Use _Alignof to query alignments in the struct module · Issue #132868 · python/cpython · GitHub

Feature or enhancement

Proposal:

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

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

Linked PRs

gh-132868: use _Alignof() operator in the struct module #132872

Considering I did use _Alignof in my recent PR (#132428), I support this proposal.

pythongh-132868: use _Alignof() operator in the struct module

PR is ready: #132872

gh-132868: use `_Alignof()` C11 operator in the `struct` module (#132872

Use _Alignof to query alignments in the struct module · Issue #132868 · python/cpython · GitHub

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Use _Alignof to query alignments in the struct module #132868

Feature or enhancement

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Has this already been discussed elsewhere?

Links to previous discussion of this feature:

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	/* 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))

Use _Alignof to query alignments in the struct module · Issue #132868 · python/cpython · GitHub

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Use _Alignof to query alignments in the struct module #132868

Description

Feature or enhancement

Proposal:

Has this already been discussed elsewhere?

Links to previous discussion of this feature:

Linked PRs

Activity

picnixz commented on Apr 24, 2025

skirpichev commented on Apr 24, 2025

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