struct pointer by using memory allocation

Question

i am stucked in that code how can make memory allocation for that struct

typedef struct {
  int a, b, c, d;
} FourInts;


void fillArray(int* array, int len) {
  printf("Filling an array at address %p with %d "
         "values\n", array, len);
  for (int i = 0; i < len; ++i) {
    array[i] = (i * 3) + 2;
    // assert() verifies that the given condition is true
    // and exits the program otherwise. This is just a
    // "sanity check" to make sure that the line of code
    // above is doing what we intend.
    assert(array[i] == ( (i * 3) + 2) );
  }
  printf("Done!\n");
}


/***********from here the problem *******/
struct FourInts *heap_struct_FourInts = (FourInts*) malloc(sizeof( FourInts) * 1);

  fillArray(heap_struct_FourInts->*a), 4);


  free(heap_struct_FourInts);

the compiler gives me that error

   arrays.c:222:43: warning: initialization from incompatible pointer type [enabled by default]
   struct FourInts *heap_struct_FourInts = (FourInts*) malloc(sizeof( FourInts) * 1);
                                           ^
arrays.c:224:33: error: dereferencing pointer to incomplete type
   fillArray(heap_struct_FourInts->a, 4);
                             ^

what is the error in the code for the struct and malloc ?

I believe the type of heap_struct_FourInts should be FourInts* not struct FourInts* but I don't know why that would cause the errors that are produced. — clcto
– clcto, Commented Jul 10, 2014 at 20:23
The warning comes from the fact, that you assign a pointer to FourInts to a pointer to struct FourInts—note, that they are distinct types the latter of which is declared at line 222. Your error message shows a line different from the line in your code. A typo? (Don't type, copy and paste). — mafso
– mafso, Commented Jul 10, 2014 at 20:24
You might want to use a cast in your call to fillArray so that it would be fillArray((int *) heap_struct_FourInts, 4);, since fillArray is declared as taking (int*, int) parameters. — Logicrat
– Logicrat, Commented Jul 10, 2014 at 20:40

Fiddling Bits · Accepted Answer · 2014-07-10 20:42:32Z

1

The following function call is incorrect:

fillArray(heap_struct_FourInts->*a), 4);

a is an int, not a pointer to an int, so you cannot dereference it. (Even if it was a pointer to an int, your syntax is incorrect).

Also, in your structure ...

typedef struct {
    int a, b, c, d;
} FourInts;

... you are not declaring an array of 4 ints, but four independent ints. If you would like a to be an array of ints with a length of 4 you need to declare it like this:

typedef struct {
    int a[4], b, c, d;
} FourInts;

Now you can call you function like this:

FourInts *heap_struct_FourInts = malloc(sizeof(*heap_struct_FourInts);
fillArray(heap_struct_FourInts->a), 4);

The following is equivalent:

fillArray((*heap_struct_FourInts).a), 4);

edited Jul 10, 2014 at 20:42

answered Jul 10, 2014 at 20:32

Fiddling Bits

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Alexander Gessler · Accepted Answer · 2014-07-10 20:30:10Z

1

To fix the first warning drop the struct from the variable type since it is not a struct, but a typedef for a struct (thus, warning for type mismatch). As for the error, use &heap_struct_FourInts->a to pass the address of the very first int in your structure.

However, the code potentially invokes undefined behaviour as the ints need not be contiguous in memory. For example, a compiler could be configured to pad to 8 byte boundaries by default, in which case there would be 4 unused bytes after each int (assuming we're on a platform that has 4 byte int). Read up on struct padding for more information. That particular padding would be a very unlikely scenario, but it is something to keep in mind.

edited Jul 10, 2014 at 20:30

answered Jul 10, 2014 at 20:24

Alexander Gessler

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3 Comments

Fiddling Bits Over a year ago

Sprechen Sie Englisch? ;-)

Alexander Gessler Over a year ago

@Fiddling Bits Ich hoffe es ... :-)

Cloud Over a year ago

To add to Alex's point, ALWAYS address a data type when using structs via the user-defined type you created (ie: as a complete struct), or by accessing its members. Alignment and padding issues will always bite you if you do otherwise.

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