How to find the sizeof a pointer pointing to an array

Question

First off  here is some code   int main          int days      1 2 3 4 5       int  ptr   days      printf   u n   sizeof days        printf   u n   sizeof ptr         return 0      Is there a way to find out the size of the array that ptr is pointing to  instead of just giving its size  which is four bytes on a 32-bit system

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No  you can t   The compiler doesn t know what the pointer is pointing to   There are tricks  like ending the array with a known out-of-band value and then counting the size up until that value  but that s not using sizeof       Another trick is the one mentioned by Zan  which is to stash the size somewhere   For example  if you re dynamically allocating the array  allocate a block one int bigger than the one you need  stash the size in the first int  and return ptr 1 as the pointer to the array   When you need the size  decrement the pointer and peek at the stashed value   Just remember to free the whole block starting from the beginning  and not just the array

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In strings there is a   0  character at the end so the length of the string can be gotten using functions like strlen  The problem with an integer array  for example  is that you can t use any value as an end value so one possible solution is to address the array and use as an end value the NULL pointer    include  lt stdio h gt     the following function will produce the warning        sizeof    on array function parameter    a    will    return size of    int       -Wsizeof-array-argument      void foo  int a           printf    lu n   sizeof a         so we have to implement something else one possible    idea is to use the NULL pointer as a control value    the same way   0  is used in strings but this way    the pointer passed to a function should address pointers    so the actual implementation of an array type will    be a pointer to pointer     typedef char   type t     line 18    typedef type t    array t  int main  void         array t initialize  int                initialize an array with four values  foo    bar    baz    foobar         if one wants to use integers rather than strings than in the typedef        declaration at line 18 the char   type should be changed with int        and in the format used for printing the array values         at line 45 and 51   s  should be changed with   i              array t array   initialize  4   foo    bar    baz    foobar          int size  array t           print array size        printf   size  i  n   size  array          void aprint  char    array t           print array values        aprint    s n   array       line 45         type t getval  array t  int           print an indexed value        int i   2      type t val   getval  array  i        printf    i   s n   i  val       line 51         void delete  array t           free some space        delete  array         return 0       the output of the program should be     size 4     foo    bar    baz    foobar    2  baz      include  lt stdarg h gt   include  lt stdlib h gt  array t initialize  int n                 here we store the array values        type t  v    type t    malloc  sizeof  type t     n        va list ap      va start  ap  n        int j      for   j   0  j  lt  n  j             v j    va arg  ap  type t        va end  ap           the actual array will hold the addresses of those        values plus a NULL pointer             array t a    array t  malloc  sizeof  type t        n   1         a n    NULL      for   j   0  j  lt  n  j             a j    v   j      return a    int size  array t a         int n   0      while    a      NULL           n        return n    void aprint  char  fmt  array t a         while    a    NULL           printf  fmt    a           type t getval  array t a  int i         return  a i     void delete  array t a         free   a        free  a

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No  you can t use sizeof ptr  to find the size of array ptr is pointing to   Though allocating extra memory more than the size of array  will be helpful if you want  to store the length in extra space

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int main          int days      1 2 3 4 5       int  ptr   days      printf   u n   sizeof days        printf   u n   sizeof ptr         return 0      Size of days   is 20 which is no of elements   size of it s data type  While the size of pointer is 4 no matter what it is pointing to  Because a pointer points to other element by storing it s address

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For dynamic arrays  malloc or C   new  you need to store the size of the array as mentioned by others or perhaps build an array manager structure which handles add  remove  count  etc  Unfortunately C doesn t do this nearly as well as C   since you basically have to build it for each different array type you are storing which is cumbersome if you have multiple types of arrays that you need to manage   For static arrays  such as the one in your example  there is a common macro used to get the size  but it is not recommended as it does not check if the parameter is really a static array  The macro is used in real code though  e g  in the Linux kernel headers although it may be slightly different than the one below    if  defined ARRAY SIZE       define ARRAY SIZE x   sizeof  x     sizeof  x  0     endif  int main         int days      1 2 3 4 5       int  ptr   days      printf   u n   ARRAY SIZE days        printf   u n   sizeof ptr        return 0      You can google for reasons to be wary of macros like this  Be careful   If possible  the C   stdlib such as vector which is much safer and easier to use

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For dynamic arrays  malloc or C   new  you need to store the size of the array as mentioned by others or perhaps build an array manager structure which handles add  remove  count  etc  Unfortunately C doesn t do this nearly as well as C   since you basically have to build it for each different array type you are storing which is cumbersome if you have multiple types of arrays that you need to manage   For static arrays  such as the one in your example  there is a common macro used to get the size  but it is not recommended as it does not check if the parameter is really a static array  The macro is used in real code though  e g  in the Linux kernel headers although it may be slightly different than the one below    if  defined ARRAY SIZE       define ARRAY SIZE x   sizeof  x     sizeof  x  0     endif  int main         int days      1 2 3 4 5       int  ptr   days      printf   u n   ARRAY SIZE days        printf   u n   sizeof ptr        return 0      You can google for reasons to be wary of macros like this  Be careful   If possible  the C   stdlib such as vector which is much safer and easier to use

