C pass int array pointer as parameter into a function

In your new code,

int func(int *B){    *B[0] = 5;}

B is a pointer to int, thus B[0] is an int, and you can't dereference an int. Just remove the *,

int func(int *B){    B[0] = 5;}

and it works.

In the initialisation

int B[10] = {NULL};

you are initialising anint with a void* (NULL). Since there is a valid conversion from void* to int, that works, but it is not quite kosher, because the conversion is implementation defined, and usually indicates a mistake by the programmer, hence the compiler warns about it.

int B[10] = {0};

is the proper way to 0-initialise an int[10].

Maybe you were trying to do this?

#include <stdio.h>int func(int * B){    /* B + OFFSET = 5 () You are pointing to the same region as B[OFFSET] */    *(B + 2) = 5;}int main(void) {    int B[10];    func(B);    /* Let's say you edited only 2 and you want to show it. */    printf("b[0] = %d\n\n", B[2]);    return 0;}

If you actually want to pass an array pointer, it's

#include <stdio.h>void func(int (*B)[10]){   // ptr to array of 10 ints.        (*B)[0] = 5;   // note, *B[0] means *(B[0])         //B[0][0] = 5;  // same, but could be misleading here; see below.}int main(void){        int B[10] = {0};   // not NULL, which is for pointers.        printf("b[0] = %d\n\n", B[0]);        func(&B);            // &B is ptr to arry of 10 ints.        printf("b[0] = %d\n\n", B[0]);        return 0;}

But as mentioned in other answers, it's not that common to do this. Usually a pointer-to-array is passed only when you want to pass a 2d array, where it suddenly looks a lot clearer, as below. A 2D array is actually passed as a pointer to its first row.

void func( int B[5][10] )  // this func is actually the same as the one above! {         B[0][0] = 5;}int main(void){    int Ar2D[5][10];    func(Ar2D);   // same as func( &Ar2D[0] )}

The parameter of func may be declared as int B[5][10], int B[][10], int (*B)[10], all are equivalent as parameter types.

Addendum: you can return a pointer-to-array from a function, but the syntax to declare the function is very awkward, the [10] part of the type has to go after the parameter list:

int MyArr[5][10];int MyRow[10];int (*select_myarr_row( int i ))[10] { // yes, really   return (i>=0 && i<5)? &MyArr[i] : &MyRow;}

This is usually done as below, to avoid eyestrain:

typedef int (*pa10int)[10];pa10int select_myarr_row( int i ) {   return (i>=0 && i<5)? &MyArr[i] : &MyRow;}