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system/Error: use std::generic_category() for errno on Windows

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It's wrong to use std::system_category() for both GetLastError() and
errno on Windows.  Apparently, everybody uses std::generic_category()
for errno values, which appears to be a safe choice.

Some discussion on this confusing topic can be found here:

 https://stackoverflow.com/questions/28746372/system-error-categories-and-standard-system-error-codes
This commit is contained in:
Max Kellermann 2016-12-04 19:50:21 +01:00
parent 30dc473697
commit c6e1ca1c22
2 changed files with 28 additions and 7 deletions
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5
src/decoder/plugins/FlacIOHandle.cxx
View File

@ -21,8 +21,7 @@
#include "FlacIOHandle.hxx"
#include "Log.hxx"
#include "Compiler.h"
#include <system_error>
#include "system/Error.hxx"
#include <errno.h>
#include <stdio.h>
@ -49,7 +48,7 @@ FlacIORead(void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)
#ifndef WIN32
} catch (const std::system_error &e) {
errno = e.code().category() == std::system_category()
errno = e.code().category() == ErrnoCategory()
? e.code().value()
/* just some random non-zero errno
value */

30
src/system/Error.hxx
View File

@ -95,10 +95,32 @@ FormatLastError(const char *fmt, Args&&... args)
#include <errno.h>
#include <string.h>
/**
* Returns the error_category to be used to wrap errno values. The
* C++ standard does not define this well, so this code is based on
* observations what C++ standard library implementations actually
* use.
*
* @see https://stackoverflow.com/questions/28746372/system-error-categories-and-standard-system-error-codes
*/
static inline const std::error_category &
ErrnoCategory()
{
#ifdef WIN32
/* on Windows, the generic_category() is used for errno
values */
return std::generic_category();
#else
/* on POSIX, system_category() appears to be the best
choice */
return std::system_category();
#endif
}
static inline std::system_error
MakeErrno(int code, const char *msg)
{
return std::system_error(std::error_code(code, std::system_category()),
return std::system_error(std::error_code(code, ErrnoCategory()),
msg);
}
@ -133,7 +155,7 @@ IsFileNotFound(const std::system_error &e)
return e.code().category() == std::system_category() &&
e.code().value() == ERROR_FILE_NOT_FOUND;
#else
return e.code().category() == std::system_category() &&
return e.code().category() == ErrnoCategory() &&
e.code().value() == ENOENT;
#endif
}
@ -146,7 +168,7 @@ IsPathNotFound(const std::system_error &e)
return e.code().category() == std::system_category() &&
e.code().value() == ERROR_PATH_NOT_FOUND;
#else
return e.code().category() == std::system_category() &&
return e.code().category() == ErrnoCategory() &&
e.code().value() == ENOTDIR;
#endif
}
@ -159,7 +181,7 @@ IsAccessDenied(const std::system_error &e)
return e.code().category() == std::system_category() &&
e.code().value() == ERROR_ACCESS_DENIED;
#else
return e.code().category() == std::system_category() &&
return e.code().category() == ErrnoCategory() &&
e.code().value() == EACCES;
#endif
}