What is the difference between the following declarations in C?

问题

what's the difference between these 2 declarations:

char (*ptr)[N];

vs.

char ptr[][N];

thanks.

回答1:

(1) declaration

char (*ptr)[N];

ptr is pointer to char array of size N following code will help you to on how to use it:

#include<stdio.h>
#define N 10
int main(){
 char array[N] = "yourname?";
 char (*ptr)[N] = &array;
 int i=0;
 while((*ptr)[i])
  printf("%c",(*ptr)[i++]);
}

output:

yourname?

See: Codepad

(2.A)

Where as char ptr[][N]; is an invalid expression gives error: array size missing in 'ptr'.

But char ptr[][2] = {2,3,4,5}; is a valid declaration that is 2D char array. Below example:

int ptr[][3] = {{1,2,3,4,5}, {5,6,7,8,9}, {6,5,4,3,2}};

Create an int array of 3 rows and 5 cols. Codepade-Example

(2.B) Special case of a function parameter!

As function parameter char ptr[][N]; is a valid expression. that means ptr can point a 2D char array of N columns.

example: Read comments in output

#include <stdio.h>
int fun(char arr[][5]){
  printf("sizeof arr is %d bytes\n", (int)sizeof arr);
}
int main(void) {
  char arr[][6] = {{'a','b'}, {'c','d'}, {'d','e'}};
  printf("sizeof arr is %d bytes\n", (int)sizeof arr);
  printf("number of elements: %d\n", (int)(sizeof arr/sizeof arr[0]));
  fun(arr);
  return 0;
}

output:

sizeof arr is 6 bytes   // 6 byte an Array 2*3 = 6
number of elements: 3   // 3 rows
sizeof arr is 4 bytes   // pointer of char 4 bytes

To view this example running: codepad

回答2:

First is declare ptr as pointer to array N of char
Second is declare ptr as array of array N of char

Plz refer link

回答3:

The first declares a pointer to an N-long array, the other declares an two dinemsional array. Note: They can be used to achieve the similar functionality, but they don't mean the same!

回答4:

1.ptr is pointer to character array of size N 2. ptr looks like a double dimensional array but number of rows is not provided.In 2-D array both number of rows and column is compulsory as compiler will decide how many bytes should be assigned to array by (number of row*number of column*size of data type).This declaration may work fine as below:

char[][2]={
           {'a','b'}
          };

as here compiler will look and understand that there is one row.or when a two dimensional array is passed on as a function argument,the number of columns(second dimension) must be provided by compulsion,passing number of rows is not compulsory in function definition.

来源：https://stackoverflow.com/questions/15157133/what-is-the-difference-between-the-following-declarations-in-c

标签

pointers

declaration