Random seed initialization

[Angelo Graziosi]

Surely I am missing something but any way I want to flag this for 
completeness.

The following code uses the subroutine example for RANDOM_SEED, cited in 
GCC manual (current development):

program rnd_init
   implicit none
   real :: rnd(9)
   call init_random_seed()
   call random_number(rnd)
   print *,rnd
contains
   SUBROUTINE init_random_seed()
     INTEGER :: i, n, clock
     INTEGER, DIMENSION(:), ALLOCATABLE :: seed

     CALL RANDOM_SEED(size = n)
     ALLOCATE(seed(n))

     CALL SYSTEM_CLOCK(COUNT=clock)

     seed = clock + 37 * (/ (i - 1, i = 1, n) /)
     CALL RANDOM_SEED(PUT = seed)

     DEALLOCATE(seed)
   END SUBROUTINE init_random_seed
end program rnd_init

Now, when I compile it using GCC-4.5.3, it prints different number 
sequences each time I run it.

Using GCC 4.9.0 20130324 it prints always the same sequence.

I have tested this on Cygwin with its GCC-4.5.3 packages,

$ gfortran -v
Using built-in specs.
COLLECT_GCC=gfortran
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-cygwin/4.5.3/lto-wrapper.exe
Target: i686-pc-cygwin
Configured with: 
/gnu/gcc/releases/respins/4.5.3-3/gcc4-4.5.3-3/src/gcc-4.5.3/configure 
--srcdir=/gnu/gcc/releases/respins/4.5.3-3/gcc4-4.5.3-3/src/gcc-4.5.3 
--prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin 
--libexecdir=/usr/lib --datadir=/usr/share --localstatedir=/var 
--sysconfdir=/etc --datarootdir=/usr/share --docdir=/usr/share/doc/gcc4 
-C --datadir=/usr/share --infodir=/usr/share/info 
--mandir=/usr/share/man -v --with-gmp=/usr --with-mpfr=/usr 
--enable-bootstrap --enable-version-specific-runtime-libs 
--libexecdir=/usr/lib --enable-static --enable-shared 
--enable-shared-libgcc --disable-__cxa_atexit --with-gnu-ld 
--with-gnu-as --with-dwarf2 --disable-sjlj-exceptions 
--enable-languages=ada,c,c++,fortran,java,lto,objc,obj-c++ 
--enable-graphite --enable-lto --enable-java-awt=gtk --disable-symvers 
--enable-libjava --program-suffix=-4 --enable-libgomp --enable-libssp 
--enable-libada --enable-threads=posix --with-arch=i686 
--with-tune=generic --enable-libgcj-sublibs CC=gcc-4 CXX=g++-4 
CC_FOR_TARGET=gcc-4 CXX_FOR_TARGET=g++-4 GNATMAKE_FOR_TARGET=gnatmake 
GNATBIND_FOR_TARGET=gnatbind --with-ecj-jar=/usr/share/java/ecj.jar
Thread model: posix
gcc version 4.5.3 (GCC)

and with a my build of 4.9.0 20130324,

$ gfortran-4.9 -v
Using built-in specs.
COLLECT_GCC=gfortran-4.9
COLLECT_LTO_WRAPPER=/usr/local/gfortran/libexec/gcc/i686-pc-cygwin/4.9.0/lto-wrapper.exe
Target: i686-pc-cygwin
Configured with: /home/angelo/work/gcc-4.9-20130324/configure 
--prefix=/usr/local/gfortran --program-suffix=-4.9 
--enable-languages=c,c++,fortran --enable-checking=release 
--enable-threads=posix --enable-libgomp --with-arch=native 
--with-tune=native --with-fpmath=sse --disable-bootstrap 
--disable-libmudflap --disable-shared
Thread model: posix
gcc version 4.9.0 20130324 (experimental) (GCC)


I suspect this behavior is to be expected for a snapshot...

May you confirm?


TIA,
  Angelo.