Thread: Not atoi, not sprintf, but "int coded into char*"

(this is to be solved in ANSI C, not really ANSI required but it'd be great not to get warnings with -Wall)

This program packs, sends and recieves Descriptors according to the Gnutella Protocol Specification...not really relevant, just read this struct for example:

Code:

struct tpong
{
struct theader header;
char port[2];
char ip[4];
char nfiles[4];
char nkbytes[4];
};

I also calculate things like nfiles and nkbytes, and get port and ip as parameters. BUT some of these data I recieve or gather come as numeric data type (int...). So I haven't been able to pack the number into the string, and get it back as well. Moreover, as you see char* size is not necessarily 4 bytes (as int).

HELP!! Thx in advance!

Mariano Lopez-Gappa (20) from Argentina


EDIT: just a tiny comment: char vector lengths are not wrong since G.P.S. says not to save space for NULL terminators (so strlen, strcat, strcpy, strcmp is NO GOOD )

I don't know what your question is, but could it be that the arrays in that struct contain the binary representation of int instead of string? For example

Code:

long n = 123;
struct tpong t;
memcpy(t.ip, &n, sizeof(long));

The above will work as long as sizeof(long) does not exceed 4.

yaya, THAT'S IT, so its memcpy, thx Ancient Dragon. But there's still an issue. What if I got an int being the port number, which is to be stored in a char[2]?

A 4-byte-int won't fit on a char[2]...should I memcpy only the first 2 bytes? I've read that this may work only if int's are saved in little endian format, cause the 2 less significant bytes would be first....no clue though...

Thanks in advance once again.

Mariano Lopez-Gappa

ok thx a lot ill be doing that. THX !

Perhaps you need to look up what endian means - particularly big endian and little endian.

Then learn about htons() and it's related functions.