Re: How to shoot yourself in the foot: kill -9 postmaster - Mailing list pgsql-hackers

On Tue, Mar 06, 2001 at 08:19:12PM +0100, Peter Eisentraut wrote:
> Alfred Perlstein writes:
> 
> > Seriously, there's some dispute on the type that 'shm_nattch' is,
> > under Solaris it's "shmatt_t" (unsigned long afaik), under FreeBSD
> > it's 'short' (i should fix this. :)).
> 
> What I don't like is that my /usr/include/sys/shm.h (through other
> headers) has:
> 
> typedef unsigned long int shmatt_t;
> 
> /* Data structure describing a set of semaphores.  */
> struct shmid_ds
>   {
>     struct ipc_perm shm_perm;           /* operation permission struct */
>     size_t shm_segsz;                   /* size of segment in bytes */
>     __time_t shm_atime;                 /* time of last shmat() */
>     unsigned long int __unused1;
>     __time_t shm_dtime;                 /* time of last shmdt() */
>     unsigned long int __unused2;
>     __time_t shm_ctime;                 /* time of last change by shmctl() */
>     unsigned long int __unused3;
>     __pid_t shm_cpid;                   /* pid of creator */
>     __pid_t shm_lpid;                   /* pid of last shmop */
>     shmatt_t shm_nattch;                /* number of current attaches */
>     unsigned long int __unused4;
>     unsigned long int __unused5;
>   };
> 
> whereas /usr/src/linux/include/shm.h has:
> 
> struct shmid_ds {
>         struct ipc_perm         shm_perm;       /* operation perms */
>         int                     shm_segsz;      /* size of segment (bytes) */
>         __kernel_time_t         shm_atime;      /* last attach time */
>         __kernel_time_t         shm_dtime;      /* last detach time */
>         __kernel_time_t         shm_ctime;      /* last change time */
>         __kernel_ipc_pid_t      shm_cpid;       /* pid of creator */
>         __kernel_ipc_pid_t      shm_lpid;       /* pid of last operator */
>         unsigned short          shm_nattch;     /* no. of current attaches */
>         unsigned short          shm_unused;     /* compatibility */
>         void                    *shm_unused2;   /* ditto - used by DIPC */
>         void                    *shm_unused3;   /* unused */
> };
> 
> 
> Not only note the shm_nattch type, but also shm_segsz, and the "unused"
> fields in between.  I don't know a thing about the Linux kernel sources,
> but this doesn't seem right.

On Linux, /usr/src/linux/include is meaningless for anything in userland; 
it's meant only for building the kernel and kernel modules.  That Red Hat 
tends to expose it to user-level builds is a long-standing bug in Red 
Hat's distribution, in violation of the File Hierarchy Standard as well 
as explicit instructions from Linus & crew and from the maintainer of the 
C library.

User-level programs see what's in /usr/include, which only has to match 
what the C library wants.  It's the C library's job to do any mapping 
needed, and it does.  The C library is maintained very, very carefully
to keep binary compatibility with all old versions.  (One sometimes
encounters commercial programs that rely on a bug or undocumented/
usupported feature that disappears in a later library version.)

That is why there is no problem with version skew in the syscall
argument structures on a correctly-configured Linux system.  (On a
Red Hat system it is very easy to get them out of sync, but RH fans 
are used to problems.)

Nathan Myers
ncm@zembu.com