Re: #pragma once

[Ian Lance Taylor <iant at google dot com>]

Drgt <dragity@mail.ru> writes:

> It seems, that "#pragma once" isn't in ISO, and will never be, especially
> because it is Microsoft (am I right ?) C extension.
> (http://gcc.gnu.org/ml/gcc/2004-06/msg01887.html)

I believe that gcc was actually the first compiler to implement
"#pragma once".  It was added back in 1989 in gcc 1.35 by RMS.  It is,
perhaps, ironic that it is now considered to be a Microsoft extension.

Then in 1991 RMS added support for automatically detecting when a
header file puts all the non-comment text inside #ifndef/#define/
#endif.  When that happens, and gcc sees another #include for the same
header file, and the relevant macro is #defined, gcc will skip reading
the header file entirely.  That first appeared in gcc 2.0.  Given that
ability, RMS decided that there was no need to support #pragma once,
and, since he doesn't like #pragma (or at least didn't at the time),
he changed #pragma once at that time to issue an unavoidable warning.
Note that #pragma once still worked; it just issued a warning.

But #pragma once was still used in practice, so the warning was
removed by Jason Merrill in 2003, for gcc 3.3.

In short, #pragma once works fine today.  What makes you think that it
doesn't work?

Ian