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[libc] Use #undef isascii in specific header

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Standard C allows all standard headers to declare macros for all
their functions.  So after possibly including any standard header
like <ctype.h>, it's perfectly normal for any and all of the
functions it declares to be defined as macros.  Standard C requires
explicit `#undef` before using that identifier in a way that is not
compatible with function-like macro definitions.

The C standard's rules for this are extended to POSIX as well for
the interfaces it defines, and it's the expected norm for
nonstandard extensions declared by standard C library headers too.

So far the only place this has come up for llvm-libc's code is with
the isascii function in Fuchsia's libc.  But other cases can arise
for any standard (or common extension) function names that source
code in llvm-libc is using in nonstandard ways, i.e. as C++
identifiers.

The only correct and robust way to handle the possible inclusion of
standard C library headers when building llvm-libc source code is to
use `#undef` explicitly for each identifier before using it.  The
easy and obvious place to do that is in the per-function header.
This requires that all code, such as test code, that might include
any standard C library headers, e.g. via utils/UnitTest/Test.h, make
sure to include those *first* before the per-function header.

This change does that for isascii and its test.  But it should be
done uniformly for all the code and documented as a consistent
convention so new implementation files are sure to get this right.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D94642
This commit is contained in:
Roland McGrath 2021-01-13 17:22:45 -08:00
parent 0975604cc0
commit e7228062b2
3 changed files with 4 additions and 8 deletions
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2
libc/src/ctype/isascii.h
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@ -9,6 +9,8 @@
#ifndef LLVM_LIBC_SRC_CTYPE_ISASCII_H
#define LLVM_LIBC_SRC_CTYPE_ISASCII_H
#undef isascii
namespace __llvm_libc {
int isascii(int c);

4
libc/test/src/ctype/isascii_test.cpp
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@ -6,10 +6,10 @@
//
//===----------------------------------------------------------------------===//
#include "src/ctype/isascii.h"
#include "utils/UnitTest/Test.h"
#include "src/ctype/isascii.h"
TEST(IsAscii, DefaultLocale) {
// Loops through all characters, verifying that ascii characters
// (which are all 7 bit unsigned integers)

6
libc/utils/UnitTest/FuchsiaTest.h
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@ -10,11 +10,5 @@
#define LLVM_LIBC_UTILS_UNITTEST_FUCHSIATEST_H
#include <zxtest/zxtest.h>
// isascii is being undef'd because Fuchsia's headers define a macro for
// isascii. that macro causes errors when isascii_test.cpp references
// __llvm_libc::isascii since the macro is applied first.
#ifdef isascii
#undef isascii
#endif
#endif // LLVM_LIBC_UTILS_UNITTEST_FUCHSIATEST_H