Segmentation fault shmat() with 2d array - arrays

Question

Trying to allocate 2d array in shared memory. Execution returns segmentation fault during assignments (I think). Should I do the assignments in another way? This is my code:
...
#define KEYSM 46378
#define X 10
#define Y 10
int main(){
int i, j;
int shm_id;
int **addressArray;
if((shm_id = shmget(KEYSM, sizeof(int[X][Y]), IPC_CREAT | 0666)) == -1){
perror("shmget");
exit(-1);
}
if((addressArray = (int **)shmat(shm_id, NULL, 0)) == (int **)-1){
perror("shmat");
exit(-1);
}
for(i = 0; i < X; i++){
for(j = 0; j < Y; j++){
if(i % 2 != 0)
addressArray[i][j] = -1;
else
addressArray[i][j] = 0;
}
}
...
}

Answer 1

Your problem is in a – (your ?) – misunderstanding of the difference of a true 2D array, and a pointer→pointer→value indirection.
When you define a int **a this is interpreted as a pointer to another pointer, that then reference an int. Assigning a pointer obtained from an allocation function like malloc or shmat to such a double pointer, then as far as the semantics of C go, it expects this allocation of memory contain further pointers. So if you do a double dereference, and there's not a valid pointer there, it will fall flat on its face.
This misunderstanding is furthered by the fact, that in C you can validly write int a[X][Y]; and dereference it with a[i][j]. The key insight to understand this is, that the first "half", i.e. that with a array defined like that, a[i]… decays into a int* pointer, that points toward the 0th element in the i "column". The other half …[j] then dereferences this implicitly "appearing" pointer.
Multidimensional arrays in C are deceiving and I strongly discourage using them like that. Also you'll have a hard time to properly implement specific padding and row alignments with them, without jumping some really annoying arithmetic.
It's easier by far, to just write down the calculations explicitly, with the added benefit of having precise control of padding and alignment.
Suppose we want to create a 2D array of int, that shall be aligned to the sizes of long long
size_t const array_height = ...;
size_t const array_width = ...;
size_t const alignment = sizeof(long long);
size_t const row_size = array_width * sizeof(int);
size_t const row_stride =
alignment * ((row_size + alignment-1) / alignment);
size_t const array_size = array_height * row_stride;
int const shm_id = shmget(KEYSM, array_size, IPC_CREAT | 0666);
if( 0 > shm_id ){
perror("shmget");
exit(-1);
}
int *const array = shmat(shm_id, NULL, 0);
if( (void*)-1 == array ){
perror("shmat");
exit(-1);
}
for(size_t j = 0; j < array_height; ++j){
int *const row = array + j * row_stride;
for(size_t i = 0; i < array_width; ++i){
row[i] = (i % 2) ? -1 : 0;
}
}

Answer 2

A segfault is a runtime error, but your code shouldn't even compile, look a this statement (paranthesis missing):
if ( ( addressArray = ( int** ) shmat ( shm_id, NULL, 0 ) == ( int** ) -1 )
And what if that assignment failed? Does perror terminate the process? No. But you still access the array below. Most likely the source of the seg fault.
As mch pointed out:
// i is not declared
for(i = 0; i < X; i++){
// j is not declared, after j < Y, use semicolon instead of comma
for(j = 0; j < Y, j++){
// if the result is not zero you're doing an assignment
if(i % 2 != 0)
addressArray[i][j] = -1;
// but immediately after you assign a zero,
// previous statement -> useless
// 'else' is missing here
addressArray[i][j] = 0;
}
}