What is the use of __packed attribute in function arguments

Question

int readint(__packed int *data)
{
    return *data;
}

I have seen __packed attribute in struct declarations to avoid padding. However, what is the benefit of using __packed attribute in function arguments.

The author says that he has used __packed to tell the compiler that the integer may possibly not be aligned. What does it means?

Edit: Will the following work with gcc compiler

int readint(__attribute__((packed)) int *data)
{
    return *data;
}

Answer 1

The __packed qualifier is a compiler-specific feature of the armcc C compiler, published by ARM. A full explanation is present in their documentation, but in brief, it indicates that no padding for alignment should be inserted into the qualified object, and that pointers with this qualifier should be accessed as if they might be misaligned. (This may cause slower code to be generated for some processors, so it should not be used gratuitously.)

Note that this is not the same as the GCC packed attribute, which only applies to struct and union type definitions.