#OS Detection

./000 core.h detects the host OS and defines a few portable helpers so the rest of the library can use one code path per platform.

#Platform macros

Macro	POSIX value	Windows value	Notes
`FIO_OS_POSIX`	`1`	`0`	Set on Linux, macOS, BSD, and other `__unix__`/`__linux__`/`__APPLE__` systems.
`FIO_OS_WIN`	`0`	`1`	Set on Windows, including MinGW, MSYS2, Cygwin, and Borland.

The Windows check is evaluated first, because MinGW/MSYS2 define both _WIN32 and __unix__. Treating the system as Windows avoids calling POSIX socket functions on Winsock SOCKET handles.

After detection, FIO_OS_WIN always wins over FIO_OS_POSIX:

#if FIO_OS_WIN
#undef FIO_OS_POSIX
#define FIO_OS_POSIX 0
#endif

#Unix-tools level

FIO_HAVE_UNIX_TOOLS tells the library how Unix-like the environment is:

0 — no Unix tools (pure MSVC).
1 — full POSIX.
2 — MinGW.
3 — Cygwin.

It is used to decide whether functions such as pipe(), fcntl(), and POSIX signal APIs are available.

#Process helpers

Macro	POSIX	Windows
`fio_getpid`	`getpid`	`_getpid`
`FIO_KILL_SELF()`	`kill(0, SIGINT)`	`GenerateConsoleCtrlEvent(CTRL_C_EVENT, 0)`

FIO_KILL_SELF is guarded by #ifndef, so you can define your own version before including the header. The default implementation is used by the assertion/debug machinery to deliver a signal to the running process.

#Thread sleep and yield

Macro	POSIX	Windows
`FIO_THREAD_WAIT(nano_sec)`	`nanosleep`	`Sleep`
`FIO_THREAD_YIELD()`	see below	see below
`FIO_THREAD_RESCHEDULE()`	`FIO_THREAD_WAIT(4)`	`FIO_THREAD_WAIT(4)`

FIO_THREAD_WAIT(nano_sec) sleeps the current thread for the requested number of nanoseconds. On POSIX it builds a struct timespec and calls nanosleep. On Windows it converts the interval to milliseconds and calls Sleep; any non-zero request is rounded up to at least 1 ms.

FIO_THREAD_YIELD() hints the processor that the thread is in a spin loop. On x86/x86-64 with GCC or Clang it emits a pause instruction; on ARM/AArch64 with GCC or Clang it emits a yield instruction; with MSVC it calls YieldProcessor(); otherwise it falls back to sched_yield() on POSIX.

FIO_THREAD_RESCHEDULE() yields the thread by sleeping for 4 nanoseconds (FIO_THREAD_WAIT(4)). It is used by the lock code to back off busy waits without fully suspending the thread.

#Portable type patches

On pure MSVC ssize_t is missing, so the core header defines it from SSIZE_T:

typedef SSIZE_T ssize_t;

SSIZE_MAX and SSIZE_MIN are also provided if the system headers do not:

#ifndef SSIZE_MAX
#define SSIZE_MAX ((ssize_t)((~(size_t)0) >> 1))
#endif
#ifndef SSIZE_MIN
#define SSIZE_MIN ((ssize_t)(~((~(size_t)0) >> 1)))
#endif

#`printf`-style format checking

The FIO___PRINTF_STYLE(string_index, check_index) macro is defined differently on each toolchain so the compiler can check printf-style arguments. MinGW/Cygwin use the MinGW format attribute, pure MSVC leaves it empty, and POSIX/GCC/Clang use the standard printf format attribute. It is documented in ./001 compiler attributes.md.