How do pointer-to-pointer s work in C and when might you use them

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How do pointers-to-pointers work in C  When might you use them

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Consider the below figure and program to understand this concept better     As per the figure  ptr1 is a single pointer which is having address of variable num   ptr1    amp num    Similarly ptr2 is a pointer to pointer double pointer  which is having the address of pointer ptr1    ptr2    amp ptr1    A pointer which points to another pointer is known as double pointer  In this example ptr2 is a double pointer   Values from above diagram    Address of variable num has   1000 Address of Pointer ptr1 is  2000 Address of Pointer ptr2 is  3000   Example    include  lt stdio h gt   int main         int  num   10     int   ptr1     int    ptr2         Take the address of var     ptr1    amp num         Take the address of ptr1 using address of operator  amp     ptr2    amp ptr1         Print the value    printf  Value of num    d n   num       printf  Value available at  ptr1    d n    ptr1       printf  Value available at   ptr2    d n     ptr2       Output   Value of num   10 Value available at  ptr1   10 Value available at   ptr2   10

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When covering pointers on a programming course at university  we were given two hints as to how to begin learning about them  The first was to view Pointer Fun With Binky  The second was to think about the Haddocks  Eyes passage from Lewis Carroll s Through the Looking-Glass     You are sad     the Knight said in an anxious tone     Let me sing you a song to comfort you        Is it very long     Alice asked  for she had heard a good deal of poetry that day     It s long     said the Knight     but it s very  very beautiful  Everybody that hears me sing it - either it brings the tears to their eyes  or else -       Or else what     said Alice  for the Knight had made a sudden pause     Or else it doesn t  you know  The name of the song is called    Haddocks  Eyes           Oh  that s the name of the song  is it  quot  Alice said  trying to feel interested     No  you don t understand     the Knight said  looking a little vexed     That s what the name is called  The name really is    The Aged Aged Man           Then I ought to have said    That s what the song is called        Alice corrected herself     No  you oughtn t  that s quite another thing  The song is called    Ways And Means     but that s only what it s called  you know        Well  what is the song  then     said Alice  who was by this time completely bewildered     I was coming to that     the Knight said     The song really is    A-sitting On A Gate     and the tune s my own invention

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it s a pointer to the pointer s address value   that s terrible I know   basically  it lets you pass a pointer to the value of the address of another pointer  so you can modify where another pointer is pointing from a sub function  like   void changeptr int   pp       pp  amp someval

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Pointers to Pointers  Since we can have pointers to int  and pointers to char  and pointers to any structures we ve defined  and in fact pointers to any type in C  it shouldn t come as too much of a surprise that we can have pointers to other pointers

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How do pointers to pointers work in C   First a pointer is a variable  like any other variable  but that holds the address of a variable   A pointer to a pointer is a variable  like any other variable  but that holds the address of a variable   That variable just happens to be a pointer   When would you use them   You can use them when you need to return a pointer to some memory on the heap  but not using the return value    Example   int getValueOf5 int  p       p   5    return 1   success    int get1024HeapMemory int   p       p   malloc 1024     if  p    0      return -1   error   else      return 0   success     And you call it like this   int x  getValueOf5  amp x    I want to fill the int varaible  so I pass it s address in   At this point x holds 5  int  p      get1024HeapMemory  amp p    I want to fill the int  variable  so I pass it s address in   At this point p holds a memory address where 1024 bytes of memory is allocated on the heap   There are other uses too  like the main   argument of every C program has a pointer to a pointer for argv  where each element holds an array of chars that are the command line options   You must be careful though when you use pointers of pointers to point to 2 dimensional arrays  it s better to use a pointer to a 2 dimensional array instead     Why it s dangerous   void test       double   a    int i1   sizeof a 0     i1    4    sizeof double      double matrix ROWS  COLUMNS     int i2   sizeof matrix 0     i2    240    COLUMNS   sizeof double      Here is an example of a pointer to a 2 dimensional array done properly   int   myPointerTo2DimArray  ROWS  COLUMNS    You can t use a pointer to a 2 dimensional array though if you want to support a variable number of elements for the ROWS and COLUMNS   But when you know before hand you would use a 2 dimensional array

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Let s assume an 8 bit computer with 8 bit addresses  and thus only 256 bytes of memory   This is part of that memory  the numbers at the top are the addresses      54   55   56   57   58   59   60   61   62   63   64   65   66   67   68   69  ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----         58             63        55             h    e    l    l    o     0         ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----    What you can see here  is that at address 63 the string  hello  starts  So in this case  if this is the only occurrence of  hello  in memory then   const char  c    hello         defines c to be a pointer to the  read-only  string  hello   and thus contains the value 63  c must itself be stored somewhere  in the example above at location 58  Of course we can not only point to characters  but also to other pointers  E g    const char   cp    amp c    Now cp points to c  that is  it contains the address of c  which is 58   We can go even further  Consider   const char    cpp    amp cp    Now cpp stores the address of cp  So it has value 55  based on the example above   and you guessed it  it is itself stored at address 60     As to why one uses pointers to pointers    The name of an array usually yields the address of its first element  So if the array contains elements of type t  a reference to the array has type t    Now consider an array of arrays of type t  naturally a reference to this 2D array will have type  t       t     and is hence a pointer to a pointer  Even though an array of strings sounds one-dimensional  it is in fact two-dimensional  since strings are character arrays  Hence  char     A function f will need to accept an argument of type t    if it is to alter a variable of type t    Many other reasons that are too numerous to list here

