4 ms·

Has anyone ever seen any behaviour other than a segfault in the wild, though? Would any real-world compiler author decide that some other behaviour was reasonab

Has anyone ever seen any behaviour other than a segfault in the wild, though? Would any real-world compiler author decide that some other behaviour was reasonable? I can't imagine making that decision myself.

sdk77 11y ago
On some microcontrollers I know a pointer to address 0 really is just that. I tested compiling the following unsigned short *a = 0; ((void (*)(void))a)(); which may cause a jump to address 0 (usually the reset vector). On R8C with the NC30 compiler there are no warnings and the output is: MOV.W:Q #0H, -2H[FB] MOV.W:G -2H[FB], R0 MOV.W:Q #0H, R2 JSRI.A R2R0 It jumps to 0. But on PIC 16 with the XC compiler a warning is emitted warning: (1471) indirect function call via a NULL pointer ignored and the statement is not compiled at all.
Kristine1975 11y ago
On MacOS Classic (no memory protection) you can happily write to address zero and replace whatever is stored there (I think Apple wisely left that memory address unused, though).
pm215 11y ago
On BSD on the VAX, address 0 used to happen to hold 0, so dereferencing NULL got you zero. That was at the time a sufficiently popular platform that some programs assumed the behaviour (eg, that a null pointer could be used as an empty string), and you can still find older docs on the web exhorting the reader to avoid "all the world's a VAX" syndrome.
TorKlingberg 11y ago
A classic problem is code like this: printf("a=%p *a=%d", a, *a); if (a == NULL) return; An optimizing compiler will likely remove the NULL check completely because the printf above it has undefined behavior if a is NULL.
krylon 11y ago
I think on DOS in real mode address 0 was not special. Some(?) compilers would add code to programs to check that location on program exit to see if its value had changed and emit an error. (I think I read about this somewhere, but it was a while ago, and I never really programmed for DOS (well, one little program to set the serial port to some specific settings, but that was about ten lines including whitespace and a little comment explaining what the code did).)

sdk77 11y ago

On some microcontrollers I know a pointer to address 0 really is just that. I tested compiling the following unsigned short *a = 0; ((void (*)(void))a)(); which may cause a jump to address 0 (usually the reset vector). On R8C with the NC30 compiler there are no warnings and the output is: MOV.W:Q #0H, -2H[FB] MOV.W:G -2H[FB], R0 MOV.W:Q #0H, R2 JSRI.A R2R0 It jumps to 0. But on PIC 16 with the XC compiler a warning is emitted warning: (1471) indirect function call via a NULL pointer ignored and the statement is not compiled at all.

Kristine1975 11y ago

On MacOS Classic (no memory protection) you can happily write to address zero and replace whatever is stored there (I think Apple wisely left that memory address unused, though).

pm215 11y ago

On BSD on the VAX, address 0 used to happen to hold 0, so dereferencing NULL got you zero. That was at the time a sufficiently popular platform that some programs assumed the behaviour (eg, that a null pointer could be used as an empty string), and you can still find older docs on the web exhorting the reader to avoid "all the world's a VAX" syndrome.

TorKlingberg 11y ago

A classic problem is code like this: printf("a=%p *a=%d", a, *a); if (a == NULL) return; An optimizing compiler will likely remove the NULL check completely because the printf above it has undefined behavior if a is NULL.

krylon 11y ago

I think on DOS in real mode address 0 was not special. Some(?) compilers would add code to programs to check that location on program exit to see if its value had changed and emit an error. (I think I read about this somewhere, but it was a while ago, and I never really programmed for DOS (well, one little program to set the serial port to some specific settings, but that was about ten lines including whitespace and a little comment explaining what the code did).)