c - Why do these two pointers access the same elements? -


i'm working on mpi program , have following code each of processes:

#define cell(a,x,y,mx,my) (*(a+y*mx+x)) int bs_x = 1; int bs_y = 1;  int *this_data=calloc((bs_y+2)*(bs_x+2), sizeof(int));  //...  int *recv_top = calloc(bs_x, sizeof(int)); int *recv_left = calloc(bs_x, sizeof(int));  // make operations , want assign  // value of recv_top , recv_left this_data  for(i=0; i<bs_x; ++i) cell(this_data, i+1, 0, bs_x+2, bs_y+2) = recv_top[i]; // sets both (0,1) , (1,0) recv_top!! for(i=0; i<bs_x; ++i) cell(this_data, 0, i+1, bs_x+2, bs_y+2) = recv_left[i]; // again sets both (0,1) , (1,0) recv_left!!

now problem comes when check value of element (1,0) , found out first call saves value of recv_top in element (0,1):

for(i=0; i<bs_x; ++i) cell(this_data, i+1, 0, bs_x+2, bs_y+2) = recv_top[i]; if (rank == 4) {     printf("rank 4 value @ (%d,0) = %d\n", 1, cell(this_data,1,0,bs_x+2,bs_y+2));     printf("rank 4 value @ (0,%d) = %d\n", 1, cell(this_data,0,1,bs_x+2,bs_y+2)); } //rank 4 value @ (1,0) = 12 //rank 4 value @ (0,1) = 12

and next:

for(i=0; i<bs_x; ++i) cell(this_data, 0, i+1, bs_x+2, bs_y+2) = recv_left[i]; if (rank == 4) {     printf("rank 4 value @ (%d,0) = %d\n", 1, cell(this_data,1,0,bs_x+2,bs_y+2));     printf("rank 4 value @ (0,%d) = %d\n", 1, cell(this_data,0,1,bs_x+2,bs_y+2)); } //rank 4 value @ (1,0) = 1024 //rank 4 value @ (0,1) = 1024

updates (1,0).

they shouldn't since not same:

cell(this_data, 0, i+1, bs_x+2, bs_y+2) = (*(this_data+(i+1)*(bs_x+2)) cell(this_data, i+1, 0, bs_x+2, bs_y+2) = (*(this_data+i+1))

any ideas i'm doing wrong?

the code #define cell(a,x,y,mx,my) (*(a+y*mx+x)) should be

#define cell(a,x,y,mx,my) ( *( (a) + (y) * (mx) + (x) ) )

note parenthesis!

btw, why not use inline function?with inline function have better code.

in version code

cell(this_data, 0, i+1, bs_x+2, bs_y+2)

expands into:

*(this_data+i+1*bs_x+2+bs_y+2)

which seems different you'd expect

update

i still suggest make inline function!

inline void cell(int* a, int x, int y, int mx, int my, int newvalue) {   int * cell = + y * mx + x;   *cell = newvalue; }

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