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I like this  quot real world quot  code example of pointer to pointer usage  in Git 2 0  commit 7b1004b   Linus once said   I actually wish more people understood the really core low-level kind of coding  Not big  complex stuff like the lockless name lookup  but simply good use of pointers-to-pointers etc  For example  I ve seen too many people who delete a singly-linked list entry by keeping track of the  quot prev quot  entry  and then to delete the entry  doing something like      if  prev       prev- gt next   entry- gt next     else      list head   entry- gt next   and whenever I see code like that  I just go  quot This person doesn t understand pointers quot   And it s sadly quite common   People who understand pointers just use a  quot pointer to the entry pointer quot   and initialize that with the address of the list head  And then as they traverse the list  they can remove the entry without using any conditionals  by just doing a  pp    entry- gt next   Applying that simplification lets us lose 7 lines from this function even while adding 2 lines of comment  - struct combine diff path  p   pprev   ptmp    struct combine diff path  p    tail    amp curr     Chris points out in the comments to the 2016 video  quot Linus Torvalds s Double Pointer Problem quot    kumar points out in the comments the blog post  quot Linus on Understanding Pointers quot   where Grisha Trubetskoy explains   Imagine you have a linked list defined as     typedef struct list entry          int val         struct list entry  next       list entry   You need to iterate over it from the beginning to end and remove a specific element whose value equals the value of to remove  The more obvious way to do this would be     list entry  entry   head     assuming head exists and is the first entry of the list       list entry  prev   NULL         while  entry       line 4           if  entry- gt val    to remove         this is the one to remove   line 5               if  prev                prev- gt next   entry- gt next     remove the entry   line 7               else                head   entry- gt next          special case - first entry   line 9                  move on to the next entry           prev   entry         entry   entry- gt next        What we are doing above is   iterating over the list until entry is NULL  which means we   ve reached the end of the list  line 4   When we come across an entry we want removed  line 5   we assign the value of current next pointer to the previous one  thus eliminating the current element  line 7    There is a special case above - at the beginning of the iteration there is no previous entry  prev is NULL   and so to remove the first entry in the list you have to modify head itself  line 9   What Linus was saying is that the above code could be simplified by making the previous element a pointer to a pointer rather than just a pointer  The code then looks like this     list entry   pp    amp head     pointer to a pointer       list entry  entry   head      while  entry           if  entry- gt val    to remove              pp   entry- gt next         else             pp    amp entry- gt next         entry   entry- gt next        The above code is very similar to the previous variant  but notice how we no longer need to watch for the special case of the first element of the list  since pp is not NULL at the beginning  Simple and clever  Also  someone in that thread commented that the reason this is better is because  pp   entry- gt next is atomic  It is most certainly NOT atomic  The above expression contains two dereference operators    and - gt   and one assignment  and neither of those three things is atomic  This is a common misconception  but alas pretty much nothing in C should ever be assumed to be atomic  including the    and -- operators

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There so many of the useful explanations  but I didnt found just a short description  so    Basically pointer is address of the variable   Short summary code        int a   p a   declaration of normal variable and int pointer variable      a   56        simply assign value      p a    amp a      save address of  a  to pointer variable       p a   15     override the value of the variable    print 0xfoo and 15    - first is address  2nd is value stored at this address  that is called dereference       printf  pointer p a is having value  d and targeting at variable value  d   p a   p a      Also useful info can be found in topic What means reference and dereference  And I am not so sure  when can be pointers useful  but in common it is necessary to use them when you are doing some manual dynamic memory allocation- malloc  calloc  etc    So I hope it will also helps for clarify the problematic

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A pointer-to-a-pointer is used when a reference to a pointer is required   For example  when you wish to modify the value  address pointed to  of a pointer variable declared in a calling function s scope inside a called function  If you pass a single pointer in as an argument  you will be modifying local copies of the pointer  not the original pointer in the calling scope   With a pointer to a pointer  you modify the latter

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A pointer to a pointer is also called a handle  One usage for it is often when an object can be moved in memory or removed  One is often responsible to lock and unlock the usage of the object so it will not be moved when accessing it  It s often used in memory restricted environment  ie the Palm OS   computer howstuffworks com Link gt  gt  www flippinbits com Link gt  gt

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You have a variable that contains an address of something  That s a pointer   Then you have another variable that contains the address of the first variable  That s a pointer to pointer

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A pointer to pointer is  well  a pointer to pointer   A meaningfull example of someType   is a bidimensional array  you have one array  filled with pointers to other arrays  so when you write  dpointer 5  6   you access at the array that contains pointers to other arrays in his 5th position  get the pointer  let fpointer his name  and then access the 6th element of the array referenced to that array  so  fpointer 6

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A 5-minute video explaining how pointers work

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How it works  It is a variable that can store another pointer   When would you use them   Many uses one of them is if your function wants to construct an array and return it to the caller     returns the array of roll nos  11  12  through paramater    return value is total number of  students int fun  int   i         int  j       i    int  malloc   2 sizeof int           i   11      e g   newly allocated memory 0x2000 store 11     j    i      j         j   12        e g   newly allocated memory 0x2004 store 12      return 2     int main         int  i      int n   fun   amp i       hey I don t know how many students are in your class please send all of their roll numbers      for   int j 0  j lt n  j             printf   roll no    d  n   i j          return 0

[c] How do pointer-to-pointer's work in C? (and when might you use them?)

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