Question

I’m not sure what I’m doing wrong. I’ve got a Ruby hash I’m passing in as the third argument for this function, Test::test().

static VALUE nm_test(VALUE self, VALUE str, VALUE from, VALUE options) {
  if (TYPE(options) != T_HASH) {
    // raises error
  }

  if (rb_funcall(options, rb_intern("has_key?"), 1, rb_intern("dtype")) == Qtrue) { // SEGFAULT
    // success
  }

  // other stuff
}

void Init_test() {
  cTest = rb_define_module("Test");

  rb_define_singleton_method(cTest, "test", nm_test, 3);
}

I’ve heard that you’ll get a segfault if you try to call a function that doesn’t exist on some object. I also checked that the object responds to has_key?:

if (TYPE(options) != T_HASH) {
  rb_raise(rb_eArgError, "third argument to repack must be hash");
} else if (!rb_respond_to(options, rb_intern("has_key?"))) {
  rb_raise(rb_eArgError, "hash does not respond to has_key?!");
}

This doesn’t trigger an error. So it’s definitely a Hash, and definitely has Hash#has_key?.

Here’s the actual backtrace from the actual code (not the code above). The segfault is on the rb_funcall line.

Program received signal SIGSEGV, Segmentation fault.
rb_type (obj=44840) at ./include/ruby/ruby.h:1344
1344    ./include/ruby/ruby.h: No such file or directory.
(gdb) bt
#0  rb_type (obj=44840) at ./include/ruby/ruby.h:1344
#1  rb_any_hash (a=44840) at hash.c:83
#2  0x080fc570 in st_lookup (table=0x8fa0470, key=44840, value=0x0) at st.c:341
#3  0x08066ef8 in rb_hash_has_key (hash=144171660, key=44840) at hash.c:1516
#4  0x08157f7d in vm_call0 (th=0x8265b88, recv=144171660, id=5127, argc=1, argv=0xbfffdf60, me=0x82ce480)
    at vm_eval.c:79
#5  0x081587c4 in rb_call (scope=CALL_FCALL, argv=0xbfffdf60, argc=1, mid=5127, recv=144171660) at vm_eval.c:456
#6  rb_funcall (recv=144171660, mid=5127, n=1) at vm_eval.c:658
#7  0xb70933f8 in nm_rbstring_matlab_repack (self=142142600, str=144171680, from=12992526, options=144171660)
    at ../../../../ext/nmatrix/util/io.cpp:210

Any ideas?

Answer 1

Figured it out. It seems that rb_intern("whatever!") returns an ID, and rb_funcall needs a VALUE. So the call to rb_funcall should look like this:

rb_funcall(options, rb_intern("has_key?"), 1, ID2SYM(rb_intern("dtype")));

It’s confusing because :has_key? gets to be an ID, and works just fine — but the arguments themselves need to be of type VALUE.