How to get the real and total length of char char array

Question

For a char     I can easily get its length by   char a      aaaaa   int length   sizeof a  sizeof char      length 6   However  I cannot do like this to get the length of a char   by   char  a   new char 10   int length   sizeof a  sizeof char     because  I know  a here is a pointer  such that length here will be always be 4  or something other in different systems    My question is that how can I get the length of a char   afterwards  I know someone may challenge me that you already know its 10 because you just created it  I want to know this because this step of getting its length may come long long way from its creation and I don t want to come long long way back to check this number  Moreover  I also want to know its real length   To be more specific   how can I get its real length 5  how can I get its total length 10    for the following example   char  a   new char 10    strcpy a   hello

User · Answer

You could try this   int lengthChar const char  chararray       int n   0     while chararray n       0        n        return n

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You can make a back-tracker character  ex  you could append any special character say     to the end of your string and then check the occurrence of that character  But this is a very risky way as that character can be in other places also in the char   char  stringVar   new char 4     stringVar 0     H     stringVar 1     E     stringVar 2             back-tracker character  int i   0   while 1       if  stringVar i               break       i            i is the length of the string     you need to make sure  that there is no other   in the char     Otherwise define a custom structure to keep track of length and allocate memory

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You can implement your own new and delete functions  as well as an additional get-size function    define CEIL DIV x y     x -1   y  1   void  my new int size        if  size  gt  0                int  ptr   new int 1 CEIL DIV size sizeof int             if  ptr                        ptr 0    size              return ptr 1                      return 0     void my delete void  mem        int  ptr    int  mem-1      delete ptr     int my size void  mem        int  ptr    int  mem-1      return ptr 0       Alternatively  you can override the new and delete operators in a similar manner

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In C      Just use std  vector lt char gt  which keep the  dynamic  size for you   Bonus  memory management for free    Or std  array lt char  10 gt  which keep the  static  size    In pure C    Create a structure to keep the info  something like   typedef struct       char  ptr      int size    my array   my array malloc array int size        my array res      res ptr    char   malloc size       res size   size      return res     void free array my array array        free array ptr

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If the char   is 0-terminated  you can use strlen  Otherwise  there is no way to determine that information

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char  a   new char 10        My question is that how can I get the length of a char     It is very simply    It is enough to add only one statement  size t N   10  char  a   new char N     Now you can get the size of the allocated array  std  cout  lt  lt   The size is    lt  lt  N  lt  lt  std  endl    Many mentioned here C standard function std  strlen  But it does not return the actual size of a character array  It returns only the size of stored string literal   The difference is the following  if to take your code snippet as an example  char a      aaaaa   int length   sizeof a  sizeof char      length 6   then std  strlen  a   will return 5 instead of 6 as in your code   So the conclusion is simple  if you need to dynamically allocate a character array consider usage of class std  string  It has methof size and its synonym length that allows to get the size of the array at any time   For example  std  string s   aaaaa      std  cout  lt  lt  s length    lt  lt  std  endl    or  std  string s  s resize  10     std  cout  lt  lt  s length    lt  lt  std  endl

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Strlen command is working for me   You can try following code      char  s  unsigned int  strLength strlen s

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There are only two ways    If the memory pointer to by your char   represents a C string  that is  it contains characters that have a 0-byte to mark its end   you can use strlen a   Otherwise  you need to store the length somewhere  Actually  the pointer only points to one char  But we can treat it as if it points to the first element of an array  Since the  length  of that array isn t known you need to store that information somewhere

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when new allocates an array  depending on the compiler  i use gnu c     the word in front of the array contains information about the number of bytes allocated   The test code    include  lt stdio h gt   include  lt stdlib h gt   int main          int arraySz      char  a      unsigned int  q       for  arraySz   5  arraySz  lt   64  arraySz               printf    02d -    arraySz            a   new char arraySz           unsigned char  p    unsigned char    a           q    unsigned int     a - 4           printf    02d n     q             delete    a              on my machine dumps out   05 - 19 06 - 19 07 - 19 08 - 19 09 - 19 10 - 19 11 - 19 12 - 19 13 - 27 14 - 27 15 - 27 16 - 27 17 - 27 18 - 27 19 - 27 20 - 27 21 - 35 22 - 35 23 - 35 24 - 35 25 - 35 26 - 35 27 - 35 28 - 35 29 - 43 30 - 43 31 - 43 32 - 43 33 - 43 34 - 43 35 - 43 36 - 43 37 - 51 38 - 51 39 - 51 40 - 51 41 - 51 42 - 51 43 - 51 44 - 51 45 - 59 46 - 59 47 - 59 48 - 59 49 - 59 50 - 59 51 - 59 52 - 59 53 - 67 54 - 67 55 - 67 56 - 67 57 - 67 58 - 67 59 - 67 60 - 67 61 - 75 62 - 75 63 - 75 64 - 75   I would not recommend this solution  vector is better   but if you are really desperate  you could find a relationship and be able to conclude the number of bytes allocated from the heap