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For this specific example  yes  there is  IF you use typedefs  see below   Of course  if you do it this way  you re just as well off to use SIZEOF DAYS  since you know what the pointer is pointing to    If you have a  void    pointer  as is returned by malloc    or the like  then  no  there is no way to determine what data structure the pointer is pointing to and thus  no way to determine its size    include  lt stdio h gt    define NUM DAYS 5 typedef int days t  NUM DAYS     define SIZEOF DAYS   sizeof  days t      int main         days t  days      days t  ptr    amp days        printf   SIZEOF DAYS    u n   SIZEOF DAYS         printf   sizeof days    u n   sizeof days         printf   sizeof  ptr    u n   sizeof  ptr         printf   sizeof ptr     u n   sizeof ptr           return 0       Output   SIZEOF DAYS   20 sizeof days   20 sizeof  ptr   20 sizeof ptr    4

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There is a clean solution with C   templates  without using sizeof    The following getSize   function returns the size of any static array    include  lt cstddef gt   template lt typename T  size t SIZE gt  size t getSize T   amp   SIZE         return SIZE      Here is an example with a foo t structure    include  lt cstddef gt   template lt typename T  size t SIZE gt  size t getSize T   amp   SIZE         return SIZE     struct foo t       int ball      int main         foo t foos3       1   2   3        foo t foos5       1   2   3   4   5        printf   u n   getSize foos3        printf   u n   getSize foos5         return 0      Output   3 5

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In strings there is a   0  character at the end so the length of the string can be gotten using functions like strlen  The problem with an integer array  for example  is that you can t use any value as an end value so one possible solution is to address the array and use as an end value the NULL pointer    include  lt stdio h gt     the following function will produce the warning        sizeof    on array function parameter    a    will    return size of    int       -Wsizeof-array-argument      void foo  int a           printf    lu n   sizeof a         so we have to implement something else one possible    idea is to use the NULL pointer as a control value    the same way   0  is used in strings but this way    the pointer passed to a function should address pointers    so the actual implementation of an array type will    be a pointer to pointer     typedef char   type t     line 18    typedef type t    array t  int main  void         array t initialize  int                initialize an array with four values  foo    bar    baz    foobar         if one wants to use integers rather than strings than in the typedef        declaration at line 18 the char   type should be changed with int        and in the format used for printing the array values         at line 45 and 51   s  should be changed with   i              array t array   initialize  4   foo    bar    baz    foobar          int size  array t           print array size        printf   size  i  n   size  array          void aprint  char    array t           print array values        aprint    s n   array       line 45         type t getval  array t  int           print an indexed value        int i   2      type t val   getval  array  i        printf    i   s n   i  val       line 51         void delete  array t           free some space        delete  array         return 0       the output of the program should be     size 4     foo    bar    baz    foobar    2  baz      include  lt stdarg h gt   include  lt stdlib h gt  array t initialize  int n                 here we store the array values        type t  v    type t    malloc  sizeof  type t     n        va list ap      va start  ap  n        int j      for   j   0  j  lt  n  j             v j    va arg  ap  type t        va end  ap           the actual array will hold the addresses of those        values plus a NULL pointer             array t a    array t  malloc  sizeof  type t        n   1         a n    NULL      for   j   0  j  lt  n  j             a j    v   j      return a    int size  array t a         int n   0      while    a      NULL           n        return n    void aprint  char  fmt  array t a         while    a    NULL           printf  fmt    a           type t getval  array t a  int i         return  a i     void delete  array t a         free   a        free  a

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You can do something like this   int days         length   5    values    1 2 3 4 5    int  ptr   days   1  printf  array length   u n   ptr -1    return 0

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The answer is   No    What C programmers do is store the size of the array somewhere   It can be part of a structure  or the programmer can cheat a bit and malloc   more memory than requested in order to store a length value before the start of the array

