-lpthread vs -pthread: does -D_REENTRANT matter?

[Jilles Tjoelker]

On Mon, Oct 08, 2012 at 12:17:08PM -0400, Eitan Adler wrote:
> The only difference between -lpthread and -pthread that I could see is
> that the latter also sets -D_REENTRANT.
> However, I can't find any uses of _REENTRANT anywhere outside of a few
> utilities that seem to define it manually.

> Testing with various manually written pthread programs resulted in
> identical binaries, let alone identical results.

> Is there an actual difference between -pthread and -lpthread or is
> this just a historical artifact?

In some cases, -pthread also affects the compiler's code generation. On
some RISC architectures, compilers may try to avoid loads and stores of
less than 32 bits.

For example (untested):
  struct { int n; char a, b, c, d; } *p;
  p->a = p->b = p->c = 0;

The compiler might load p->d and then store the 32 bits containing a, b,
c and d at once. This causes a race condition if p->d is written
concurrently.

Because C99 does not specify threading, it allows these transformations.
In C11, they are forbidden. Passing -pthread disables them as well.

-- 
Jilles Tjoelker