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Use _Alignof to query alignments in the struct module #132868

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Use _Alignof to query alignments in the struct module#132868
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extension-modulesC modules in the Modules dirtype-featureA feature request or enhancement
@skirpichev

Description

@skirpichev
skirpichev
opened on Apr 24, 2025

Feature or enhancement

Proposal:

The struct module uses pre-C11 hacks to determine alignment for types:

cpython/Modules/_struct.c

Lines 79 to 106 in 5808889

/* Define various structs to figure out the alignments of types */
typedef struct { char c; short x; } st_short;
typedef struct { char c; int x; } st_int;
typedef struct { char c; long x; } st_long;
typedef struct { char c; float x; } st_float;
typedef struct { char c; double x; } st_double;
#ifdef Py_HAVE_C_COMPLEX
typedef struct { char c; float complex x; } st_float_complex;
typedef struct { char c; double complex x; } st_double_complex;
#endif
typedef struct { char c; void *x; } st_void_p;
typedef struct { char c; size_t x; } st_size_t;
typedef struct { char c; _Bool x; } st_bool;
#define SHORT_ALIGN (sizeof(st_short) - sizeof(short))
#define INT_ALIGN (sizeof(st_int) - sizeof(int))
#define LONG_ALIGN (sizeof(st_long) - sizeof(long))
#define FLOAT_ALIGN (sizeof(st_float) - sizeof(float))
#define DOUBLE_ALIGN (sizeof(st_double) - sizeof(double))
#ifdef Py_HAVE_C_COMPLEX
# define FLOAT_COMPLEX_ALIGN (sizeof(st_float_complex) - sizeof(float complex))
# define DOUBLE_COMPLEX_ALIGN (sizeof(st_double_complex) - sizeof(double complex))
#endif
#define VOID_P_ALIGN (sizeof(st_void_p) - sizeof(void *))
#define SIZE_T_ALIGN (sizeof(st_size_t) - sizeof(size_t))
#define BOOL_ALIGN (sizeof(st_bool) - sizeof(_Bool))

Now we can use _Alignof operator, as C11-compatible compiler is a build requireent.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

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