1. 23
    "(null)" is a bad idea c compilers programming daemonology.net
    pushcx avatar via pushcx 6 years ago |
    Archive.org Archive.today Ghostarchive
    | 7 comments
  1. 3
    tedu avatar tedu 6 years ago | link

    Heh. http://www.tedunangst.com/flak/post/printf-null-in-the-wild

    1. 4
      inactive-user avatar inactive-user 6 years ago | link

      It also happens on OpenBSD.

      1. 4
        mulander avatar mulander 6 years ago | link

        What happens on OpenBSD?

        $ vi null.c
$ make null
cc -O2 -pipe    -o null null.c 
$ ./null                                                                                                                                      
Segmentation fault (core dumped) 
$ sysctl kern.version                                                                                                                         
kern.version=OpenBSD 5.8 (GENERIC) #1153: Mon Aug  3 05:31:43 MDT 2015
    deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC

$ cat null.c                                                                                                                                  
null.c     null.core  
$ cat null.c 
#include <stdio.h>

int
main(int argc, char * argv[])
{

        printf("%s\n", NULL);
}

$

        From gdb on the core file:

        Core was generated by `null'.
Program terminated with signal 11, Segmentation fault.

#0  strlen () at /usr/src/lib/libc/arch/amd64/string/strlen.S:124
124     /usr/src/lib/libc/arch/amd64/string/strlen.S: No such file or directory.
        in /usr/src/lib/libc/arch/amd64/string/strlen.S
(gdb) bt
#0  strlen () at /usr/src/lib/libc/arch/amd64/string/strlen.S:124
#1  0x00001e30183ddade in puts (s=0x0) at /usr/src/lib/libc/stdio/puts.c:45
#2  0x00001e2d521009f1 in _start () from /home/mulander/code/c/null
#3  0x0000000000000000 in ?? ()
Current language:  auto; currently asm
(gdb) Quit
        1. 13
          tedu avatar tedu 6 years ago | link

          That’s because gcc helpfully replaces printf with fputs. Try a format string like “null prints like %s\n”.

          1. 3
            mulander avatar mulander 6 years ago | link

            Hah! Thanks learned something interesting today :)

            printf("test: %s\n", NULL);


$ ./null                                                                                                                                      
test: (null)
            1. 3
              friendlysock avatar friendlysock 6 years ago | link

              Wait, what? wtf?

              If I asked for printf, I want printf.

              1. 10
                owen avatar owen 6 years ago | link

                I mean it’s an optimizing compiler, and it knows lots of things that can be optimized.