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This may sound Evil    and I haven t tested it  but how about initializing all values in an array at allocation to   0  and then using strlen     This would give you your so-called real value since it would stop counting at the first   0  it encounters   Well  now that I think about it though  please don t Ever    do this  Unless  you want to land in a pile of dirty memory   Also  for the allocated memory or the total memory you may use the following functions if your environment provides them    msize    malloc usable size

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So the thing with the sizeof operator is that it returns you the amount of storage needed  in bytes  to store the operand    The amount of storage needed to store a char is always 1 byte  So the sizeof char  will always return 1   char a      aaaaa    int len1   sizeof a  sizeof char      length   6 int len2   sizeof a                   length   6    This is the same for both len1 and len2 because this division of 1 does not influence the equation    The reason why both len1 and len2 carry the value 6 has to do with the string termination char   0   Which is also a char which adds another char to the length  Therefore your length is going to be 6 instead of the 5 you were expecting    char  a   new char 10   int length   sizeof a  sizeof char     You already mentioned that the length turns out to be 4 here  which is correct  Again  the sizeof operator returns the storage amount for the operand and in your case it is a pointer a  A pointer requires 4 bytes of storage and therefore the length is 4 in this case  Since you probably compile it to a 32-bit binary  If you d created a 64-bit binary the outcome would be 8   This explanation might be here already be here  Just want to share my two cents

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Legit question  I personally think people confuse pointers with arrays as a result of character pointers  char    which serve almost the same purpose as character arrays  char    X    This means that pointers and arrays are not the same  so pointers of course don t contain a specific size  only an address if I could say so  But nonetheless you can try something similar to strlen   int ssize const char  s        for  int i   0    i            if  s i     0              return i       return 0

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Given just the pointer  you can t  You ll have to keep hold of the length you passed to new   or  better  use std  vector to both keep track of the length  and release the memory when you ve finished with it   Note  this answer only addresses C    not C

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You can t  Not with 100  accuracy  anyway  The pointer has no length size but its own  All it does is point to a particular place in memory that holds a char  If that char is part of a string  then you can use strlen to determine what chars follow the one currently being pointed to  but that doesn t mean the array in your case is that big  Basically   A pointer is not an array  so it doesn t need to know what the size of the array is  A pointer can point to a single value  so a pointer can exist without there even being an array  It doesn t even care where the memory it points to is situated  Read only  heap or stack    doesn t matter   A pointer doesn t have a length other than itself  A pointer just is    Consider this   char beep     a   void alert user const char  msg  char  signal     for some reason alert user  Hear my super-awsome noise     amp beep     passing pointer to single char   void alert user const char  msg  char  signal        printf   s c n   msg   signal       A pointer can be a single char  as well as the beginning  end or middle of an array    Think of chars as structs  You sometimes allocate a single struct on the heap  That  too  creates a pointer without an array   Using only a pointer  to determine how big an array it is pointing to is impossible  The closest you can get to it is using calloc and counting the number of consecutive  0 chars you can find through the pointer  Of course  that doesn t work once you ve assigned reassigned stuff to that array s keys and it also fails if the memory just outside of the array happens to hold  0  too  So using this method is unreliable  dangerous and just generally silly  Don t  Do  It   Another analogy  Think of a pointer as a road sign  it points to Town X  The sign doesn t know what that town looks like  and it doesn t know or care  or can care  who lives there  It s job is to tell you where to find Town X  It can only tell you how far that town is  but not how big it is  That information is deemed irrelevant for road-signs  That s something that you can only find out by looking at the town itself  not at the road-signs that are pointing you in its direction  So  using a pointer the only thing you can do is   char a str      hello     h e l l o  0  char  arr ptr    amp a str 0   printf  Get length of string - gt   d n   strlen arr ptr      But this  of course  only works if the array string is  0-terminated   As an aside   int length   sizeof a  sizeof char    sizeof char is guaranteed 1  so sizeof a  is enough   is actually assigning size t  the return type of sizeof  to an int  best write   size t length   sizeof a  sizeof  a    best use ptr s type - gt  good habit   Since size t is an unsigned type  if sizeof returns bigger values  the value of length might be something you didn t expect

User · Answer

You can find the length of a char  string like this   char  mystring    Hello World   int length   sprintf mystring    s   mystring     sprintf   prints mystring onto itself  and returns the number of characters printed

[c++] How to get the real and total length of char * (char array)?

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