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[libc++][NFC] Improve documentation of the various random_device implementations 

Also, move the setting of the macro closer to its point of use, which
also has the benefit of uncluttering `__config`.
This commit is contained in:
Louis Dionne 2021-12-21 15:31:18 -05:00
parent 1674d9b6b2
commit 346ef5e587
2 changed files with 44 additions and 19 deletions
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19
libcxx/include/__config
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@ -333,25 +333,6 @@
# define _LIBCPP_SHORT_WCHAR 1
#endif
#if defined(__OpenBSD__)
// Certain architectures provide arc4random(). Prefer using
// arc4random() over /dev/{u,}random to make it possible to obtain
// random data even when using sandboxing mechanisms such as chroots,
// Capsicum, etc.
# define _LIBCPP_USING_ARC4_RANDOM
#elif defined(__Fuchsia__) || defined(__wasi__)
# define _LIBCPP_USING_GETENTROPY
#elif defined(__native_client__)
// NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
// including accesses to the special files under /dev. C++11's
// std::random_device is instead exposed through a NaCl syscall.
# define _LIBCPP_USING_NACL_RANDOM
#elif defined(_LIBCPP_WIN32API)
# define _LIBCPP_USING_WIN32_RANDOM
#else
# define _LIBCPP_USING_DEV_RANDOM
#endif
#if !defined(_LIBCPP_LITTLE_ENDIAN) && !defined(_LIBCPP_BIG_ENDIAN)
# include <endian.h>
# if __BYTE_ORDER == __LITTLE_ENDIAN

44
libcxx/include/__random/random_device.h
View File

@ -23,6 +23,50 @@ _LIBCPP_BEGIN_NAMESPACE_STD
#if !defined(_LIBCPP_HAS_NO_RANDOM_DEVICE)
// Libc++ supports various implementations of std::random_device.
//
// _LIBCPP_USING_DEV_RANDOM
// Read entropy from the given file, by default `/dev/urandom`.
// If a token is provided, it is assumed to be the path to a file
// to read entropy from. This is the default behavior if nothing
// else is specified. This implementation requires storing state
// inside `std::random_device`.
//
// _LIBCPP_USING_ARC4_RANDOM
// Use arc4random(). This allows obtaining random data even when
// using sandboxing mechanisms. On some platforms like Apple, this
// is the recommended source of entropy for user-space programs.
// When this option is used, the token passed to `std::random_device`'s
// constructor *must* be "/dev/urandom" -- anything else is an error.
//
// _LIBCPP_USING_GETENTROPY
// Use getentropy().
// When this option is used, the token passed to `std::random_device`'s
// constructor *must* be "/dev/urandom" -- anything else is an error.
//
// _LIBCPP_USING_NACL_RANDOM
// NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
// including accesses to the special files under `/dev`. This implementation
// uses the NaCL syscall `nacl_secure_random_init()` to get entropy.
// When this option is used, the token passed to `std::random_device`'s
// constructor *must* be "/dev/urandom" -- anything else is an error.
//
// _LIBCPP_USING_WIN32_RANDOM
// Use rand_s(), for use on Windows.
// When this option is used, the token passed to `std::random_device`'s
// constructor *must* be "/dev/urandom" -- anything else is an error.
#if defined(__OpenBSD__)
# define _LIBCPP_USING_ARC4_RANDOM
#elif defined(__Fuchsia__) || defined(__wasi__)
# define _LIBCPP_USING_GETENTROPY
#elif defined(__native_client__)
# define _LIBCPP_USING_NACL_RANDOM
#elif defined(_LIBCPP_WIN32API)
# define _LIBCPP_USING_WIN32_RANDOM
#else
# define _LIBCPP_USING_DEV_RANDOM
#endif
class _LIBCPP_TYPE_VIS random_device
{
#ifdef _LIBCPP_USING_DEV_RANDOM