Check for NULL pointer doesn't work for char pointer

Question

I know it must be silly to ask about, but my if statement that checks for null pointer doesn't seem to work. The code continues, and once I dereference the pointer, an error occurs. The code is kernel code, if that matters.

// issue.h
static char *sys_efile[FILE_PATH_SIZE]; // 100

// issue_main.c
#include "issue.h"

if (sys_efile == NULL)
    return -EFAULT;

file = filp_open(*sys_efile, O_RDWR, 0);

BUG: kernel NULL pointer dereference, address: 0000000000000000
[   32.262950] #PF: supervisor read access in kernel mode
[   32.262952] #PF: error_code(0x0000) - not-present page

Do I miss smth?

Answer 1

You declared an array of pointers with static storage duration.

static char *sys_efile[FILE_PATH_SIZE];

So all elements of the array are implicitly initialized as null-pointers. But the array itself can not be implicitly converted to a null pointer because it occupies memory. So this statement

if (sys_efile == NULL)

will always evaluate to false.

Either actually instead of the array of pointers you want to declare a character array like

static char sys_efile[FILE_PATH_SIZE]; // 100

and then you may write for example

// issue_main.c
#include "issue.h"

if ( *sys_efile == '\0' )
    return -EFAULT;

file = filp_open( sys_efile, O_RDWR, 0);

Or if you are going to use an array of pointers then the if statement should look like

static char *sys_efile[FILE_PATH_SIZE]; // 100

// issue_main.c
#include "issue.h"

if ( *sys_efile == NULL)
    return -EFAULT;

file = filp_open(*sys_efile, O_RDWR, 0);