c - Pointers, Arrays and Functions -

i programmed function can rotate 4 x 4 arrays, , want output it... bunch warnings, compiles , works, guess did "minor/formal" mistakes.

#include <stdio.h>  //----------------------------------------------------------------------------- /// /// function prints 4 4x4 matrix /// /// @param m1, m2, m3, m4 input matrix want print. /// /// @return 0 //-----------------------------------------------------------------------------  void *print_matrix(char m1[4][4], char m2[4][4], char m3[4][4], char m4[4][4])     {         for(int = 0; < 4; i++)         {             for(int j = 0; j < 4; j++)             {                 printf("%c ", m1[i][j]);             }             printf(" ");             for(int j = 0; j < 4; j++)             {                 printf("%c ", m2[i][j]);             }             printf(" ");             for(int j = 0; j < 4; j++)             {                 printf("%c ", m3[i][j]);             }             printf(" ");             for(int j = 0; j < 4; j++)             {                 printf("%c ", m4[i][j]);             }             printf("\n");         }         printf("\n");         return 0;     }  //----------------------------------------------------------------------------- /// /// function rotates 4x4 matrix  /// (and collects tears overworked students @2am, because wtf  ) /// /// @param m, matrix want rotate /// /// @return m_r rotated matrix //-----------------------------------------------------------------------------  char *rotate_matrix(char m[4][4]) {     static char m_r[4][4];     for(int = 0; < 4; i++)     {         for(int j = 0; j < 4; j++)         {             m_r[i][j] = m[3-j][i];         }     }     return *m_r; }  int main(void) { char matrix_t [4][4] = {{ '-', '-', '-', '-' },                         { '-', 'o', '-', '-' },                         { 'o', 'o', 'o', '-' },                         { '-', '-', '-', '-' }                        }; char matrix_z [4][4] = {{   '-', '-', '-', '-' },                         { '-', 'o', 'o', '-' },                         { 'o', 'o', '-', '-' },                         { '-', '-', '-', '-' }                        }; char matrix_l [4][4] = {{   '-', '-', '-', '-' },                         { '-', 'o', '-', '-' },                         { '-', 'o', '-', '-' },                         { '-', 'o', 'o', '-' }                        }; char matrix_i [4][4] = {{   '-', '-', 'o', '-' },                         { '-', '-', 'o', '-' },                         { '-', '-', 'o', '-' },                         { '-', '-', 'o', '-' }                        }; char (*array)[4][4];    print_matrix(matrix_t, matrix_z, matrix_l, matrix_i);   array = rotate_matrix(matrix_t);   print_matrix(array, matrix_z, matrix_l, matrix_i);     return 0; } 

this errors get:

102:9: warning: assignment incompatible pointer type [-wincompatible-pointer-types]   array = rotate_matrix(matrix_t); 

,

103:16: warning: passing argument 1 of ‘print_matrix’ incompatible pointer type [-wincompatible-pointer-types]   print_matrix(array, matrix_z, matrix_l, matrix_i); 

and

note: expected ‘char (*)[4]’ argument of type ‘char (*)[4][4]’   void *print_matrix(char m1[4][4], char m2[4][4], char m3[4][4], char m4[4][4]) 

and honest, don't these errors, because code seems work, i'm never out of boundaries , i'm not doing memory violations...

the warnings because these:

char *            // pointer char         or first element of char[] char (*)[4]       // pointer char[4]      or first element of char[][4] char (*)[4][4]    // pointer char[4][4]   or first element of char[][4][4] 

are different types. pointer doesn't denote memory location, (via type) how memory location should interpreted.

one tricky bit array name (or other expression evaluates array) decays pointer. example, valid:

char abc[4] = "abc";    // abc array of 4 characters char = *abc;          // 'a', first character of abc 

. . . , conversely, can use array notation on pointer (to perform pointer arithmetic , dereference result):

char * abc = "abc";     // abc pointer (into static memory) char c = abc[2];        // c 'c', third character of abc 

so char * interconvertible char ()[], char (*)[4] interconvertible char ()[][4], , char (*)[4][4] interconvertible char ()[][4][4].

and makes extra tricky c doesn't quite support passing array function parameter (though if want that, can achieve same effect wrapping array in struct , passing that). let declare function parameter having array type, syntactic sugar function parameter pointer type. these 3 signatures equivalent:

void *print_matrix(char m1[4][4], char m2[4][4], char m3[4][4], char m4[4][4]) void *print_matrix(char m1[][4], char m2[][4], char m3[][4], char m4[][4]) void *print_matrix(char (*m1)[4], char (*m2)[4], char (*m3)[4], char (*m4)[4]) 

your code works because various warnings sort of cancel out:

• rotate_matrix should return either char (*)[4] (a pointer first row of char[][4]) or char (*)[4][4] (a pointer complete char[4][4]). instead, have return char *, , accomplish that, dereference desired pointer, thereby getting (after array-to-pointer decay) pointer first element of first row of matrix. of course, of these pointers point same memory location, different types.
  • to declare return-type char (*)[4], can write char (*rotate_matrix(char m[4][4]))[4] . . . it's better declare row type, typedef char row[4], , write row * rotate_matrix(char m[4][4]). in case return m_r @ end, decays pointer first row of m_r.
  • to declare return-type char (*)[4][4], same thing, [4]'s (and name matrix rather row). in case return &m_r @ end, meaning pointer entirety of m_r.
  • either of above approaches fine imho, though of course you'll have adjust calling sites match. rest of answer, i'm going assume we're doing second version, 1 return-type char (*)[4][4].
• array has type char (*)[4][4], fine. code warning in array = rotate_matrix(matrix_t) because of mis-declaration of rotate_matrix. actual value of returned pointer fine, @ least on typical platforms.
  • that said, if you're going use type array, should call array_ptr, since it's pointer usual array type.
• but pass array argument print_matrix, takes parameter of type char [4][4], meaning char (*)[4]. can't convert char (*)[4][4] (pointer array of 4 rows of 4 chars) char (*)[4] (pointer array of 4 chars). fix this, need dereference array, passing *array instead.

(note: i'm not sure of above implicit conversions guaranteed work; pointers different types of objects might not interconvertible on platforms. , on typical modern platforms, are interconvertible, might run afoul of strict aliasing rules, means compiler might end performing optimizations totally break code. usually work.)