The entire struct can be declared as __packed. For example:

__packed struct mystruct
{
    char c;
    short s;
} // not recommended

Each field of the structure inherits the __packed qualifier.

Declaring an entire struct as __packed typically incurs a penalty both in code size and performance.

Individual non-aligned fields within the struct can be declared as __packed. For example:

struct mystruct
{
    char c;
    __packed short s; // recommended
}

This is the recommended approach to packing structures.