StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PORuby C Extension - 64bit pointer being truncated causing a segfault
text
Body
copied!<p>I'm working on trying to get a ruby extension written in c, working with ruby 1.9.2 on Mac OSX. I keep getting a segfault caused by a 64-bit pointer returned from a function being truncated to a 32-bit value and trying to access low memory.</p> <p>Here's what I've figured out about it. VALUE is a long, defined in the ruby source code, and the code won't compile unless sizeof(long) is 8 bytes.</p> <p>In file_b.c:</p> <pre><code>VALUE rb_helper_function_foo(VALUE file) { VALUE blah; ... //Using gdb, blah right here is a 64bit address, ex: 0x1 009d 62a0 return blah; } </code></pre> <p>In file_a.c</p> <pre><code>VALUE func_do_stuff(int argc, VALUE *argv, VALUE obj) { VALUE filename, file_path; ... // But here, the size of what is returned is 4 bytes // file_path has a size of 8 bytes though file_path = rb_helper_function_foo(filename); //This statement will segfault :-( if(!RTEST(file_path)) { ... } ... } </code></pre> <p>At the end of the function call, the return value has the correct address, but only the lower 4 bytes get returned: 0x009d52a0 instead of 0x1009d52a0. Accessing the low memory address is causing a segfault.</p> <p>In the assembly code that gets called during the return, there's this instruction</p> <pre><code>movslq %eax, %r12 </code></pre> <p>which only copies the lower 4 bytes, but I need all 8 bytes of %rax to be copied.</p> <p>I'm on ruby 1.9.2-p290 (though it did the same with p180), Mac OSX 10.6.8, using xcode 4.0.1 (with gcc 4.2.1), but I also tried upgrading to gcc 4.6.1. It has also previously tried xcode 3.2.</p> <p>Thanks for any help you can give me.</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload