C++: Casting unsigned char to a Structure

What I am trying to do

typedef struct {
    unsigned char a;
    unsigned char b;
    unsigned int  c;
} Packet;

unsigned char buffer[] = {1, 1, 0, 0, 0, 1};
Packet pkt = (Packet)buffer;

Basically I am trying to cast a byte array to a structure in C++, when compiling I get:

No matching function call for Packet::Packet(unsigned char[6])

Is this not possible or do I have to manually index into the array?

>Solution :

There are a few ways to do this:

// packet.h
////////////////
struct Packet {
    unsigned char a;
    unsigned char b;
    unsigned int  c;
};

If you compile and dump the structs with pahole you will see the paddings

$ pahole -dr --structs main.o
struct Packet {
        unsigned char              a;                    /*     0     1 */
        unsigned char              b;                    /*     1     1 */

        /* XXX 2 bytes hole, try to pack */

        unsigned int               c;                    /*     4     4 */

        /* size: 8, cachelines: 1, members: 3 */
        /* sum members: 6, holes: 1, sum holes: 2 */
        /* last cacheline: 8 bytes */
};

So it’s basically the 2 chars, 2 padding bytes and 4 bytes of an int for a total of 8 bytes.

Because Intel is a little endian platform, the least significant byte comes first as in

void print_packet( Packet* pkt ) {
    printf( "a:%d b:%d c:%d\n", int(a), int(b), c );
}
int main() {
    unsigned char buffer[] = {1, 1, 0, 0, 1, 0, 0, 0};
    print_packet( (Packet*) buffer );
    print_packet( reinterpret_cast<Packet*>(buffer));
}

Produces:

$ g++ main.cpp -o main
$ ./main
a:1 b:1 c:1
a:1 b:1 c:1

However one can change the packing from the command line as below where we set the alignment to 2 bytes.

$ g++ -ggdb  main.cpp -o main -fpack-struct=2
$ pahole -dr --structs main
struct Packet {
        unsigned char              a;                    /*     0     1 */
        unsigned char              b;                    /*     1     1 */
        unsigned int               c;                    /*     2     4 */

        /* size: 6, cachelines: 1, members: 3 */
        /* last cacheline: 6 bytes */
} __attribute__((__packed__));

Then you can see that the Packet struct is only 6 bytes and the result of running main is completely different

$ ./main
a:1 b:1 c:65536
a:1 b:1 c:65536

This is because the value of c is now 0x00000100 or 65536

So not to be at mercy of these compiler shenanigans, it is better to define your packet in code with the right packing as

// packet.h
////////////////
struct [[gnu::packed]] Packet {
    unsigned char a;
    unsigned char b;
    unsigned char reserved[2];
    unsigned int  c;
};

Then execution becomes

$ g++ -ggdb  main.cpp x.cpp -o main -fpack-struct=2
$ ./main
a:1 b:1 c:1
a:1 b:1 c:1
$ g++ -ggdb  main.cpp x.cpp -o main -fpack-struct=4
$ ./main
a:1 b:1 c:1
a:1 b:1 c:1
$ g++ -ggdb  main.cpp x.cpp -o main -fpack-struct=8
$ ./main
a:1 b:1 c:1
a:1 b:1 c:1
$ g++ -ggdb  main.cpp x.cpp -o main -fpack-struct=16
$ ./main
a:1 b:1 c:1
a:1 b:1 c:1