Re: File/Record locking on Linux with GPC

4 Dec 1997

      According to Pismo:
...
Perl and C have Flock, and GPC 
should (does?) have a similar call [...]
I just have discovered the following C header for `flock'
(see `man flock' under Linux; I hope that DJGPP provides
an equivalent function):

    int flock ( int fd, int operation )

The equivalent Pascal declaration is

    Function FLock ( fd, operation: Integer ): Integer; C;

You can look up the values for `operation' from the
C include file.

The remaining problem is how to get the file descriptor `fd'
from a Pascal `file'.  First, you can get a C-style `FILE'
pointer from a Pascal file using the `GetFile' function
(built-in in gpc-971001):

    Function GetFile ( Const F: <file type> ): Pointer;

(With GPC before gpc-971001, say

    Function GetFile ( Const F: <file type> ): Pointer;
    AsmName '_p_getfile';

to declare the function.)

Having that you can use the `filenum' function to get the
file number:

    int fileno ( FILE *stream );

or, in Pascal, 

    Function FileNo ( Stream: Pointer ): Integer; C;

With this declarations, the following should do what you wish:

    Function GetFile ( Const F: <file type> ): Pointer;
    AsmName '_p_getfile';

    Function FileNo ( Stream: Pointer ): Integer; C;

    Const
      SharedLock = 1;      (* From <sys/file.h> under Linux *)
      ExclusiveLock = 2;
      Unlock = 8;

    Var
      Foo: File of Bar;

    ...

    Flock ( FileNo ( GetFile ( Foo ) ), SharedLock );

Good luck,

    Peter

 Dipl.-Phys. Peter Gerwinski, Essen, Germany, free physicist and programmer
peter.gerwinski@uni-essen.de - http://home.pages.de/~peter.gerwinski/ [971005]
 maintainer GNU Pascal [971001] - http://home.pages.de/~gnu-pascal/ [971005]