#pragma pack(...)

Syntax

#pragma pack([n])

#pragma pack(push[,n])

#pragma pack(pop)

Where:

n

Is the alignment in bytes, valid alignment values are 1, 2, 4, and 8. If omitted, sets the alignment to the one that was in effect when compilation started.

push[,n]

Pushes the current alignment setting on an internal stack and then optionally sets the new alignment.

pop

Restores the alignment setting to the one saved at the top of the internal stack, then removes that stack entry.

Note:

#pragma pack([n]) does not influence this internal stack. Therefore, it is possible to have #pragma pack(push) followed by multiple #pragma pack([n]) instances, then finalized by a single #pragma pack(pop).

Example

This example shows how pack(2) aligns integer variable b to a 2-byte boundary.

typedef struct
{ 
    char a;
    int b;
} S;

#pragma pack(2)

typedef struct
{ 
    char a;
    int b;
} SP;

S var = { 0x11, 0x44444444 };
SP pvar = { 0x11, 0x44444444 };

The layout of S is:

Figure B5-1 Nonpacked structure S

The layout of SP is:

Figure B5-2 Packed structure SP

Note:

In this layout, x denotes one byte of padding.

SP is a 6-byte structure. There is no padding after b.