The value of null in C language is 0.

The following is a piece of test code written. With the result tracked by GDB, GDB can even calculate the variable to be tracked before output:

1 # include "stdio. H"
2 int main ()
3 {
4 char * A = NULL;
5
6 if (! A ){
7 printf ("AA \ n ");
8}
9
10
(GDB) B 4
Breakpoint 1 at 0x40052c: file main. C, line 4.
(GDB) S
The program is not being run.
(GDB) r
Starting program:/home/yjbo/temp/main

Breakpoint 1, main () at main. C: 4
4 char * A = NULL;
(GDB) N
6 if (! A ){
(GDB) P
$1 = 0x0
(GDB) P! A
$2 = 1
(GDB) P! A + 1
$3 = 2