Thread: structure into string?

I have a small question about the code's output.

Code:

main()
 {
    struct data {
       unsigned char first;
       unsigned short int second;
    } *info;
    char *msg;
    int i;

    msg = (char *) malloc(sizeof(struct data));
    info = (struct data *) msg;

    info->first = 0x4a;
    info->second = (unsigned short int)0x4d5e;

    for (i=0; i<sizeof(struct data); ++i)
       printf("%x\n", msg[i]);
 }

the output i get is:

4a
0
5e
4d


why do i have a 0 after 4a?




thank you.

Because structures can have internal padding to preserve the alignment restrictions of the types

but what if i want to use this structure as part of an ip header (for example)... that 0 will corrupt the header structure, because the protocol does not have any padding in it.
what can i do to remove the padding?

Well there are no good ways - in fact if you want to be portable, you shouldn't use a structure at all, but compose the external structure one byte at a time

Say

Code:

void make_ip_header ( unsigned char *ip_header_buff );

But for quick hacks, try whatever compiler specific extension applies for packing structures
VC++ would be #pragma pack(1)
GCC would be __attribute__((packed))

compose the external structure one byte at a time

you mean to read the structure byte by byte and create a string?

But for quick hacks, try whatever compiler specific extension applies for packing structures
VC++ would be #pragma pack(1)
GCC would be __attribute__((packed))

how does it work? where do i apply it?
can you please give me an example for both platforms?


thank you.

Originally Posted by Devil Panther

how does it work? where do i apply it?
can you please give me an example for both platforms?


thank you.

I sure do wonder why noone knows how to do any searching for answers themselves.
Really, how hard is it to visit Google and click a few links? *shakes fist*

Quzah.

fair enough... just one question.

once I change the way the structures are packed to byte by byte ( #pragma pack(1) ), could it effect something else?

If so, how can I restore the old settings?

> could it effect something else?
Everything else which follows it.

Which if you'd read the rest of the manual, you would have seen
#pragma pack() // restore compiler default
#pragma push() // save current setting
#pragma pop() // restore previously saved setting

is this code right?

Code:

#pragma pack( push, backup_id, 1 )    // saves the old value, and sets a new one

. . .    // some code

#pragma pack( pop, backup_id )          // sets back the old value