Re: [libgpiod] Segmentation faults with

[Michael Beach <michaelb@xxxxxxxx>]

Hi all, when I tried running gpiodetectcxx (one of the example
programs for the C++ bindings) I was getting a segmentation fault on a
system with no GPIO devices. The problem seemed to occur in a call to
gpiod_chip_close() with a null pointer argument, which is made from
~chip_iter().

The reason for this seems to be that if we called
gpiod::make_chip_iter() when there were no GPIO devices in the system,
the _m_current field of chip_iter gets constructed as a
std::shared_ptr with a null pointer value, and so on destruction
chip_deleter() ends up calling gpiod_chip_close() with a null pointer
argument.

The solution seems to be greater care in how we initialise
chip_iter::_m_current, so that we never construct a std::share_ptr
that points to null. Please see the patch below...

diff --git a/bindings/cxx/iter.cpp b/bindings/cxx/iter.cpp
index 0f11057..3598dbc 100644
--- a/bindings/cxx/iter.cpp
+++ b/bindings/cxx/iter.cpp
@@ -58,10 +58,11 @@ bool chip_iter::operator!=(const chip_iter& rhs)
const noexcept
 }

 chip_iter::chip_iter(::gpiod_chip_iter *iter)
-       : _m_iter(iter, chip_iter_deleter),
-         _m_current(chip(::gpiod_chip_iter_next_noclose(this->_m_iter.get())))
+       : _m_iter(iter, chip_iter_deleter)
 {
+       ::gpiod_chip* current =
::gpiod_chip_iter_next_noclose(this->_m_iter.get());

+       this->_m_current = current ? chip(current) : chip();
 }

 chip_iter& chip_iter::operator++(void)