When using C++ exceptions to transport errno state, the compiled code that gets generated by g++ (4.5.3) for code such as the following
#include <cerrno>
#include <stdexcept>
#include <string>
class oserror : public std::runtime_error {
private:
static std::string errnotostr(int errno_);
public:
explicit oserror(int errno_) :
std::runtime_error(errnotostr(errno_)) {
}
};
void test() {
throw oserror(errno);
}
is rather unexpectedly (on Linux, x86_64)
.type _Z4testv, @function
...
movl $16, %edi
call __cxa_allocate_exception
movq %rax, %rbx
movq %rbx, %r12
call __errno_location
movl (%rax), %eax
movl %eax, %esi
movq %r12, %rdi
call _ZN7oserrorC1Ei
What this basically means is that errno as an argument to a C++ exception is pretty much useless due to the call to __cxa_allocate_exception preceding the call to __errno_location (which is the macro content of errno), where the former calls std::malloc and does not save errno state (at least as far as I understood the sources of __cxa_allocate_exception in eh_alloc.cc of libstdc++).
This means that in the case that memory allocation fails, the error number that was actually to be passed into the exception object gets overwritten with the error number that std::malloc set up. std::malloc gives no guarantee to save an existing errno state, anyway, even in the case of successful exit - so the above code is definitely broken in the general case.
On Cygwin, x86, the code that gets compiled (also using g++ 4.5.3) for test() is okay, though:
.def __Z4testv; .scl 2; .type 32; .endef
...
call ___errno
movl (%eax), %esi
movl $8, (%esp)
call ___cxa_allocate_exception
movl %eax, %ebx
movl %ebx, %eax
movl %esi, 4(%esp)
movl %eax, (%esp)