How to convert struct to char array in C

Question

I\'m trying to convert a struct to a char array to send over the network. However, I get some weird output from the char array when I do.

#include 

        

Answer 1

Unless you have very convincing measurements showing that every octet is precious, don't do this. Use a readable ASCII protocol like SMTP, NNTP, or one of the many other fine Internet protocols codified by the IETF.

If you really must have a binary format, it's still not safe just to shove out the bytes in a struct, because the byte order, basic sizes, or alignment constraints may differ from host to host. You must design your wire protcol to use well-defined sizes and to use a well defined byte order. For your implementation, either use macros like ntohl(3) or use shifting and masking to put bytes into your stream. Whatever you do, make sure your code produces the same results on both big-endian and little-endian hosts.

Answer 2

You may want to convert to a unsigned char array.

Answer 3

char is a signed type; so with two's complement, 0x80 is -128 for an 8-bit integer (i.e. a byte)

Answer 4

char is a signed type so what you are seeing is the two-compliment representation, casting to (unsigned char*) will fix that (Rowland just beat me).

On a side note you may want to change

for (i=0; i<4; i++) {
//...
}

to

for (i=0; i<sizeof(x); i++) {
//...
}

Answer 5

What you see is the sign preserving conversion from char to int. The behavior results from the fact that on your system, char is signed (Note: char is not signed on all systems). That will lead to negative values if a bit-pattern yields to a negative value for a char. Promoting such a char to an int will preserve the sign and the int will be negative too. Note that even if you don't put a (int) explicitly, the compiler will automatically promote the character to an int when passing to printf. The solution is to convert your value to unsigned char first:

for (i=0; i<4; i++)
   printf("%02x ", (unsigned char)b[i]);

Alternatively, you can use unsigned char* from the start on:

unsigned char *b = (unsigned char *)&a;

And then you don't need any cast at the time you print it with printf.

Answer 6

Converting your structure to characters or bytes the way you're doing it, is going to lead to issues when you do try to make it network neutral. Why not address that problem now? There are a variety of different techniques you can use, all of which are likely to be more "portable" than what you're trying to do. For instance:

• Sending numeric data across the network in a machine-neutral fashion has long been dealt with, in the POSIX/Unix world, via the functions htonl, htons, ntohl and ntohs. See, for example, the byteorder(3) manual page on a FreeBSD or Linux system.
• Converting data to and from a completely neutral representation like JSON is also perfectly acceptable. The amount of time your programs spend converting the data between JSON and native forms is likely to pale in comparison to the network transmission latencies.