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Basic data types in ARM C and C++

Describes the basic data types implemented in ARM C and C++:

Size and alignment of basic data types

The following table gives the size and natural alignment of the basic data types.

Table 11-2 Size and alignment of data types

Type Size in bits Natural alignment in bytes
char 8 1 (byte-aligned)
short 16 2 (halfword-aligned)
int 32 4 (word-aligned)
long 32 4 (word-aligned)
long long 64 8 (doubleword-aligned)
float 32 4 (word-aligned)
double 64 8 (doubleword-aligned)
long double 64 8 (doubleword-aligned)
All pointers 32 4 (word-aligned)
bool (C++ only) 8 1 (byte-aligned)
_Bool (C onlya) 8 1 (byte-aligned)
wchar_t (C++ only) 16 2 (halfword-aligned)

Type alignment varies according to the context:

  • Local variables are usually kept in registers, but when local variables spill onto the stack, they are always word-aligned. For example, a spilled local char variable has an alignment of 4.

  • The natural alignment of a packed type is 1.


Integers are represented in two's complement form. The low word of a long long is at the low address in little-endian mode, and at the high address in big-endian mode.


Floating-point quantities are stored in IEEE format:

  • float values are represented by IEEE single-precision values

  • double and long double values are represented by IEEE double-precision values.

For double and long double quantities the word containing the sign, the exponent, and the most significant part of the mantissa is stored with the lower machine address in big-endian mode and at the higher address in little-endian mode.

Arrays and pointers

The following statements apply to all pointers to objects in C and C++, except pointers to members:

  • Adjacent bytes have addresses that differ by one.

  • The macro NULL expands to the value 0.

  • Casting between integers and pointers results in no change of representation.

  • The compiler warns of casts between pointers to functions and pointers to data.

  • The type size_t is defined as unsigned int.

  • The type ptrdiff_t is defined as signed int.


stdbool.h lets you define the bool macro in C.

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