For example this code is broken (I've just fixed it in actual code..).
uint64_t a = 1 << 60
It can be fixed as,
uint64_t a = (uint64_t)1 << 60
but then this passed my brain.
uint64_t a = UINT64_C(1) << 60
I know that UINT64_C(1)
is a macro that expands usually as 1ul
in 64-bit systems, but then what makes it different than just doing a type cast?
(uint64_t)1
is formally an int
value 1
casted to uint64_t
1ul
is a constant 1
of type unsigned long
which is probably the same as uint64_t
on a 64-bit system.
The macro is a portable way to specify the correct suffix for a constant (literal) of type uint64_t
.
As you are dealing with constants, all calculations will be done by the compiler and the result is the same.
The suffix appended by the macro ( ul
, system specific) can be used for literal constants only.
The cast (uint64_t)
can be used for both constant and variable values. With a constant it will have the same effect as the suffix/macro, with a variable of a different type it may perform a truncation or extension of the value, eg fill the higher bits with 0 when changing from 32 bits to 64 bits.
It's a matter of taste if you use UINT64_C(1)
or (uint64_t)1
. The macro makes it a bit more clear that you are dealing with a constant.
There is no obvious difference or advantage, these macros are kind of redundant. There are some minor, subtle differences between the cast and the macro:
(uintn_t)1
might be cumbersome to use for preprocessor purposes, whereas UINTN_C(1)
expands into a single pp token.
The resulting type of the UINTN_C
is actually uint_leastn_t
and not uintn_t
. So it is not necessarily the type you expected.
Static analysers for coding standards like MISRA-C might moan if you type 1
rather than 1u
in your code, since shifting signed integers isn't a brilliant idea regardless of their size.
(uint64_t)1u
is MISRA compliant, UINT64_c(1)
might not be, or at least the analyser won't be able to tell since it can't expand pp tokens like a compiler. And UINT64_C(1u)
will likely not work, since this macro implementation probably looks something like this:
#define UINT64_C(n) ((uint_least64_t) n ## ull) // BAD: 1u##ull = 1uull
In general, I would recommend to use an explicit cast. Or better yet wrap all of this inside a named constant:
#define MY_BIT ( (uint64_t)1u << 60 )
UINT64_C(1)
produces a single token via token pasting, whereas ((uint64_t)1)
is a constant expression with the same value.
They can be used interchangeably in the sample code posted, but not in preprocessor directives such as #if
expressions.
XXX_C
macros should be used to define constants that can be used in #if
expressions. They are only needed if the constant must have a specific type, otherwise just spelling the constant in decimal or hexadecimal without a suffix is sufficient.
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