Initialization of 3D global arrays in C

Question

I have global 3D arrays defined as

double*** arr;

in a file common.c

I have the declaration

extern double*** arr;

in a file common.h

Now when I am initializing this array dynamically at runtime, I am running into a segmentation fault I executed the code

 exs =malloc(sizeof(double)*nx*ny*nz);

where nx,ny and nz are known at runtime prior to executing this statement.

But when i try to initialize this array as

  for(i=0;i<nx;i++)  
      for(j=0;j<ny;j++)  
          for(k=0;k<nz;k++)  
              arr[i][j][k]=0.0e0;

I get a segfault.

What am I doing wrong ?

off-by-one error? your for loops should have < instead of <= — tay10r
– tay10r, Commented Jun 28, 2013 at 6:49
You don't have a 3D array, you have a pointer to pointer(s) to pointer(s) to int(s). You need to malloc at every level. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 28, 2013 at 6:50

Some programmer dude · Accepted Answer · 2013-06-28 07:13:14Z

7

You need to allocate all dimensions:

arr = malloc(sizeof(*arr) * nx);
for (int i = 0; i < nx; i++)
{
    arr[i] = malloc(sizeof(**arr) * ny);
    for (int j = 0; j < ny; j++)
    {
        arr[i][j] = malloc(sizeof(***arr) * nz);
        for (int k = 0; k < nz; k++)
        {
            arr[i][j][k] = 0.0;
        }
    }
}

And of course don't forget to free all the data you allocated.

edited Jun 28, 2013 at 7:13

answered Jun 28, 2013 at 6:51

Some programmer dude

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3 Comments

user529758 Over a year ago

@TaylorFlores Fortunately I did. :P

user529758 Over a year ago

@JoachimPileborg Better say malloc(sizeof(*arr)) instead of an explicit type, it's safer in case the base type of the pointer changes.

jnovacho Over a year ago

Would it be possible to use calloc to get the zeros right at init time?

score 6 · Accepted Answer · 2013-06-28 10:36:51Z

6

First of all, arrays are not pointers.

Second, let me show you another solution which tries to avoid memory fragmentation and uses one single call to the malloc() function (which is expensive):

double (*arr)[ny][nz] = malloc(sizeof(*arr) * nx);

After this, you can use arr as an array of dimensions nx * ny * nz.

And if you want this to be used at file scope: declare it as void *, allocate memory for nx * ny * nz elements, then assign it to a pointer-to-array when used for convenience:

// common.h
extern void *ptr;

// common.c
ptr = malloc(sizeof(double) * nx * ny * nz);

// foo.c
double (*arr)[ny][nz] = ptr;

But if you need such a hack... you should be thinking about what you have done wrong. Well, in this case, you've done wrong one thing: you are using a global variable. Don't use it.

edited Jun 28, 2013 at 10:36

answered Jun 28, 2013 at 6:54

user529758

4 Comments

user529758 Over a year ago

@TaylorFlores Thanks. (I hope also for the one-malloc approach.)

Armali Over a year ago

Looks nicer - too bad that this declaration cannot be global or extern when ny and nz are not constant.

user529758 Over a year ago

@Armali Yes, that's why one should declare it as double *ptr, then convert it to double (*)[ny][nz] at usage-time.

user529758 Over a year ago

@Armali But anyway, why does OP use a global variable at all?

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