memory leak in eval

lightspeed has asked for the wisdom of the Perl Monks concerning the following question:

All, I'm making calls to eval_pv in an application with an embedded interpreter. The final code was leaking memory, so I chopped it down to the point where I have only a call to eval_pv in a tight loop:


   #include <EXTERN.h>
   #include <perl.h>
   static PerlInterpreter *my_perl;
   main (int argc, char **argv, char **env)
   {
       STRLEN n_a;
       int i;
       char *embedding[] = { "", "-e", "0" };
       my_perl = perl_alloc();
       perl_construct( my_perl );
       perl_parse(my_perl, NULL, 3, embedding, NULL);
       perl_run(my_perl);
       for(i=0; i < 10000000; i++) {
         eval_pv("1", TRUE);
       }
       perl_destruct(my_perl);
       perl_free(my_perl);
   }
[download]

This doesn't create any variables in perl space (I don't think) and I throw away the SV* returned by eval_pv. Watching memory use as it runs, this still leaks memory compiled agains perl 5.8.3 and 5.6.1. As an experiment, I tried eval_sv with:

   #include <EXTERN.h>
   #include <perl.h>
   static PerlInterpreter *my_perl;
   main (int argc, char **argv, char **env)
   {
       STRLEN n_a;
       int i;
       SV* my_string;
       char *embedding[] = { "", "-e", "0" };
       my_perl = perl_alloc();
       perl_construct( my_perl );
       perl_parse(my_perl, NULL, 3, embedding, NULL);
       perl_run(my_perl);
       my_string = newSVpv("2", 0);
       for(i=0; i < 10000000; i++) {
         eval_sv(my_string, TRUE);
       }
       perl_destruct(my_perl);
       perl_free(my_perl);
   }
[download]

which leaks in the same way. However, if I use call_pv instead, this doesnt leak at all:


   #include <EXTERN.h>
   #include <perl.h>
   static PerlInterpreter *my_perl;
   main (int argc, char **argv, char **env)
   {
       STRLEN n_a;
       int i;
       char *embedding[] = { "", "-e", "sub fred {1;};" };
       my_perl = perl_alloc();
       perl_construct( my_perl );
       perl_parse(my_perl, NULL, 3, embedding, NULL);
       perl_run(my_perl);
       for(i=0; i < 10000000; i++) {
         dSP;
         ENTER;
         SAVETMPS;
         PUSHMARK(sp);
         call_pv("fred", G_DISCARD);
         SPAGAIN;
         LEAVE;
       }
       perl_destruct(my_perl);
       perl_free(my_perl);
   }
[download]

This seems fairly fundamental, so I must missing something. In my application, it is possible that eval_pv be called a large number of times (hundreds of millions). Any ideas? Many thanks!

Comment on memory leak in eval_pv? Select or Download Code

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Re: memory leak in eval_pv? by Trizor (Pilgrim) on May 15, 2007 at 16:58 UTC
`eval_(...)` ~~take the same flags as their cousins `call_(...)` in that you must specify `G_DISCARD` to discard the return value of the~~ `eval` or you'll leak SV's with refcount one. Update: After speaking with ysth I learned that `eval_pv(...)` doesn't care about G_DISCARD. While my initial diagnosis was correct, you're not freeing the SV returned by `eval_*(...)`, a solution that works is: `SvREFCNT_dec(eval_pv(...));` [download]	[reply] [d/l] [select]
Re^2: memory leak in eval_pv? by lightspeed (Initiate) on May 16, 2007 at 11:13 UTC
Hi Trizor, Many thanks for your reply - alas this did not seem to fix the leak. The code I have now is: `#include <EXTERN.h> #include <perl.h> static PerlInterpreter my_perl; main (int argc, char argv, char env) { STRLEN n_a; int i; char embedding[] = { "", "-e", "sub fred {1;};" }; my_perl = perl_alloc(); perl_construct( my_perl ); perl_parse(my_perl, NULL, 3, embedding, NULL); perl_run(my_perl); for(i=0; i < 10000000; i++) { SvREFCNT_dec(eval_pv("1;", TRUE)); } perl_destruct(my_perl); perl_free(my_perl); }` [download] and compiled with perl 5.8.3 still leaks. In a previous attempt (before I posted) I tried assigning the eval_pv() to an SV* and explicitly calling SV_free on it (and also setting it mortal with sv_2motral) but neither of these helped. I was coming to the conclusion that the leak may be inside eval_pv/sv iteself so I thought I'd post here to see if anyone else had seen similar issues. I guess the thing to try next is to build the latest 5.8.8 perl and try it with that. Many thanks, Shaun.	[reply] [d/l]
Re^3: memory leak in eval_pv? by lightspeed (Initiate) on May 17, 2007 at 12:33 UTC
Just to wrap this one up.... It appears that eval_pv (and eval_sv) return a mortal SV* that must be cleaned up by "topping and tailing" the call to eval_pv with: `ENTER; SAVETMPS; value = (eval_pv("1;", TRUE)); FREETMPS; LEAVE;` [download] The assignment SV* value is required, else errors about freeing unreferenced value are generated. Shaun.	[reply] [d/l]