Re: [Libguestfs] [PATCH nbdkit 2/2] common/include/checked-overflow.h: Provide fallback

On RHEL 7 (GCC 4.8.5) we don't have __builtin_add_overflow and similar functions. They were first added in GCC 5. Provide a fallback path for these older compilers. Note that the fallback path does not actually implement overflow detection. --- configure.ac | 6 ++++++ common/include/checked-overflow.h | 8 ++++++++ 2 files changed, 14 insertions(+)

diff --git a/configure.ac b/configure.ac index 8980fdd4..30330813 100644 --- a/configure.ac +++ b/configure.ac @@ -220,6 +220,12 @@ CFLAGS="$old_CFLAGS" AC_MSG_RESULT([$supports_std_c90]) AM_CONDITIONAL([CAN_TEST_ANSI_C], [test "x$supports_std_c90" = "xyes"]) +dnl Check for __builtin_*_overflow. We need the dummy parameters +dnl else detection doesn't work correctly for some reason. +AC_CHECK_DECLS([__builtin_add_overflow(int, int, int *), + __builtin_mul_overflow(int, int, int *)], + [], [], []) + dnl On Haiku we must use BSD-compatibility headers to get the endian dnl macros we use. AC_MSG_CHECKING(whether OS-dependent include paths are required) diff --git a/common/include/checked-overflow.h b/common/include/checked-overflow.h index ddc4b487..c683b5a9 100644 --- a/common/include/checked-overflow.h +++ b/common/include/checked-overflow.h @@ -49,11 +49,19 @@ /* Add two values. *r = a + b * Returns true if overflow happened. */ +#if HAVE_DECL___BUILTIN_ADD_OVERFLOW #define ADD_OVERFLOW(a, b, r) __builtin_add_overflow((a), (b), (r)) +#else +#define ADD_OVERFLOW(a, b, r) (*(r) = (a) + (b), 0) +#endif /* Multiply two values. *r = a * b * Returns true if overflow happened. */ +#if HAVE_DECL___BUILTIN_MUL_OVERFLOW #define MUL_OVERFLOW(a, b, r) __builtin_mul_overflow((a), (b), (r)) +#else +#define MUL_OVERFLOW(a, b, r) (*(r) = (a) * (b), 0) +#endif #endif /* NBDKIT_CHECKED_OVERFLOW_H */