How to create a block using the C API? mrb_yield keeps crashing!

I'm trying to create a block using the C API, but I keep getting a segfault. This is the code below:

mrb_value proc_test(mrb_state *mrb, mrb_value self) {
        return mrb_false_value();                                            
}

struct RProc *p = mrb_proc_new_cfunc(mrb, proc_test);

mrb_value b = mrb_obj_value(p);

mrb_yield(mrb, b, mrb_nil_value());

The problem seems to the variable mrb_value b. I have no idea why!

Even when I use mrb_p trying to see if I'm getting the expected object value it just crashes! What's going on here?

Valgrind gives me this output in regard to the error:

Invalid read of size 8
0x12DE5A: mrb_yield.cold (vm.c:999)
0x130C45: data_manager_parse (MRB_DataManager.c:54)
0x15A42B: mrb_vm_exec (vm.c:1792)
0x162CF5: mrb_vm_run (vm.c:1282)
0x1630AD: mrb_run (vm.c:3100)
0x1630AD: mrb_funcall_with_block (vm.c:681)
0x1636E9: mrb_funcall_with_block (vm.c:637)
0x163AAD: mrb_funcall_argv (vm.c:692)
0x163AAD: mrb_funcall (vm.c:484)
0x1307C5: RunGame (CORE_Main.c:142)
0x1308FB: CORE_Main (CORE_Main.c:45)
0x12E068: main (Main.c:17)
Address 0x18 is not stack'd, malloc'd or (recently) free'd

Why is it an invalid read?? And what is mrb_yield.cold??

Can I get an example of how to create a block using the C API?

Solved it!

[matz]

Since your code snippet is incomplete, I can only guess the situation. Probably, you passed the wrong value to mrb_yield function.

[dearblue]

I have come to the conclusion that this issue should be addressed.

This is because functions defined as methods can cause SIGSEGV if they use mrb_yield() or mrb_yield_argv() to call block arguments.
The block object to be given to such a method is created by mrb_proc_new_cfunc() as in this issue.

The place where SIGSEGV occurs is MRB_PROC_ENV(p)->stack[0] in the mrb_yield() function.
This is because MRB_PROC_ENV(p) is NULL for block objects created by mrb_proc_new_cfunc().

The following is an example for reference:

% cat 5932a.c
#include <mruby.h>
#include <mruby/compile.h>
#include <mruby/proc.h>
#include <stdlib.h>

static mrb_value
reporter(mrb_state *mrb, mrb_value self)
{
  mrb_value o = mrb_get_arg1(mrb);
  mrb_p(mrb, o);
  return mrb_nil_value();
}

int
main(int argc, char *argv[])
{
  int mode = (argc > 1) ? strtol(argv[1], NULL, 0) : 0;

  mrb_state *mrb = mrb_open();

  mrb_value b;
  if (mode == 0) {
    // Generate block objects that wrap C functions
    b = mrb_obj_value(mrb_proc_new_cfunc(mrb, reporter));
  } else {
    // Generate block object as top-level scope
    mrbc_context *mrbc = mrbc_context_new(mrb);
    mrbc->no_exec = TRUE;
    mrb_load_string_cxt(mrb, "a = nil", mrbc); // Allocate as the first argument
    b = mrb_load_string_cxt(mrb, "p a", mrbc);
    mrbc_context_free(mrb, mrbc);
  }

  mrb_value objspace = mrb_obj_value(mrb_module_get(mrb, "ObjectSpace"));
  mrb_sym mid = mrb_intern_lit(mrb, "each_object");
  mrb_funcall_with_block(mrb, objspace, mid, 0, NULL, b);
  if (mrb->exc) {
    mrb_print_error(mrb);
  }

  mrb_close(mrb);

  return 0;
}

%  $(bin/mruby-config --cc --cflags) 5932a.c $(bin/mruby-config --ldflags --libs)

% ./a.out 0    # case using mrb_proc_new_cfunc()
zsh: segmentation fault (core dumped)  ./a.out 0

% ./a.out 1    # case using mrb_load_string_cxt() with mrbc_context::no_exec
zsh: segmentation fault (core dumped)  ./a.out 1

@dearblue Thank you.