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My solution to this problem is to save the length of the array into a struct Array as a meta-information about the array    include  lt stdio h gt   include  lt stdlib h gt   struct Array       int length       double  array      typedef struct Array Array   Array  NewArray int length           Allocate the memory for the struct Array        Array  newArray    Array   malloc sizeof Array            Insert only non-negative length s       newArray- gt length    length  gt  0    length   0       newArray- gt array    double   malloc length sizeof double         return newArray     void SetArray Array  structure int length double  array        structure- gt length   length      structure- gt array   array     void PrintArray Array  structure               if structure- gt length  gt  0                int i          printf  length   d n   structure- gt length           for  i   0  i  lt  structure- gt length  i                printf   g n   structure- gt array i              else         printf  Empty Array  Length 0 n       int main         int i      Array  negativeTest   days   NewArray 5        double moreDays      1 2 3 4 5 6 7 8 9 10        for  i   0  i  lt  days- gt length  i            days- gt array i    i 1       PrintArray days        SetArray days 10 moreDays        PrintArray days        negativeTest   NewArray -5        PrintArray negativeTest        return 0      But you have to care about set the right length of the array you want to store  because the is no way to check this length  like our friends massively explained

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int main          int days      1 2 3 4 5       int  ptr   days      printf   u n   sizeof days        printf   u n   sizeof ptr         return 0      Size of days   is 20 which is no of elements   size of it s data type  While the size of pointer is 4 no matter what it is pointing to  Because a pointer points to other element by storing it s address

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define array size 10   struct        int16 size       int16 array array size        int16 property1  array size 16  1       int16 property2  array size 16  1     array1    array size  0  1  2  3  4  5  6  7  8  9      undef array size   array size is passing to the size variable    define array size 30  struct       int16 size      int16 array array size       int16 property1  array size 16  1      int16 property2  array size 16  1    array2    array size     undef array size   Usage is   void main          int16 size   array1 size      for  int i 0  i  size  i               array1 array i     2

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The answer is   No    What C programmers do is store the size of the array somewhere   It can be part of a structure  or the programmer can cheat a bit and malloc   more memory than requested in order to store a length value before the start of the array

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You can do something like this   int days         length   5    values    1 2 3 4 5    int  ptr   days   1  printf  array length   u n   ptr -1    return 0

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There is a clean solution with C   templates  without using sizeof    The following getSize   function returns the size of any static array    include  lt cstddef gt   template lt typename T  size t SIZE gt  size t getSize T   amp   SIZE         return SIZE      Here is an example with a foo t structure    include  lt cstddef gt   template lt typename T  size t SIZE gt  size t getSize T   amp   SIZE         return SIZE     struct foo t       int ball      int main         foo t foos3       1   2   3        foo t foos5       1   2   3   4   5        printf   u n   getSize foos3        printf   u n   getSize foos5         return 0      Output   3 5

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For dynamic arrays  malloc or C   new  you need to store the size of the array as mentioned by others or perhaps build an array manager structure which handles add  remove  count  etc  Unfortunately C doesn t do this nearly as well as C   since you basically have to build it for each different array type you are storing which is cumbersome if you have multiple types of arrays that you need to manage   For static arrays  such as the one in your example  there is a common macro used to get the size  but it is not recommended as it does not check if the parameter is really a static array  The macro is used in real code though  e g  in the Linux kernel headers although it may be slightly different than the one below    if  defined ARRAY SIZE       define ARRAY SIZE x   sizeof  x     sizeof  x  0     endif  int main         int days      1 2 3 4 5       int  ptr   days      printf   u n   ARRAY SIZE days        printf   u n   sizeof ptr        return 0      You can google for reasons to be wary of macros like this  Be careful   If possible  the C   stdlib such as vector which is much safer and easier to use

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There is no magic solution  C is not a reflective language  Objects don t automatically know what they are   But you have many choices    Obviously  add a parameter Wrap the call in a macro and automatically add a parameter Use a more complex object  Define a structure which contains the dynamic array and also the size of the array  Then  pass the address of the structure

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There is no magic solution  C is not a reflective language  Objects don t automatically know what they are   But you have many choices    Obviously  add a parameter Wrap the call in a macro and automatically add a parameter Use a more complex object  Define a structure which contains the dynamic array and also the size of the array  Then  pass the address of the structure

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As all the correct answers have stated  you cannot get this information from the decayed pointer value of the array alone  If the decayed pointer is the argument received by the function  then the size of the originating array has to be provided in some other way for the function to come to know that size   Here s a suggestion different from what has been provided thus far that will work  Pass a pointer to the array instead  This suggestion is similar to the C   style suggestions  except that C does not support templates or references    define ARRAY SZ 10  void foo  int   arr  ARRAY SZ         printf   u n    unsigned sizeof  arr  sizeof   arr        But  this suggestion is kind of silly for your problem  since the function is defined to know exactly the size of the array that is passed in  hence  there is little need to use sizeof at all on the array   What it does do  though  is offer some type safety  It will prohibit you from passing in an array of an unwanted size   int x 20   int y 10   foo  amp x      error    foo  amp y      ok      If the function is supposed to be able to operate on any size of array  then you will have to provide the size to the function as additional information

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The answer is   No    What C programmers do is store the size of the array somewhere   It can be part of a structure  or the programmer can cheat a bit and malloc   more memory than requested in order to store a length value before the start of the array

