Expand array of structs using realloc

Question

I am passing a struct through a function like so...

expandArrayofStructs(Container *container, int n)

This container is one struct and inside of that struct is an array of another type of struct I'll call it inner.

I would like to expand the size of the array of inner by some value and then continue in my original function after the array has been expanded. So this expand function does not return anything rather it is just called and expands the data and finishes and the program continues with a new larger array than before.

My understanding of the situation would be something like this but this does not work properly...

int expandArrayofStructs(Container *container, int n)
{
   container->inner = realloc(container->inner, sizeof(inner) * 50);
                                                                ^
                                                   Just a number i picked. so if i 
                                                   already had an  array of 50
                                                   I would be increasing by 50 here.

  if(Container->inner == NULL)
    //HANDLE ERROR IF REALLOC FAILS

  //Update the container length
  container->length = container->length + 50;

  //For some reason the specs of the program say I need to return 
  //the array length which is an attribute of container
  return container->length;
}

But when I realloc in this manner I am not even getting segfault error I am getting: realloc(): invalid next size 0x463829

the numbers at the end vary.

Not sure what I am doing wrong, but if there is a better way to dynamically realloc an array of structs then I am open to suggestion. This particular code does not have to be exactly what it is.

The only stipulation is that this function returns type int which is = the new array length

so `Container` has two fields inside: length and inner, isn't it? I suppose length is the initial size of the array `inner`. — Kyrol, Jun 15 '13 at 11:01
You probably need this to be realloc(container->inner, sizeof(*container->inner) * (50 + container->length)); — John Sheridan, Jun 15 '13 at 11:03

score 2 · Answer 1 · answered Jun 15 '13 at 11:03

2

This error happens because you are not "reallocating" more memory. You are allocating the same the size.

To fix it, just allocating total size that you will need:

container->inner = realloc(container->inner, sizeof(inner) * (50 + container->length);

answered Jun 15 '13 at 11:03

Thiago Lewin

2,810
14
18

score 0 · Answer 2 · answered Jun 15 '13 at 10:59

The new allocated size should be based on the size of the elements of the array, and not based on the size of the pointer to the beginning of the array. Also you want to resize it relative to the current length:

container->inner = realloc(container->inner, sizeof(*container->inner) * (container->length + 50));

score 0 · Answer 3 · answered Jun 15 '13 at 11:07

Assuming your struct definition looks something like this:

typedef struct { ... } Inner;
typedef struct { ...; Inner *inner;  size_t length; } Container;

then your function would look something like

size_t expandArrayOfStruct( Container *container, size_t extent )
{
  Inner *tmp = realloc( container->inner, 
                        sizeof *container->inner * (container->length + extent) );
  if ( tmp )
  {
    container->length += extent;
    container->inner = tmp;
  }

  return container->length;
}

You want to assign the result of realloc to a temporary variable; if the operation fails, realloc will return NULL, and if you assign that to your original pointer you'll lose your reference to the memory that was already allocated, causing a memory leak.

You can use this function for the original allocation as well:

int main( void )
{
  Container c = {..., NULL, 0 };
  size_t len = expandArrayOfStruct( &c, 50 );
  if ( len != 50 )
  {
    // allocation failed
  }
  ...
}

score 0 · Answer 4 · answered Jun 15 '13 at 11:10

You have to allocate new memory, and you don't do that.

int expandArrayofStructs(Container *container, int n)
{
  container->inner = realloc(container->inner, sizeof(inner) * (50 + container->length));
  if(Container->inner == NULL)
  /* return -1; (set realloc handle errors properly)*/

  container->length = container->length + 50;

  return container->length;
}

score 0 · Answer 5 · answered Jun 15 '13 at 11:10

0

Why not read the manual page (see http://linux.die.net/man/3/realloc). You are using the same size each time. Besides - Why not use a linked list?

answered Jun 15 '13 at 11:10

Ed Heal

59,252
17
87
127

Expand array of structs using realloc

5 Answers5