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PACK Compiler Directive

Abbreviation

None.

Arguments

A decimal number (1 or 2) enclosed in parentheses.

Default

PACK(1)

µVision

Options — C166 — Misc Controls.

Description

The PACK directive causes the compiler to generate byte-aligned or half-word-aligned structures. This is useful when exchanging data with other systems where no alignment is required.

Note

  • The compiler generates considerably more code to access byte-aligned words. For this reason, PACK(1) should be used only when necessary.
  • By default, the compiler assumes that structure members are aligned on word boundaries. If you use PACK(1) some structure members may be aligned on byte boundaries. The BYTEALIGN directive supports pointers to structure members that are aligned on byte boundaries.
See Also

BYTEALIGN

Example
#pragma pack(1)     /* byte alignment */
#pragma bytealign   /* ptrs to byte-aligned objects */

struct s1  {
  int  i1;     // i1 has offset 0
  char c1;     // c1 has offset 2
  int  i2;     // i2 has offset 3
  char c2;     // c2 has offset 5
  int  i3;     // i3 has offset 6
  char z1;     // z1 has offset 8
};

#pragma pack()    /* reset to default alignment */

struct s2  {
  int  i1;     // i1 has offset 0
  char c1;     // c1 has offset 2
  int  i2;     // i2 has offset 4
  char c2;     // c2 has offset 6
  int  i3;     // i3 has offset 8
  char z1;     // z1 has offset 10
};

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