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#if __STDC__ | |
#else | |
# error A standard C compiler is required! | |
#endif |
#include "conf.h" | |
#include <stddef.h> /* size_t; offsetof. */ | |
extern int uMyth_( void ) | |
{ | |
typedef const struct { const char Trap; const size_t Gold; } BOX; | |
auto const size_t Rule[ 2 ] = { offsetof( BOX, Gold ), sizeof(size_t) }; | |
register const size_t | |
ConA = ( Rule[ 0 ] - 1 ) & ( Rule[ 0 ] | Rule[ 1 ] | (size_t) ( Rule + 0 ) ), | |
ConS = ( Rule[ 1 ] - 0 ) ^ ( (size_t) ( Rule + 1 ) - (size_t) ( Rule + 0 ) ); | |
return !( ConA | ConS ); | |
} |
#include "conf.h" | |
#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS. */ | |
extern int main( void ) | |
{ | |
extern int uMyth_( void ); | |
return uMyth_( )? EXIT_SUCCESS: EXIT_FAILURE; | |
} |
Motivation
Memory alignment cannot be dynamically determined in a standard manner, since the standard does not define how the casting from a pointer to integer shall be implemented.
§ 6.3.2.3 Pointers - ISO/IEC 9899:2018
- Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.
cast operator - cppreference.com
- Any pointer type can be cast to any integer type. The result is implementation-defined, even for null pointer values (they do not necessarily result in the value zero). If the result cannot be represented in the target type, the behavior is undefined (unsigned integers do not implement modulo arithmetic on a cast from pointer).
How to allocate aligned memory only using the standard library? - Stack Overflow
Unfortunately, in C99 it seems pretty tough to guarantee alignment of any sort in a way which would be portable across any C implementation conforming to C99. Why? Because a pointer is not guaranteed to be the "byte address" one might imagine with a flat memory model. Neither is the representation of
uintptr_t
so guaranteed, which itself is an optional type anyway.
Can I determine whether a pointer is aligned in a fully standard manner? - Reddit
But yes, you are right, there is no standard compliant way to check if a pointer is aligned. If you need to align an object that might be misaligned, you can
memcpy
it into amalloc
-ed buffer.
Modify the functions' name, scope, signature, specification, implementation, or anything as you want. #
uMyth_
int uMyth_( void );
Testing whether an integer, casted from a pointer, shows an alignment and a size as expected:
- Returns
int
-1
for true,0
for false.
- Remarks
- Note that the returned value could vary for each compiler, or even for each call.
- The implementation has same meaning as below, where the type of
SArray
issize_t[ 2 ]
:
( /*--------*/ 0 == /*---*/ alignof(size_t) & ( alignof(size_t) - 1 ) ) && ( /*--------*/ 0 == /*----*/ sizeof(size_t) & ( alignof(size_t) - 1 ) ) && ( /*--------*/ 0 == (size_t) ( SArray + 0 ) & ( alignof(size_t) - 1 ) ) && ( sizeof(size_t) == (size_t) ( SArray + 1 ) - (size_t) ( SArray + 0 ) );
Compiler Tests
Special thanks to Compiler Explorer.
GCC
-Wall -Wextra -pedantic-errors -O2 -std=c89
x86-64 gcc 10.2
uMyth_:
mov eax, 1
ret
ARM64 gcc 8.2
uMyth_:
mov w0, 1
ret
RISC-V rv64gc gcc 8.2.0
uMyth_:
li a0,1
ret
PowerPC64 gcc 6.3.0
uMyth_:
li 3,1
blr
.long 0
.byte 0,0,0,0,0,0,0,0
MIPS64 gcc 5.4
uMyth_:
j $31
li $2,1 # 0x1
MSP430 gcc 6.2.1
uMyth_:
MOV.B #1, R12
RET
Clang
-Wall -Wextra -pedantic-errors -O2 -std=c89
x86-64 clang 11.0.1
uMyth_: # @uMyth_
mov eax, 1
ret
armv8-a clang 11.0.1
uMyth_: // @uMyth_
mov w0, #1
ret
RISC-V rv64gc clang 11.0.1
uMyth_: # @uMyth_
addi a0, zero, 1
ret
MSVC
-WX -Wall -wd4820 -O2 -TC -Za
x64 msvc v19.14 (WINE)
Rule$ = 0
uMyth_ PROC ; COMDAT
$LN4:
sub rsp, 24
lea rax, QWORD PTR Rule$[rsp]
test al, 7
mov eax, 0
sete al
add rsp, 24
ret 0
uMyth_ ENDP
ARM64 msvc v19.14 (WINE)
;Flags[SingleProEpi] functionLength[24] RegF[0] RegI[0] H[0] frameChainReturn[UnChained] frameSize[16]
|uMyth_| PROC
|$LN4|
sub sp,sp,#0x10
mov x8,sp
tst x8,#7
cseteq w0
add sp,sp,#0x10
ret
ENDP ; |uMyth_|
ICC
-Wall -Wextra -pedantic-errors -O2 -std=c89
x86-64 icc 19.0.1
uMyth_:
mov eax, 1 #20.19
ret #20.19
Tab-Size Preference: { default, 2, 3, 4, 6, 8 }