This page is a draft. Please help enhancing it by adding lines and filling tables. Comments at the bottom of the text are also welcome.

This is just a quick array to recall some of the specifics of the architectures found in the Debian project.

feature	alpha	any-amd64¹	arm	armel	armhf	arm64	avr32	hppa	any-i386²	ia64	m68k	mips	mipsel	powerpc	s390	s390x	sparc	sh4
size of short	2	2	2			2	2	2	2	2	2	2		2	2		2	2
size of int	4	4	4			4	4	4	4	4	4	4		4	4		4	4
size of long	8	8	4			8	4	4	4	8	4	4		4	4	8	4	4
size of long long	8	8	8			8	8	8	8	8	8	8		8	8		8	8
alignment of long long	?	-	8			8	4	-	4	?	?	?		?	-		?	4
size of float	4	4	4			4	4	4	4	4	4	4		4	4		4	4
size of double	8	8	8			8	8	8	8	8	8	8		8	8		8	8
size of long double	16³	16	8			16	8	8	12	16	12	8		16³	16³	16	16³	8
alignment of long double	?	-	?		8	?	4	-	4	?	?	?		?	8		?	4
size of data pointer	8	8	4			8	4	4	4	8	4	4		4	4	8	4	4
max alignment	8	8	4	8	?	?	?	4 (16)	8	?	?	8		?	?	?	?	4
unaligned access OK	no	yes	no		yes	yes	yes	no	yes	no	yes	no		yes	yes	yes	no	no
endian	little	little	little			little	big	big	little	little	big	big	little	big	big	big	big	little
char	s	s	u			u	u	s	s	s	s	s		u	u	u	s	s
stack grows	down	down	down			down	down	up	down	down	down	down		down	down	down	down	down
func param location	regs	regs	regs			regs	?	?	stack	regs	stack	regs		regs	regs	regs	regs	regs
func return ptr location	reg	stack	reg			reg	?	?	stack	reg	stack	reg		reg	?	?	reg	reg
page size(s)	8K	4K, 2M	4K			4K, 64K	?	?	4K, 2M/4M	?	4K	4K-64K		?	4K		?	4k

amd64, kfreebsd-amd64 (1)
i386, kfreebsd-i386, hurd-i386 (2)
long double changed size from 8 to 16 in Lenny (3 4 5 6)

C/C++ Preprocessor Symbols

Generally, GNU C tends to define the symbol

__arch__

A list of some common arch-specific symbols can be found on the Pre-defined Architecture Macros page.

Signedness

When programming in C, variables can be signed or unsigned.

Example:

unsigned char c;

explicit unsigned, 0 <= c <= 255

signed char c;

explicit signed, -128 <= c <= 127

char c;

implicit (un)signedness, depending on architecture

To force a signedness, either declare it explicitly or use the gcc command line option -f(un)signed-char. The gcc defaults differ only due to optimisation. Other compilers may not have this issue.

Obtaining this information

via a special tool

#include <alloca.h>
#include <endian.h>
#include <stddef.h>
#include <stdio.h>

static void test_size_align(const char *name, size_t size, size_t align) {
  printf("sizeof(%s) = %zu\n", name, size);
  if (size != align) printf("alignment(%s) = %zu\n", name, align);
}

#define TEST_SIZE_ALIGN(type, name) \
  struct test_align_##name { char a; type b; }; \
  test_size_align(#type, sizeof(type), offsetof(struct test_align_##name, b))

static void test_endian(void) {
#if __BYTE_ORDER == __LITTLE_ENDIAN
  printf("byte order = little endian\n");
#elif __BYTE_ORDER == __BIG_ENDIAN
  printf("byte order = big endian\n");
#endif
}

static void test_char(void) {
  if ((int)(char)-1 == -1) printf("char signedness = signed\n");
  else if ((int)(char)-1 == 255) printf("char signedness = unsigned\n");
}

static void test_stack(void) {
  void *a = alloca(8);
  void *b = alloca(8);
  if (a > b) printf("stack = grows down\n");
  else printf("stack = grows up\n");
}

int main(void) {
  TEST_SIZE_ALIGN(short, short);
  TEST_SIZE_ALIGN(int, int);
  TEST_SIZE_ALIGN(long, long);
  TEST_SIZE_ALIGN(long long, long_long);
  TEST_SIZE_ALIGN(float, float);
  TEST_SIZE_ALIGN(double, double);
  TEST_SIZE_ALIGN(long double, long_double);
  TEST_SIZE_ALIGN(void *, pointer);
  test_endian();
  test_char();
  test_stack();
  return 0;
}

via autoconf

AC_INIT(archtest, 0.1)
AC_CHECK_SIZEOF(short)
AC_CHECK_SIZEOF(int)
AC_CHECK_SIZEOF(long)
AC_CHECK_SIZEOF(long long)
AC_CHECK_SIZEOF(float)
AC_CHECK_SIZEOF(double)
AC_CHECK_SIZEOF(long double)
AC_CHECK_SIZEOF(void*)
AC_C_BIGENDIAN()
AC_C_CHAR_UNSIGNED()

CategoryDeveloper

ArchitectureSpecificsMemo

C/C++ Preprocessor Symbols

Signedness

Obtaining this information

via a special tool

via autoconf