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From: "geoffreydgr at icloud dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug analyzer/107289] New: - -Wanayzer-null-dereference false
 positive with  f = *b
Date: Mon, 17 Oct 2022 09:59:10 +0000
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X-Bugzilla-Component: analyzer
X-Bugzilla-Version: 12.1.0
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107289

            Bug ID: 107289
           Summary: - -Wanayzer-null-dereference false positive with  f =3D
                    *b
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: geoffreydgr at icloud dot com
  Target Milestone: ---

I got a false positive error when compiling the following "minimal, complete
and verifiable example (MCVE)" program

```
int a=3D1;
int *b =3D &a;
void c () {
    int f;
    f =3D *b;
}=20=20=20
void e(){
    if (0 =3D=3D b){
        int *g =3D 0;
    }
}
void d() {  e(); c();}
int main(){
    d();
}
```

Compiling the above code with gcc 12.1 with `-O0 -fanalyzer` in
https://godbolt.org/ results in :

```
<source>: In function 'void c()':
<source>:5:7: warning: dereference of NULL 'b' [CWE-476]
[-Wanalyzer-null-dereference]
    5 |     f =3D *b;
      |     ~~^~~~
  'void d()': events 1-2
    |
    |   12 | void d() {  e(); c();}
    |      |      ^      ~~~
    |      |      |       |
    |      |      |       (2) calling 'e' from 'd'
    |      |      (1) entry to 'd'
    |
    +--> 'void e()': events 3-6
           |
           |    7 | void e(){
           |      |      ^
           |      |      |
           |      |      (3) entry to 'e'
           |    8 |     if (0 =3D=3D b){
           |      |     ~~
           |      |     |
           |      |     (4) following 'true' branch...
           |    9 |         int *g =3D 0;
           |      |              ~
           |      |              |
           |      |              (5) ...to here
           |      |              (6) 'b' is NULL
           |
    <------+
    |
  'void d()': events 7-8
    |
    |   12 | void d() {  e(); c();}
    |      |             ~^~  ~~~
    |      |              |    |
    |      |              |    (8) calling 'c' from 'd'
    |      |              (7) returning to 'd' from 'e'
    |
    +--> 'void c()': events 9-10
           |
           |    3 | void c () {
           |      |      ^
           |      |      |
           |      |      (9) entry to 'c'
           |    4 |     int f;
           |    5 |     f =3D *b;
           |      |     ~~~~~~
           |      |       |
           |      |       (10) dereference of NULL 'b'
```

It should not be a null pointer deference, because b is a pointer to a. GCC
seems to be fooled by line 8  if (0 =3D=3D b){...}. But if I change the exa=
mple
program to the following one, the false positive error disappears:

```
int a=3D1;
int *b =3D &a;
void c () {
    int f;
    f =3D *b;
}=20=20=20
void d() { c();}
int main(){
    if (0 =3D=3D b){
        int *g =3D 0;
    }
    d();
}
```=