Forgetful Interpreter

[Bill Orcutt]

Having seen the number of lost souls asking questions about embedding
Python in the archives of this group, I hesitate to add myself to
their number, but I've hit a problem I can't quite get my head around.

I have a simple C program that embeds a python interpreter to execute
python commands and return stdout, and to a point everything works as
intended. The problem that each command seems to be executed in its
own context and knows nothing of what's come before. So if enter
"print 2+2", I get back "4", but if I enter "a=2+2" then enter "print
a", I get back the error, "a is undefined." My guess is that this has
something to calling PyRun_SimpleStr ing inside of a child process, but
I can't get much further than this.

Any assistance is appreciated.

thanks

-Bill

//call this once at the beginning to initialize
void * nyptho_doinit(t _gogo* x, short flag)
{
Py_Initialize() ;
char capture_stdout[] ="import sys,
StringIO\n\sys. stderr=sys._cap ture=StringIO.S tringIO()\n";
int rc = PyRun_SimpleStr ing(capture_std out);
return x;
}

//this executes the command
FILE * nyptho_exec(t_g ogo* x, char * cmd, short bflag, short fflag)
{
int pfd[2],i,z=0;
pid_t pid;
FILE *fp;
extern int errno;

if (pipe(pfd) < 0)
return(NULL); // errno set by pipe()

if ( (pid = fork()) < 0)
return(NULL); // errno set by fork()

else if (pid == 0) // child
{
close(pfd[0]);

// close all descriptors in childpid[]
if (x->childpid[0] > 0)
close(0);

PyRun_SimpleStr ing(cmd);

//PyRun_SimpleStr ing("import sys");
PyObject* sysmod = PyImport_Import Module("sys");
PyObject* capobj =
PyObject_GetAtt rString(sysmod, "_capture") ;
PyObject* strres =
PyObject_CallMe thod(capobj,"ge tvalue",0);
char * dodo = PyString_AsStri ng(strres);
strcpy(globalBu f,dodo);

write(pfd[1], globalBuf, (long)strlen(gl obalBuf));
globalBuf[0]='\0';

_exit(127); //child exits
}
// parent
close(pfd[1]);

if ( (fp = fdopen(pfd[0], "r")) == NULL)
return(NULL);

x->childpid[fileno(fp)] = pid; // remember child pid for this
fd
return(fp);

}

[Dave Kuhlman]

Re: Forgetful Interpreter

Bill Orcutt wrote:
[color=blue]
> Having seen the number of lost souls asking questions about
> embedding Python in the archives of this group, I hesitate to add
> myself to their number, but I've hit a problem I can't quite get
> my head around.
>
> I have a simple C program that embeds a python interpreter to
> execute python commands and return stdout, and to a point
> everything works as intended. The problem that each command seems
> to be executed in its own context and knows nothing of what's come
> before. So if enter "print 2+2", I get back "4", but if I enter
> "a=2+2" then enter "print a", I get back the error, "a is
> undefined." My guess is that this has something to calling
> PyRun_SimpleStr ing inside of a child process, but I can't get much
> further than this.[/color]

Take a look at Python-2.3.2/Demo/embed/demo.c in the Python source
code distribution for clues. That program calls
PyRun_SimpleStr ing() several times with the same context. If you
do not switch interpreters (thread state, whatever) or
re-initialize etc, then each call to PyRun_SimpleStr ing() should
have the same global variables etc.

Why are you calling fork()? Someone else will have to tell you
the consequences of doing that.

Dave

[snip]

--
Dave Kuhlman

http://www.rexx.com/~dkuhlman

dkuhlman@rexx.c om

[Bill Orcutt]

Calling PyRun_SimpleStr ing in a Child Process. [Was Forgetful Interpreter]

Thanks for your reply. I agree that PyRun_SimpleStr ing should be
running in the same context-- its worked that way for me in the past--
The only thing that's different this time is fork() and that's what
I'm hoping someone will tell me how to deal with, since I'm stuck with
it this time.

I'm updating the subject line to something more descriptive.

thanks

-Bill

Dave Kuhlman <dkuhlman@rexx. com> wrote in message news:<bmprfj$pe m2q$1@ID-139865.news.uni-berlin.de>...[color=blue]
> Bill Orcutt wrote:
>[color=green]
> > Having seen the number of lost souls asking questions about
> > embedding Python in the archives of this group, I hesitate to add
> > myself to their number, but I've hit a problem I can't quite get
> > my head around.
> >
> > I have a simple C program that embeds a python interpreter to
> > execute python commands and return stdout, and to a point
> > everything works as intended. The problem that each command seems
> > to be executed in its own context and knows nothing of what's come
> > before. So if enter "print 2+2", I get back "4", but if I enter
> > "a=2+2" then enter "print a", I get back the error, "a is
> > undefined." My guess is that this has something to calling
> > PyRun_SimpleStr ing inside of a child process, but I can't get much
> > further than this.[/color]
>
> Take a look at Python-2.3.2/Demo/embed/demo.c in the Python source
> code distribution for clues. That program calls
> PyRun_SimpleStr ing() several times with the same context. If you
> do not switch interpreters (thread state, whatever) or
> re-initialize etc, then each call to PyRun_SimpleStr ing() should
> have the same global variables etc.
>
> Why are you calling fork()? Someone else will have to tell you
> the consequences of doing that.
>
> Dave
>
> [snip][/color]

[Jeff Epler]

Re: Forgetful Interpreter

What you are doing won't work -- fork() behaves as though it makes a
copy of all the data in the current process's address spaces, so changes
in the child aren't mirrored in the parent.

Example C program:

#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>

int i = 0;

void increment_i_in_ forked_subproce ss() {
pid_t pid = fork();
if (pid == -1) {
perror("fork"); exit(1);
}
if (pid == 0) {
i++;
printf("In subprocess %d, i=%d\n", getpid(), i);
exit(0);
}
wait(NULL);
printf("Subproc ess exited\n");
}

int main(void) {
int j;
for(j=0; j<10; j++) {
increment_i_in_ forked_subproce ss();
}
return 0;
}

$ gcc orcutt.c && ./a.out
In subprocess 16632, i=1
Subprocess exited
In subprocess 16633, i=1
Subprocess exited
In subprocess 16634, i=1
Subprocess exited
In subprocess 16635, i=1
Subprocess exited
In subprocess 16636, i=1
Subprocess exited
In subprocess 16637, i=1
Subprocess exited
In subprocess 16638, i=1
Subprocess exited
In subprocess 16639, i=1
Subprocess exited
In subprocess 16640, i=1
Subprocess exited
In subprocess 16641, i=1
Subprocess exited