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My solution to this problem is to save the length of the array into a struct Array as a meta-information about the array    include  lt stdio h gt   include  lt stdlib h gt   struct Array       int length       double  array      typedef struct Array Array   Array  NewArray int length           Allocate the memory for the struct Array        Array  newArray    Array   malloc sizeof Array            Insert only non-negative length s       newArray- gt length    length  gt  0    length   0       newArray- gt array    double   malloc length sizeof double         return newArray     void SetArray Array  structure int length double  array        structure- gt length   length      structure- gt array   array     void PrintArray Array  structure               if structure- gt length  gt  0                int i          printf  length   d n   structure- gt length           for  i   0  i  lt  structure- gt length  i                printf   g n   structure- gt array i              else         printf  Empty Array  Length 0 n       int main         int i      Array  negativeTest   days   NewArray 5        double moreDays      1 2 3 4 5 6 7 8 9 10        for  i   0  i  lt  days- gt length  i            days- gt array i    i 1       PrintArray days        SetArray days 10 moreDays        PrintArray days        negativeTest   NewArray -5        PrintArray negativeTest        return 0      But you have to care about set the right length of the array you want to store  because the is no way to check this length  like our friends massively explained

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For dynamic arrays  malloc or C   new  you need to store the size of the array as mentioned by others or perhaps build an array manager structure which handles add  remove  count  etc  Unfortunately C doesn t do this nearly as well as C   since you basically have to build it for each different array type you are storing which is cumbersome if you have multiple types of arrays that you need to manage   For static arrays  such as the one in your example  there is a common macro used to get the size  but it is not recommended as it does not check if the parameter is really a static array  The macro is used in real code though  e g  in the Linux kernel headers although it may be slightly different than the one below    if  defined ARRAY SIZE       define ARRAY SIZE x   sizeof  x     sizeof  x  0     endif  int main         int days      1 2 3 4 5       int  ptr   days      printf   u n   ARRAY SIZE days        printf   u n   sizeof ptr        return 0      You can google for reasons to be wary of macros like this  Be careful   If possible  the C   stdlib such as vector which is much safer and easier to use

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The answer is   No    What C programmers do is store the size of the array somewhere   It can be part of a structure  or the programmer can cheat a bit and malloc   more memory than requested in order to store a length value before the start of the array

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As all the correct answers have stated  you cannot get this information from the decayed pointer value of the array alone  If the decayed pointer is the argument received by the function  then the size of the originating array has to be provided in some other way for the function to come to know that size   Here s a suggestion different from what has been provided thus far that will work  Pass a pointer to the array instead  This suggestion is similar to the C   style suggestions  except that C does not support templates or references    define ARRAY SZ 10  void foo  int   arr  ARRAY SZ         printf   u n    unsigned sizeof  arr  sizeof   arr        But  this suggestion is kind of silly for your problem  since the function is defined to know exactly the size of the array that is passed in  hence  there is little need to use sizeof at all on the array   What it does do  though  is offer some type safety  It will prohibit you from passing in an array of an unwanted size   int x 20   int y 10   foo  amp x      error    foo  amp y      ok      If the function is supposed to be able to operate on any size of array  then you will have to provide the size to the function as additional information

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For this specific example  yes  there is  IF you use typedefs  see below   Of course  if you do it this way  you re just as well off to use SIZEOF DAYS  since you know what the pointer is pointing to    If you have a  void    pointer  as is returned by malloc    or the like  then  no  there is no way to determine what data structure the pointer is pointing to and thus  no way to determine its size    include  lt stdio h gt    define NUM DAYS 5 typedef int days t  NUM DAYS     define SIZEOF DAYS   sizeof  days t      int main         days t  days      days t  ptr    amp days        printf   SIZEOF DAYS    u n   SIZEOF DAYS         printf   sizeof days    u n   sizeof days         printf   sizeof  ptr    u n   sizeof  ptr         printf   sizeof ptr     u n   sizeof ptr           return 0       Output   SIZEOF DAYS   20 sizeof days   20 sizeof  ptr   20 sizeof ptr    4

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define array size 10   struct        int16 size       int16 array array size        int16 property1  array size 16  1       int16 property2  array size 16  1     array1    array size  0  1  2  3  4  5  6  7  8  9      undef array size   array size is passing to the size variable    define array size 30  struct       int16 size      int16 array array size       int16 property1  array size 16  1      int16 property2  array size 16  1    array2    array size     undef array size   Usage is   void main          int16 size   array1 size      for  int i 0  i  size  i               array1 array i     2

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No  you can t use sizeof ptr  to find the size of array ptr is pointing to   Though allocating extra memory more than the size of array  will be helpful if you want  to store the length in extra space

[c] How to find the 'sizeof' (a pointer pointing to an array)?

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