Replace traditional `#ifndef`/`#define`/`#endif` include guards with
`#pragma` once.
`#pragma once` is a widely supported preprocessor directive that
prevents header files from being included multiple times. It is
supported by all toolchains used to build edk2: GCC, Clang/LLVM, and
MSVC.
Compared to macro-based include guards, `#pragma once`:
- Eliminates the risk of macro name collisions or copy/paste errors
where two headers inadvertently use the same guard macro.
- Eliminate inconsistency in the way include guard macros are named
(e.g., some files use `__FILE_H__`, others use `FILE_H_`, etc.).
- Reduces boilerplate (three lines replaced by one).
- Avoids polluting the macro namespace with guard symbols.
- Can improve build times as the preprocessor can skip re-opening the
file entirely, rather than re-reading it to find the matching
`#endif` ("multiple-include optimization").
- Note that some compilers may already optimize traditional include
guards, by recognzining the idiomatic pattern.
This change is made acknowledging that overall portability of the
code will technically be reduced, as `#pragma once` is not part of the
C/C++ standards.
However, this is considered acceptable given:
1. edk2 already defines a subset of supported compilers in
BaseTools/Conf/tools_def.template, all of which have supported
`#pragma once` for over two decades.
2. There have been concerns raised to the project about inconsistent
include guard naming and potential macro collisions.
Approximate compiler support dates:
- MSVC: Supported since Visual C++ 4.2 (1996)
- GCC: Supported since 3.4 (2004)
(http://gnu.ist.utl.pt/software/gcc/gcc-3.4/changes.html)
- Clang (LLVM based): Since initial release in 2007
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Struct packing is only necessary for data structures whose in-memory
representation is covered by the PI or UEFI specs, and may deviate
from the ordinary C rules for alignment.
So in case of FileExplorerLib, this applies to the device path struct
only, and other structures used to carry program data should not be
packed, or we may end up with alignment faults on architectures such
as ARM, which don't permit load/store double or multiple instructions
to access memory locations that are not 32-bit aligned.
E.g., the following call in FileExplorerLibConstructor()
InitializeListHead (&gFileExplorerPrivate.FsOptionMenu->Head);
which is emitted as follows for 32-bit ARM/Thumb2 by Clang-5.0
3de0: b510 push {r4, lr}
3de2: 4604 mov r4, r0
...
3de8: e9c4 4400 strd r4, r4, [r4]
3dec: bd10 pop {r4, pc}
will perform a double-word store on the first argument, passed in
register r0, assuming that the pointer type of the argument is
enough to guarantee that the value is suitably aligned.
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
1. Do not use tab characters
2. No trailing white space in one line
3. All files must end with CRLF
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
