointer2  280          pointer to an array of 280 integers int   pointer3  100  280     pointer to an 2D array of 100 280 integers   Using pointer2 or pointer3 produce the same binary except manipulations as   pointer2 as pointed out by WhozCraig   I recommend using typedef  producing same binary code as above pointer3   typedef int myType 100  280   myType  pointer3    Note  Since C  11  you can also use keyword using instead of typedef  using myType   int 100  280   myType  pointer3    in your example   myType  pointer                    pointer creation pointer    amp tab1                    assignation   pointer  5  12    517            set  write  int myint     pointer  5  12       get  read    Note  If the array tab1 is used within a function body    this array will be placed within the call stack memory  But the stack size is limited  Using arrays bigger than the free memory stack produces a stack overflow crash   The full snippet is online-compilable at gcc godbolt org  int main           defines an array of 280 pointers  1120 or 2240 bytes      int   pointer1  280       static assert  sizeof pointer1     2240               defines a pointer  4 or 8 bytes depending on 32 64 bits platform      int   pointer2  280          pointer to an array of 280 integers     int   pointer3  100  280     pointer to an 2D array of 100 280 integers       static assert  sizeof pointer2     8            static assert  sizeof pointer3     8                Use  typedef   or  using  if you use a modern C   compiler      typedef int myType 100  280         using myType   int 100  280        int tab1 100  280        myType  pointer                    pointer creation     pointer    amp tab1                    assignation       pointer  5  12    517            set  write      int myint     pointer  5  12       get  read       return myint

[c] Pointer to 2D arrays in C

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