Portability of #warning preprocessor directive

Question

I know that the #warning directive is not standard C/C++, but several compilers support it, including gcc/g++. But for those that don't support it, will they silently ignore it or will it result in a compile failure? In other words, can I safely use it in my project without breaking the build for compilers that don't support it?

Answer 1

Actually most compilers that I know about ignore unknown #pragma directives, and output a warning message - so in the worst case, you'll still get a warning.

Answer 2

It is likely that if a compiler doesn't support #warning, then it will issue an error. Unlike #pragma, there is no recommendation that the preprocessor ignore directives it doesn't understand.

Having said that, I've used compilers on various different (reasonably common) platforms and they have all supported #warning.

Answer 3

You are likely to get at least an unrecognized directive warning from compilers that don't recognize #warning, even if the code block is not included in your compilation. That might or might not be treated as an error - the compiler could legitimately treat it as an error, but many would be more lax.

Are you aware of (can you name) a compiler other than GCC/G++ that provides #warning? [Edited: Sun Solaris 10 (Sparc) and the Studio 11 C/C++ compilers both accept #warning.]

Answer 4

I had this problem once with a compiler for an Atmel processor. And it did generate preprocessor errors due to the unknown #warning token.

Unfortunately the solution seemed to be to convert the whole source tree to use the #pragma equivalent and accept that the build behavior was going to differ if using gcc.

Answer 5

It should be noted that MSVC uses the syntax:

#pragma message ( "your warning text here" )

The usual #warning syntax generates a fatal error

C1021: invalid preprocessor command 'warning'

so it is not portable to those compilers.