Use of #ifdef for portability - a specific case

[hzmonte]

In Solaris, the protoptype of psignal() is in siginfo.h. However, under Linux and Mac OS X, the protoptype of psignal() is in signal.h; siginfo.h does not exist. Some advocate writing 2 different .c files. Let's say I do not want to do that for such a minor discrepancy. How do I write a C program that can run on all 3 operating systems, by using #ifdef for example? That is, how can I do something like:

#ifdef SOLARIS
#  include <siginfo.h>
#else
#  include <signal.h>
#endif
int main ()
{ ...
   ... psignal() ...
   ...
}

Thanks.

[cbastard]

hope it will helphttp://www.cs.cf.ac.uk/Dave/C/node14.html

[hzmonte]

Thanks. I found another article Notes on Writing Portable Programs in C (http://www.literateprogramming.com/portableC.pdf) that is also good. It gives an example:
#ifdef SYSV
# include <fcntl.h>
#else
# include <sys/file.h>
#endif
Now my first uestion is: How do I find out whether a particular constant, e.g. SYSV, is defined in my OS? Barring reading the manual, is there a commonly available OS command that lists all the constants that are defined? And if an OS defines SYSV, does it absolutely mean that <fcntl.h> exists, for example? I mean, is it possible that a "SYSV-based" operating system does not have <fcntl.h>? In Spencer and Collyer's "#ifdef Considered Harmful" (1992), they argue that "the UNIX world is NOT cleanly split into System V and 4BSD camps, particularly with the advent of SVR4. Hybrid UNIXes are the rule, not the exception, nowadys." Therefore, one should not base the test on generalizations about an OS type (e.g. all SYSV systems have <fcntl.h>) . However, unfortunately, they do not offer an alternative way to handle a case like the above, without using #ifdef. Any suggestion?

[Ancient Dragon]

symbols are usually compiler dependent, not os dependent. one compiler may define SYSV while another compiler may or may not. So if you always use the same compiler on all operating systems you should be ok.

It would be impossible to get a list of all defined constants because programs are allowed to define their own and undefine those that may have been previously defined in header files.

[hzmonte]

Do you mean that a compiler for a System V-based OS would define SYSV, for example? GCC does not seem to define any OS-specific constants - see http://gcc.gnu.org/onlinedocs/gcc-3....ed-Macros.html.
"So if you always use the same compiler on all operating systems you should be ok. " What do you mean? Assumed that I always use GCC on all OSes, I still need to know whether the OS I am compiling the code on has fcntl.h or sys/file.h, say for example.

[Dave_Sinkula]

Of interest: Pre-defined C/C++ Compiler Macros?

[Ancient Dragon]

Do you mean that a compiler for a System V-based OS would define SYSV, for example? GCC does not seem to define any OS-specific constants - see http://gcc.gnu.org/onlinedocs/gcc-3....ed-Macros.html.
"So if you always use the same compiler on all operating systems you should be ok. " What do you mean? Assumed that I always use GCC on all OSes, I still need to know whether the OS I am compiling the code on has fcntl.h or sys/file.h, say for example.

all those constants are specific to a compiler -- there is no standard set of constants. If you use g++ or gcc on all platforms then I think the constants will normally be consistant from one platform to another. But don't expect any of the constants defined by gcc or g++ to be used by the cc compiler on AIX Unix or by any other compiler.

This lack of consistency among compilers makes writing cross-platform and cross-compiler programs very problematic. Just download any open source library that has been ported to different platforms and you will probably find the source code littered with conditional compile preprocessor statements. www.DataReel.com has a good example that supports quite a few compilers on both *nix and MS-Windows.

The link that Dave posted previously contains many of the most commonly used macros, but it is by far not complete. Each of the compilers have many more predefined macros that are not listed.

[hzmonte]

I found that with GCC, the command "touch foo.h; cpp -dM foo.h", assumed foo.h does not exist, prints out all GCC's predefined macros. And in Solaris 9, a macro named SVR4 is predefined with a value of 1. In GNU/Linux 2.4 and Mac OS X (the Darwin kernel), no such macro is defined, nor something like BSD for example. (On the other hand, sparc, __sparc__, unix, __unix, __unix__, sun, __sun, etc are predefined in Soalris; __unix, __linux, etc are predefined in Linux; __APPLE__, __MACH__, __POWERPC__, etc are predefined in Mac OS.)
So I guess in my particular case, I simply have to write
#ifdef SVR4
# include <siginfo.h>
#else
# include <signal.h>
#endif
unless and until I encounter an OS that predefines SVR4 but does not have <siginfo.h>, or an OS that does not predefine SVR4 but has psignal() declared in <signal.h>. Then I have to find a way to somehow further distinguish among them, assumed I do not use a tool such as autoconf.