RealView Compilation Tools Compiler Reference Guide - ARM ...

C and C++ Implementation Details
• In RVCT v2.0 and above, double and long long data types are
eight-byte aligned. This enables efficient use of the LDRD and STRD
instructions in ARMv5TE and above.
• Bitfield alignment depends on how the bitfield is declared. See
Bitfields in packed structures on page 5-12 for more information.
• All other types are aligned on word boundaries.
Structures can contain padding to ensure that fields are correctly aligned and that the
structure itself is correctly aligned. Figure 5-1 shows an example of a conventional,
non-packed structure. Bytes 1, 2, and 3 are padded to ensure correct field alignment.
Bytes 11 and 12 are padded to ensure correct structure alignment. The sizeof() function
returns the size of the structure including padding.
Figure 5-1 Conventional non-packed structure example
The compiler pads structures in one of the following ways, according to how the
structure is defined:
• Structures that are defined as static or extern are padded with zeros.
• Structures on the stack or heap, such as those defined with malloc() or auto, are
padded with whatever is previously stored in those memory locations. You cannot
use memcmp() to compare padded structures defined in this way (see Figure 5-1).
Use the --remarks option to view the messages that are generated when the compiler
inserts padding in a struct.
Structures with empty initializers are permitted in C++:
struct
{
int x;
} X = { };
struct {char c; int x; short s} ex1;
0 1 2 3
However, if you are compiling C, or compiling C++ with the -cpp and--c90 options, an
error is generated.
c
s
padding
4 5 7 8
x
9 10 11 12
padding
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