如何检查指针（void）的值不为零（尝试取消引用通用指针）

Question

Sorry it must be very simple question , but since I tried in diffrenet ways without any success I have to ask here to be sure. C programming :

There is a struct name rtg . EDIT: type of mtch is LLIST type of initial is LL_NODE typr of obj is pointer (void)

. Using gdb when I check

(gdb)  print *rtg->mtch->initial->obj
Attempt to dereference a generic pointer.


(gdb) print rtg->mtch->initial->obj

$10 = (void *) 0x4cc660
(gdb) x 0x4cc660
0x4cc660:       0x00000000

This null pointer causes segfault in my program. What I am looking for is simply how to check the value of what rtg->mtch->initial->obj is pointing not be zero? (to prevent above segfault) I mean if I check with if (rtg->mtch->initial->obj) , it would just check if pointer obj , adress not be zero (this is not what I intend , I intend to check the value of that pointer not be zero (but when I use * before checking in gdb it says "Attempt to dereference a generic pointer". So what is the correct way to check that value not be zero (and prevent this segfault)?

Edit : i had tried this

if (*((char *) rtg->mtch->initial->obj) != NULL)

but i got compile warning :

warning: comparison between pointer and integer

EDIT2 , here what are these defined in the source code

ECM_REQUEST is struct ECM_REQUEST rtg; in this struct defind mtch as LLIST mtch;

initial is LL_NODE

obj is a pointer i want to check obj value not be zero

so now everything are clear about my question isn't it?

Thanks

Answer 1

You cannot dereference a generic pointer. The only solution I can think of is to make a temporary pointer to integer to check the value. Basically

int *tmp = rtg->mtch->initial->obj;
if (*tmp != 0)
/* the rest of your code here */

A cast could also work, but having a temporary pointer makes the code easier to read in my opinion.

Answer 2

The warning is because the left hand side is a dereferenced char * (in other words, a char ) and the right hand side is a null pointer constant, in the form of a void * ; You're converting a char value to a void * value... Perhaps you meant char ** ?

void *o = rtg == NULL                ? NULL
        : rtg->mtch == NULL          ? NULL
        : rtg->mtch->initial == NULL ? NULL
        : rtg->mtch->initial->obj;

if (o != NULL) { /* ... */ }

... Do note that it should be safe to change the type of o to any other object pointer (eg. your question seems to suggest char ** ?)