How can I determine if the operating system is POSIX in C?

Question

Related questions
How can I detect the operating system in C/C++?

How can I find out what operating system I am running under in GCC or in ANSI C?

I'd be fine If I can know if I'm running on POSIX.

---

UPDATE:

It doesn't make a difference to me whether it is at compile time or run time. I'm using this in a debug routine, so performance isn't that important.

I'm looking for the path separator. Windows & Unix/Linux/BSD would be fine.

And, I'm trying to find the basename on a path. I found some solutions, but the solutions include a lot of includes which I do not want. I'm going to mod up this solution.

---

I'm on Mac OS X 10.4.11, and following this URL, I executed and got the following output:

mac $ touch myfile.c mac $ gcc -std=c99 -E -dM myfile.c #define __DBL_MIN_EXP__ (-1021) #define __FLT_MIN__ 1.17549435e-38F #define __CHAR_BIT__ 8 #define __WCHAR_MAX__ 2147483647 #define __DBL_DENORM_MIN__ 4.9406564584124654e-324 #define __FLT_EVAL_METHOD__ 0 #define __DBL_MIN_10_EXP__ (-307) #define __FINITE_MATH_ONLY__ 0 #define __GNUC_PATCHLEVEL__ 1 #define __SHRT_MAX__ 32767 #define __LDBL_MAX__ 1.79769313486231580793728971405301e+308L #define __APPLE_CC__ 5367 #define __UINTMAX_TYPE__ long long unsigned int #define __LDBL_MAX_EXP__ 1024 #define __SCHAR_MAX__ 127 #define __USER_LABEL_PREFIX__ _ #define __STDC_HOSTED__ 1 #define __LDBL_HAS_INFINITY__ 1 #define __DBL_DIG__ 15 #define __FLT_EPSILON__ 1.19209290e-7F #define __LDBL_MIN__ 2.00416836000897277799610805135016e-292L #define __ppc__ 1 #define __strong  #define __APPLE__ 1 #define __DECIMAL_DIG__ 33 #define __LDBL_HAS_QUIET_NAN__ 1 #define __DYNAMIC__ 1 #define __GNUC__ 4 #define __DBL_MAX__ 1.7976931348623157e+308 #define __DBL_HAS_INFINITY__ 1 #define __STRICT_ANSI__ 1 #define __weak  #define __DBL_MAX_EXP__ 1024 #define __LONG_LONG_MAX__ 9223372036854775807LL #define __GXX_ABI_VERSION 1002 #define __FLT_MIN_EXP__ (-125) #define __DBL_MIN__ 2.2250738585072014e-308 #define __DBL_HAS_QUIET_NAN__ 1 #define __REGISTER_PREFIX__  #define __NO_INLINE__ 1 #define _ARCH_PPC 1 #define __FLT_MANT_DIG__ 24 #define __VERSION__ "4.0.1 (Apple Computer, Inc. build 5367)" #define __BIG_ENDIAN__ 1 #define __SIZE_TYPE__ long unsigned int #define __FLT_RADIX__ 2 #define __LDBL_EPSILON__ 4.94065645841246544176568792868221e-324L #define __NATURAL_ALIGNMENT__ 1 #define __FLT_HAS_QUIET_NAN__ 1 #define __FLT_MAX_10_EXP__ 38 #define __LONG_MAX__ 2147483647L #define __FLT_HAS_INFINITY__ 1 #define __STDC_VERSION__ 199901L #define _BIG_ENDIAN 1 #define __LDBL_MANT_DIG__ 106 #define __WCHAR_TYPE__ int #define __FLT_DIG__ 6 #define __INT_MAX__ 2147483647 #define __LONG_DOUBLE_128__ 1 #define __FLT_MAX_EXP__ 128 #define __DBL_MANT_DIG__ 53 #define __WINT_TYPE__ int #define __LDBL_MIN_EXP__ (-968) #define __MACH__ 1 #define __LDBL_MAX_10_EXP__ 308 #define __DBL_EPSILON__ 2.2204460492503131e-16 #define __INTMAX_MAX__ 9223372036854775807LL #define __FLT_DENORM_MIN__ 1.40129846e-45F #define __PIC__ 1 #define __FLT_MAX__ 3.40282347e+38F #define __FLT_MIN_10_EXP__ (-37) #define __INTMAX_TYPE__ long long int #define __GNUC_MINOR__ 0 #define __DBL_MAX_10_EXP__ 308 #define __LDBL_DENORM_MIN__ 4.94065645841246544176568792868221e-324L #define __PTRDIFF_TYPE__ int #define __LDBL_MIN_10_EXP__ (-291) #define __LDBL_DIG__ 31 #define __POWERPC__ 1 mac $ 

Answer 1

The Single UNIX Specification requires the existence of unistd.h, which can tell you the POSIX version (via the _POSIX_VERSION macro).

But how can you include unistd.h if you don't know yet that you are in fact compiling on a UNIX?

That is where this GCC document comes handy. According to it, testing for the presence, or evaluation-to-true of __unix__ should tell you that the system is a UNIX. So:

#ifdef __unix__ /* Yes it is a UNIX because __unix__ is defined.  */  #include <unistd.h>  /* You can find out the version with _POSIX_VERSION. .. ..  */  #endif 

__unix__ is not defined on Mac OS X, so to account for that, you could instead do:

#if defined (__unix__) || (defined (__APPLE__) && defined (__MACH__)) 

To get a list of system specific predefined macros on your system, you may execute:

cpp -dM /dev/null 

For example, my GNU/Linux system also additionally defines __linux__ and __gnu_linux__ apart from __unix__ and a bunch of other stuff.

---

Another useful document that you must look at is this Wiki.

It goes on to present a way of detecting the presence and version of POSIX in a way similar to the one I described above.

---

EDIT: Since you really want to do all this because you want to decide which directory separator to use, look at this URL. It says:

Note File I/O functions in the Windows API convert "/" to "\" as part of converting the name to an NT-style name, except when using the "\?\" prefix as detailed in the following sections.

I don't program on Windows, or know much anything about it, so I can't say I've banked on this.