#311 set rndseed does not work

[marcel hendrix]

The attached circuit does not work as expected.
A "set rndseed=33" in a control section does not have
the expected effect that multiple runs of the file all
give exactly the same output. Using the command in
spinit is OK, but less convenient.

-marcel

test rndseed
v1 1 0 DC 150 trnoise ( 1V 10m 0 0 )
.tran 0.01 0.4

.control
    set rndseed=33
    run
    plot -i(v1)
.endc

.end

[marcel hendrix]

Here is a possible fix that allows to set rndseed on the
OS shell commandline before running ngspice. Setting rndseed
in the spinit file should also be OK. Setting rndseed in
a control section will not work.

/* Check if a seed has been set by the command 'set rndseed=value'
   in spinit (or on the OS commandline) with integer value > 0. 
   If available, call srand(value).
   This will override the call to srand in main.c.
   Checkseed should be put in front of any call to rand or CombLCGTaus.. .
*/
void checkseed(void) {
   int newseed;
   static int oldseed;
   static int gotit = 0, myseed = 0;
   if (gotit == 0) { 
        char *varseed = getenv("rndseed"); 
        if (varseed != NULL) myseed = (unsigned int)atoi(varseed); gotit = 1; }
   if (myseed > 0 && oldseed != myseed) { 
        srand((unsigned int)myseed); TausSeed(); oldseed = myseed; return; }
   if (cp_getvar("rndseed", CP_NUM, &newseed)) {
      if ((newseed > 0) && (oldseed != newseed)) { 
        srand((unsigned int)newseed); TausSeed(); oldseed = newseed; }
   }
}

[marcel hendrix]

The observed behavior of rndseed is not a bug as
it is clearly documented in the manual that rndseed
should be changed in the spinit or .spiceinit file.
Setting it in a .control section is not mentioned
anywhere.

That the patch causes rndseed to be (also) under
control of the OS shell is a convenient new feature,
not a bugfix.

This bug will be closed.

[Robert Larice]

~~~~~
:::html
Hello marcel,

There is an obscure feature in the ngspice frontend
which allows prefixing .control commands with "pre_"

I forgot the exact syntax,
but would you please try something like
pre_set rndseed = 42

Regards,
Robert