Chapter 2 Storage and Alignment Comparisons

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This chapter focuses on the different ways that internal data storage is allocated on various platforms.

In the following discussions, data storage refers to the size of data types. Data alignment refers to the way a system or language aligns data structures in memory. Data alignment and storage differences can cause problems when moving data between systems that have different alignment and storage schemes. These differences become apparent when data within a structure is exchanged between systems using files or inter-process communication.

“The Purpose of Alignment Modes” discusses what alignment modes are and why they are needed.
“Data Type Size and Alignments ” discusses the alignment rules for the different data types.
“The Alignment Pragmas” discusses how to use the HP_ALIGN and PACK alignment pragmas to control alignment of structs, unions, bit-fields, and typedefs.
“Aligning Structures Between Architectures” describes the storage and alignment rules of HP C on HP 9000 workstations and servers as compared with those of other systems. (Note that the storage and alignment rules on the HP 3000 Series 900 are the same as those on the HP 9000 workstations and servers.)

The storage and alignment rules for the following systems are compared:

HP C on the HP 9000 workstations and servers
HP C on the HP 9000 Series 300/400.
HP Apollo Series 3000/4000.
HP Apollo Series 10000.
CCS/C on the HP 1000.
VAX/VMS C.




	NOTE: As of the C compiler release 11.00, HP C supports a 64-bit data model in which the `long` and pointer data types are 64-bits long. In the 32-bit model, the `long` and pointer data types are the same size as the `int` data type—32 bits. In this chapter and throughout the HP C documentation, where differences between these two modes occur, they are noted. The term used to specify the 64-bit mode in which the long and pointer types are 64-bits is LP64, and the term for 32-bit mode in which the `int`, `long` and pointer types are 32-bits is called ILP32.

Chapter 2 Storage and Alignment Comparisons

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