I found this code in a book, trying to explain how generic pointers works using this to copy one matrix to another one.

#include <stdio.h>

#define FILAS 2
#define COLS 3

void copiarMatrices(void *, void *, int);

int main(){

    int m1[FILAS][COLS] = {24, 30, 15, 45, 34, 7};
    int m2[FILAS][COLS];

    copiarMatrices(m2, m1, sizeof(m1));

    for (int i = 0; i < FILAS; i  ){
        for (int j = 0; j < COLS; j  ) printf("%d ", m2[i][j]);
        printf("\n");
    }
}

void copiarMatrices(void *dest, void *orig, int n){
    char *destino = (char *)dest;
    char *origen = (char *)orig;

    for (int i = 0; i < n; i  ) {
        destino[i] = origen[i];
    }
}

The book said that the only data type that we could cast the void pointers was char because other data types are not allowed because of the bytes size.

I don't understand this and also if I execute the program with either int or double data type on void pointers the program get stuck for a few seconds and then finish well.

CodePudding user response：

What the book is saying is that the result of the "sizeof" operator is the expression argument's size in bytes. If instead you were to specify "sizeof(m1) / sizeof(int)" then instead of "char *" you would need to cast the parameters to "int *" because you would then be passing the number of ints to copy instead of the number of bytes.

CodePudding user response：

Function copiarMatrices copies buffer of any size. Let's imagine that buffer orig is 5 bytes. If you cast to int * inside copiarMatrices and int is 4 bytes and I want to copy all 5 bytes from orig to dest than n will not be integer (if n=1 than 4 bytes will be copied, if n=2 than 8 bytes will be copied but I need to copy 5 bytes). So, we can agree that copying will be by bytes and n means number of bytes which will be copied. In general you have to call copiarMatrices(m2, m1, FILAS * COLS * sizeof(int));

CodePudding user response：

You do not need any special functions. You function copiarMatrices can be reduced to memcpy(m2, m1, sizeof(m1));

Some remarks.

1. Do not use int for sizes. Use the correct size_t type instead.
2. Try to be const correct.
3. Return a reference to the destination object, It will allow the function to be used in the expressions.

Your function prototype should be: void *copiarMatrices(void *, const void *, const size_t);. If you want to make the compiler life easier you may: void *copiarMatrices(void restrict *, const void * restrict, const size_t);