c/4247: gcc fills variable with random data

c/4247: gcc fills variable with random data

[twiine]

>Number:         4247
>Category:       c
>Synopsis:       gcc fills variable with random data
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Thu Sep 06 05:06:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     twiine@web.de
>Release:        gcc version 2.95.2 19991024 (release)
>Organization:
>Environment:
SuSE Linux 7.0 (Kernel 2.2.16)
>Description:
The expression result[c++] = files; sets the c variable to a random value. (Seems to be c = files + 1). The problem only appears when c is initialized with -1. 
The error also occurs with the latest snapshot of gcc.

Command line: gcc -Wall -o test test.c
>How-To-Repeat:
int main(int argc, char *argv[])
{
  char *files, *result[5];
  int c=-1;

  result[c++] = files;
  return 0;
}
>Fix:
The expression 
    c++;
    result[c] = files;
works fine or if you enable optimzing.
>Release-Note:
>Audit-Trail:
>Unformatted:

Re: c/4247: gcc fills variable with random data

[aoliva]

Synopsis: gcc fills variable with random data

State-Changed-From-To: open->closed
State-Changed-By: aoliva
State-Changed-When: Sun Sep  9 14:51:29 2001
State-Changed-Why:
    This is the kind of thing to be expected when you invoke undefined behavior.  After c=-1, result[c++] indexes result[-1], and then increments c.  -1 is not a valid index into result.  From that point on, all bets are off.  Perhaps you meant result[++c] ?

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=4247&database=gcc