What should I #include to use 'htonl'?

Question

I want to use the htonl function in my ruby c extension, but don't want to use any of the other internet stuff that comes with it. What would be the most minimalistic file to #include that is still portable? Looking through the header files on my computer, I can see that either machine/endian.h or sys/_endian.h would let me use them, although I am not sure if that is a good idea.

Answer 1

The standard header is:

#include <arpa/inet.h>

You don't have to worry about the other stuff defined in that header. It won't affect your compiled code, and should have only a minor effect on compilation time.

EDIT: You can test this. Create two files, htonl_manual.c

// non-portable, minimalistic header
#include <byteswap.h>
#include <stdio.h>
int main()
{
    int x = 1;
    x = __bswap_32(x);
    printf("%d\n", x);
}

and htonl_include.c:

// portable
#include <arpa/inet.h>
#include <stdio.h>
int main()
{
    int x = 1;
    x = htonl(x);
    printf("%d\n", x);
}

Assemble them at -O1, then take the difference:

gcc htonl_manual.c -o htonl_manual.s -S -O1
gcc htonl_include.c -o htonl_include.s -S -O1
diff htonl_include.s htonl_manual.s

For me, the only difference is the filename.

Answer 2

If you don't want to include anything network-related, it's perfectly valid to declare htonl yourself. Just #include <stdint.h> to get uint32_t and use the following prototype:

uint32_t htonl(uint32_t);

Reference: POSIX: http://www.opengroup.org/onlinepubs/9699919799/functions/htonl.html

You can also just implement your own by testing byte order (at compiletime) using unions. This doesn't require any odd ENDIAN macros or #ifdefs.