malloc an array of struct pointers

Question

I have the following struct   typedef struct  chess      int   array     int size     struct  chess  parent    chess    and I have   typedef struct  chess  Chess    Now  I want to create an array of dynamic length to store pointers to the chess struct so I do the following   Chess array      malloc size   sizeof Chess      This gives me an error  invalid initializer   And if I drop the    and do this   Chess array   malloc size   sizeof Chess      it compiles without error but when I try to set an element of this array to NULL by doing   array i  NULL    I get an error  incompatible types when assigning to type    struct  chess    from type    void       Any idea what am I doing wrong  Thanks

User · Accepted Answer

array is a slightly misleading name  For a dynamically allocated array of pointers  malloc will return a pointer to a block of memory  You need to use Chess  and not Chess   to hold the pointer to your array   Chess  array   malloc size   sizeof Chess    array i    NULL    and perhaps later      create new struct chess    array i    malloc sizeof struct chess        set up its members    array i - gt size   0     etc

User · Answer

IMHO  this looks better   Chess  array   malloc size   sizeof Chess       array of pointers of size  size   for   int i  0  i  lt  SOME VALUE    i         array i     Chess  malloc sizeof Chess

User · Answer

I agree with  maverik above  I prefer not to hide the details with a typedef   Especially when you are trying to understand what is going on   I also prefer to see everything instead of a partial code snippet   With that said  here is a malloc and free of a complex structure   The code uses the ms visual studio leak detector so you can experiment with the potential leaks    include  stdafx h    include  lt string h gt   include  msc-lzw h    define  CRTDBG MAP ALLOC  include  lt stdlib h gt   include  lt crtdbg h gt         32-bit version int hash fun unsigned int key  int try num  int max        return  key   try num    max      the hash fun returns a number bounded by the number of slots         this hash table has    key is int    value is char buffer struct key value pair       int key     use this field as the key     char  pValue      use this field to store a variable length string      struct hash table       int max      int number of elements      struct key value pair   elements      This is an array of pointers to mystruct objects      int hash insert struct key value pair  data  struct hash table  hash table         int try num  hash      int max number of retries   hash table- gt max        if  hash table- gt number of elements  gt   hash table- gt max            return 0     FULL            for  try num   0  try num  lt  max number of retries  try num               hash   hash fun data- gt key  try num  hash table- gt max            if  NULL    hash table- gt elements hash        an unallocated slot             hash table- gt elements hash    data              hash table- gt number of elements                return RC OK                      return RC ERROR         returns the corresponding key value pair struct    If a value is not found  it returns null       32-bit version struct key value pair  hash retrieve unsigned int key  struct hash table  hash table         unsigned int try num  hash      unsigned int max number of retries   hash table- gt max       for  try num   0  try num  lt  max number of retries  try num               hash   hash fun key  try num  hash table- gt max            if  hash table- gt elements hash     0                return NULL     Nothing found                    if  hash table- gt elements hash - gt key    key                return hash table- gt elements hash                       return NULL         Returns the number of keys in the dictionary    The list of keys in the dictionary is returned as a parameter   It will need to be freed afterwards int keys struct hash table  pHashTable  int   ppKeys         int num keys   0        ppKeys    int    malloc  pHashTable- gt number of elements   sizeof int          for  int i   0  i  lt  pHashTable- gt max  i              if  NULL    pHashTable- gt elements i                   ppKeys  num keys    pHashTable- gt elements i - gt key              num keys                        return num keys        The dictionary will need to be freed afterwards int allocate the dictionary struct hash table  pHashTable             Allocate the hash table slots     pHashTable- gt elements    struct key value pair     malloc pHashTable- gt max   sizeof struct key value pair        allocate max number of key value pair entries     for  int i   0  i  lt  pHashTable- gt max  i              pHashTable- gt elements i    NULL                  alloc all the slots       struct key value pair  pa slot        for  int i   0  i  lt  pHashTable- gt max  i                 all that he could see was babylon         pa slot    struct key value pair     malloc sizeof struct key value pair            if  NULL    pa slot                printf  alloc of slot failed n                while  1                     pHashTable- gt elements i    pa slot          pHashTable- gt elements i - gt key   0               return RC OK         This will make a dictionary entry where      o  key is an int      o  value is a character buffer       The buffer in the key value pair will need to be