Re: Suspicion of a compiler bug in clang: using ternary operator in ereport()

Hi,

On 28/01/14 16:43, Christian Kruse wrote:
> ereport(FATAL,
> (errmsg("could not map anonymous shared memory: %m"),
> (errno == ENOMEM) ?
> errhint("This error usually means that PostgreSQL's request "
> "for a shared memory segment exceeded available memory "
> "or swap space. To reduce the request size (currently "
> "%zu bytes), reduce PostgreSQL's shared memory usage, "
> "perhaps by reducing shared_buffers or "
> "max_connections.",
> *size) : 0));
>
> did not emit a errhint when using clang, although errno == ENOMEM was
> true. The same code works with gcc.

According to http://llvm.org/bugs/show_bug.cgi?id=18644#c5 this is not
a compiler bug but a difference between gcc and clang. Clang seems to
use a left-to-right order of evaluation while gcc uses a right-to-left
order of evaluation. So if errmsg changes errno this would lead to
errno == ENOMEM evaluated to false. I added a watch point on errno and
it turns out that exactly this happens: in src/common/psprintf.c line
114

nprinted = vsnprintf(buf, len, fmt, args);

errno gets set to 0. This means that we will miss errhint/errdetail if
we use errno in a ternary operator and clang.

Should we work on this issue?

Best regards,

--
Christian Kruse http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services