How to send 00 (aka "binary zero") in C?

Barca · 02-08-2007, 01:48 PM

Hi I send data using socket like this:

Code:

    strcpy(send, "\x2f\xcd\xd3\xbe"); 
    tmp_file = fdopen(socket, "w"); 
    fprintf(file, send); 
    fclose(file);

And it works fine -> it sends data specified in hex and Wireshark shows in hex: 2fcdd3be.
But I need to send "00". I tried many ways: Code:

Code:

"\x2f\xcd\xd3\xbe\x000" 
"\x2f\xcd\xd3\xbe\x00" 
"\x2f\xcd\xd3\xbe\x0" 
"\x2f\xcd\xd3\xbe\0"

etc.

Nothing works. I want that Wireshark show: 2fcdd3be00 but every time it stops after be and doesn't show any more data. Thanks in advance.

anon276 · 02-08-2007, 02:00 PM

Can you check the string for a certain value before you read it into where it needs to go? If you can, try a strcmp(<string>, "00\0"); for instance.

Barca · 02-08-2007, 02:04 PM

Ha

I just solved that problem. I used fwrite():

Code:

    unsigned char test[] = "\xbe\x6c\xcd\x2f\x00\xf2";
    file = fdopen(socket, "w");
    fwrite(test, 1, 6, file);
    fflush(file);
    fclose(file);

Thank you for your post

KenJennings · 02-08-2007, 02:12 PM

Quote:

Originally Posted by Barca

Hi I send data using socket like this: ...

Nothing works. I want that Wireshark show: 2fcdd3be00 but every time it stops after be and doesn't show any more data. Thanks in advance.

It doesn't work they way you expect , because you're sending data using the higher level stdio functions which recognize a 0 byte as the end of string flag which is then ignored/not sent.

If you must do it this way, you may have to write the 0 byte by itself, say with fputc() or something.

Ideally you should deal with a socket at a lower level and use the send() and recv() functions. Beej's Super Fantastic Guide to Networks page is the best explanation of socket programming: http://beej.us/guide/bgnet/

Barca · 02-09-2007, 05:19 PM

I'm back :?
I still wants to easily sends hexadecimal data by socket. I already has this piece of code:

Code:

file = fdopen(socket, "w");
fwrite("\x15\x00", 1, 2, file);
char data_hex[] = "\x20\x30\x40\x50";
fwrite(data_hex, 1, 4, file);
fwrite("\x14\x24\xbc", 1, 3, file);
fclose(tmp_file);

Code works fine, I see in my Wireshark that it's send exactly as I want, except that data_hex variable. It's sent not as hexadecimal value but mhm "plain text".
And it's my exact question: How can I pass to fwrite() some variable and cause fwrite() to send it as hexadecimal? In other words:
fwrite("\x20\x30\x40\x50", 1, 4, file); -> WORKS
fwrite(data_hex, 1, 3, file); -> DOESN'T WORKS
data_hex will have different values so I can't pass it as "\x20\x30\x40\x50"