Calling function in C with array and values are changed?

Question

the code below changes the values of arr in function check and prints out values of "2", even though I didn't pass the array in check function in a pointer. How is that possible?

 #include <stdio.h>
 #include <stdlib.h>
 void check(int n,int array[]);

 int main()
 {
     int arr[]={1,2,3,4};
     int i;
     check(4,arr);

     for(i=0;i<4;i++){
         printf("%d\n",arr[i]);
     }
     return 0;
 }

 void check(int n,int array[])
 {
     int j=0;

     while(j<n){
         array[j]=2;
         j++;
     }
 }

It looks like you did? stackoverflow.com/questions/1641957/… — Prof. Falken
– Prof. Falken, Commented Sep 10, 2015 at 11:24
Compile with all warnings & debug info (gcc -Wall -Wextra -g). Use a debugger (gdb). Beware of undefined behavior. — Basile Starynkevitch
– Basile Starynkevitch, Commented Sep 10, 2015 at 11:24
Read a good C programming book explaining what are pointers and arrays, what is pointer aliasing, why arrays are decaying into pointers. — Basile Starynkevitch
– Basile Starynkevitch, Commented Sep 10, 2015 at 11:25
So you call a function in C. "Calling a function in C" is not your problem. Why don't you use a descriptive topic? — harper
– harper, Commented Sep 10, 2015 at 11:34

Community · Accepted Answer · 2017-05-23 11:58:08Z

9

Keep in mind that

void check(int n,int array[]);

is the same as

void check(int n,int *array);

and so, when you use

check(4,arr);

you are actually doing

check(4,&arr[0]); /* This is called array "decay" */

and because array decays to a pointer which points to the address of its first element. So, it means that the "array" is passed by reference.

edited May 23, 2017 at 11:58

CommunityBot

11 silver badge

answered Sep 10, 2015 at 11:28

Spikatrix

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2 Comments

Belal Fathy Over a year ago

thank you! I have another inquiry! When I create a pointer with a zero value in check function and assign it's adress to each of array elements, the arr prints it's original values not zeros. any explanations?

Spikatrix Over a year ago

@BelalFathy I did not understand. Could you show some code?

Prof. Falken · Accepted Answer · 2015-09-10 11:26:12Z

4

In C, arrays are converted ("decaying") automatically to pointers when sent to functions.

answered Sep 10, 2015 at 11:26

Prof. Falken

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