freed afterwards int make dict entry int a key  char   buffer  struct key value pair  pMyStruct            determine the len of the buffer assuming it is a string     int len   strlen buffer           alloc the buffer to hold the string     pMyStruct- gt pValue    char    malloc len   1      add one for the null terminator byte     if  NULL    pMyStruct- gt pValue            printf  Failed to allocate the buffer for the dictionary string value             return RC ERROR            strcpy pMyStruct- gt pValue  buffer       pMyStruct- gt key   a key       return RC OK         Assumes the hash table has already been allocated  int add key val pair to dict struct hash table  pHashTable  int key  char  pBuff         int rc      struct key value pair  pKeyValuePair       if  NULL    pHashTable            printf  Hash table is null  n            return RC ERROR                Allocate the dictionary key value pair struct     pKeyValuePair    struct key value pair    malloc sizeof struct key value pair        if  NULL    pKeyValuePair            printf  Failed to allocate key value pair struct  n            return RC ERROR              rc   make dict entry key  pBuff  pKeyValuePair       a hash table 1221     abba      if  RC ERROR    rc            printf  Failed to add buff to key value pair struct  n            return RC ERROR              rc   hash insert pKeyValuePair  pHashTable       if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR             return RC OK      void dump hash table struct hash table  pHashTable            Iterate the dictionary by keys     char   pValue      struct key value pair  pMyStruct      int  pKeyList      int num keys       printf  i tKey tValue n        printf  ----------------------------- n        num keys   keys pHashTable   amp pKeyList       for  int i   0  i  lt  num keys  i              pMyStruct   hash retrieve pKeyList i   pHashTable           pValue   pMyStruct- gt pValue          printf   d t d t s n   i  pKeyList i   pValue                 Free the key list     free pKeyList       int main int argc  char  argv           int rc      int i        struct hash table a hash table      a hash table max   20       The dictionary can hold at most 20 entries      a hash table number of elements   0      The intial dictionary has 0 entries      allocate the dictionary  amp a hash table        rc   add key val pair to dict  amp a hash table  1221   abba        if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR            rc   add key val pair to dict  amp a hash table  2211   bbaa        if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR            rc   add key val pair to dict  amp a hash table  1122   aabb        if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR            rc   add key val pair to dict  amp a hash table  2112   baab        if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR            rc   add key val pair to dict  amp a hash table  1212   abab        if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR            rc   add key val pair to dict  amp a hash table  2121   baba        if  RC ERROR    rc            printf  insert has failed  n            return RC ERROR                  Iterate the dictionary by keys     dump hash table  amp a hash table           Free the individual slots     for  i   0  i  lt  a hash table max  i                 all that he could see was babylon         if  NULL    a hash table elements i                 free a hash table elements i - gt pValue       free the buffer in the struct             free a hash table elements i        free the key value pair entry             a hash table elements i    NULL                           Free the overall dictionary     free a hash table elements          CrtDumpMemoryLeaks        return 0

User · Answer

There s a lot of typedef going on here  Personally I m against  hiding the asterisk   i e  typedef ing pointer types into something that doesn t look like a pointer  In C  pointers are quite important and really affect the code  there s a lot of difference between foo and foo     Many of the answers are also confused about this  I think   Your allocation of an array of Chess values  which are pointers to values of type chess  again  a very confusing nomenclature that I really can t recommend  should be like this   Chess  array   malloc n   sizeof  array     Then  you need to initialize the actual instances  by looping   for i   0  i  lt  n    i    array i    NULL    This assumes you don t want to allocate any memory for the instances  you just want an array of pointers with all pointers initially pointing at nothing   If you wanted to allocate space  the simplest form would be   for i   0  i  lt  n    i    array i    malloc sizeof  array i      See how the sizeof usage is 100  consistent  and never starts to mention explicit types  Use the type information inherent in your variables  and let the compiler worry about which type is which  Don t repeat yourself   Of course  the above does a needlessly large amount of calls to malloc    depending on usage patterns it might be possible to do all of the above with just one call to malloc    after computing the total size needed  Then you d still need to go through and initialize the array i  pointers to point into the large block  of course

[c] malloc an array of struct pointers

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