#pragma pack(n)

where:

The default is #pragma pack(8).

Taking the address of a field in a #pragma packed struct does not yield a __packed pointer, so the compiler will not produce an error if you assign this address to a non-__packed pointer. However, the field might not be properly aligned for its type, and dereferencing such an unaligned pointer results in Undefined behavior.

This example demonstrates 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 as shown in Figure 1, while the layout of SP is as shown in Figure 2. In Figure 2, x denotes one byte of padding.

Figure 1. Nonpacked structure S

Figure 2. Packed structure SP