用C初始化3D全局数组

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 ?

Answer 1

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.

Answer 2

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.