malloc for struct and pointer in C

Question

Suppose I want to define a structure representing length of the vector and its values as   struct Vector      double  x      int n       Now  suppose I want to define a vector y and allocate memory for it   struct Vector  y    struct Vector  malloc sizeof struct Vector      My search over the internet show that I should allocate the memory for x separately   y- gt x    double  malloc 10 sizeof double      But  it seems that I am allocating the memory for y- x twice  one while allocating memory for y and the other while allocating memory for y- x  and it seems a waste of memory  It is very much appreciated if let me know what compiler really do and what would be the right way to  initialize both y  and y- x   Thanks in advance

User · Answer

When you malloc sizeof struct name   it automatically allocates memory for the full size of the struct  you don t need to malloc each element inside  Use -fsanitize address flag to check how you used your program memory

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No  you re not allocating memory for y- gt x twice   Instead  you re allocating memory for the structure  which includes a pointer  plus something for that pointer to point to   Think of it this way            1          2          -----      ------  y------ gt    x------ gt     x               n        ------           -----    So you actually need the two allocations  1 and 2  to store everything   Additionally  your type should be struct Vector  y since it s a pointer  and you should never cast the return value from malloc in C since it can hide certain problems you don t want hidden - C is perfectly capable of implicitly converting the void  return value to any other pointer   And  of course  you probably want to encapsulate the creation of these vectors to make management of them easier  such as with   struct Vector       double  data        no place for x and n in readable code  -      size t size      struct Vector  newVector  size t sz           Try to allocate vector structure       struct Vector  retVal   malloc  sizeof  struct Vector        if  retVal    NULL          return NULL          Try to allocate vector data  free structure if fail       retVal- gt data   malloc  sz   sizeof  double        if  retVal- gt data    NULL            free  retVal           return NULL                Set size and return       retVal- gt size   sz      return retVal     void delVector  struct Vector  vector           Can safely assume vector is NULL or fully built       if  vector    NULL            free  vector- gt data           free  vector             By encapsulating the creation like that  you ensure that vectors are either fully built or not built at all - there s no chance of them being half-built  It also allows you to totally change the underlying data structures in future without affecting clients  for example  if you wanted to make them sparse arrays to trade off space for speed

User · Answer

Few points   struct Vector y    struct Vector  malloc sizeof struct Vector    is wrong  it should be struct Vector  y    struct Vector  malloc sizeof struct Vector    since y holds pointer to struct Vector   1st malloc   only allocates memory enough to hold Vector structure  which is pointer to double   int   2nd malloc   actually allocate memory to hold 10 double

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When you allocate memory for struct Vector you just allocate memory for pointer x  i e  for space  where its value  which contains address  will be placed  So such way you do not allocate memory for the block  on which y x will reference

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You could actually do this in a single malloc by allocating for the Vector and the array at the same time  Eg   struct Vector y    struct Vector  malloc sizeof struct Vector    10 sizeof double    y- gt x    double    char  y   sizeof struct Vector    y- gt n   10    This allocates Vector  y   then makes y- x point to the extra allocated data immediate after the Vector struct  but in the same memory block    If resizing the vector is required  you should do it with the two allocations as recommended  The internal y- x array would then be able to be resized while keeping the vector struct  y  intact

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The first time around  you allocate memory for Vector  which means the variables x n   However x doesn t yet point to anything useful   So that is why second allocation is needed as well

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In principle you re doing it correct already  For what you want you do need two malloc  s   Just some comments   struct Vector y    struct Vector  malloc sizeof struct Vector    y- gt x    double  malloc 10 sizeof double      should be  struct Vector  y   malloc sizeof  y      Note the pointer    y- gt x   calloc 10  sizeof  y- gt x     In the first line  you allocate memory for a Vector object  malloc   returns a pointer to the allocated memory  so y must be a Vector pointer  In the second line you allocate memory for an array of 10 doubles    In C you don t need the explicit casts  and writing sizeof  y instead of sizeof struct Vector  is better for type safety  and besides  it saves on typing   You can rearrange your struct and do a single malloc   like so   struct Vector          int n      double x       struct Vector  y   malloc sizeof  y   10   sizeof double

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First malloc allocates memory for struct  including memory for x  pointer to double   Second malloc allocates memory for double value wtich x points to

[c] malloc for struct and pointer in C

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