Re: mmap() return value when length == 0

[Justin Guyett]

On 23 Aug 2001, jacob berkman wrote:

> on linux (2.4.9 and 2.2.18), the mmap() syscall will return NULL if the
> length argument is 0 rather than returning MAP_FAILED (-1).  this is
> different than both solaris and hp-ux, and the linux man page doesn't
> indicate that it should do this.
>
> so, is this indeed the desired behaviour or a longstanding bug?

Surely the posix standard would say.

The sol8 manpage says specifically that EINVAL is returned if len <= 0;

And an openbsd and freebsd box i tried it on happily do the mmaps as
well.

All three (linux, sol8, fbsd) segfault with the following code for various
values of x and y.

fd = open();
marea = mmap(0, x, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
munmap(marea, y);
fclose(fd);

mmap:
 linux
  succeeds
 sol8
  succeeds if x>0, fails otherwise
 fbsd4.3
  succeeds

munmap:
 linux
  succeeds unless x>0 and y>x, in which case it segfaults
 sol8
  succeeds
 fbsd
  succeeds

close:
 linux
  succeeds
 sol8
  segfaults if y>x
 fbsd
  segfaults if y>x

justin