MatrixRot.c

#include <stdio.h>
#include <math.h>
 
int
main(void) {
   
  void rotate(int *mat, int side);
   
  #define Side 4
   
  int mat[Side][Side] = {
    { 1,  2,  3,  4},
    { 5,  6,  7,  8},
    { 9, 10, 11, 12},
    {13, 14, 15, 16}
  };
   
  #define PrintMatrix                                                                   \
  for (int y = 0; y < Side; y++) {                                                      \
    for (int x = 0; x < Side; x++)                                                      \
      mat[y][x] < 10 ?                                                                  \
      printf(" %d ", mat[y][x]) :                                                       \
      printf( "%d ", mat[y][x]);                                                        \
    printf("\n");                                                                       \
  }
   
  PrintMatrix;
   
  rotate(&mat[0][0], Side);
   
  printf("\n");
   
  PrintMatrix;
   
}
void
rotate(int *mat, const int side) {
   
  const int xLim = (side - 1) / 2, yLim = (side - 2) / 2;
   
  const int rotId = side - 1;
   
  int *from, *to, y, x, lY, lX, tY;
   
  #define Swap(a, b)  (((a) ^= (b)), ((b) ^= (a)), ((a) ^= (b)))
   
  for (x = 0; x <= xLim; ++x) for (y = 0; y <= yLim; ++y)
 
    for (lY  = y,   lX  = x,  tY = rotId - lX;
         tY != y || lY != x;
         lX  = lY,  lY  = tY, tY = rotId - lX) {
       
      from = mat + lX + side * lY, to = mat + lY + side * tY;
       
      Swap(*from, *to);
       
    }
   
}