Wednesday, September 19, 2007

Endianness, beautifully done

My performance enthusiastic friend wrote an excellent article (with working code) in how to determine the system's endianness and how to do swap bytes.

Out of curiosity, I browsed the Tcl source code to see how Tcl work out the global variable tcl_platform(byteOrder). Below code is extracted from tclBasic.c under the Tcl8.4.x source base. It is done beautifully and it really showed what Beautiful Code is all about.


     union {
         char c[sizeof(short)];
         short s;
     } order;


      * Compute the byte order of this machine.

     order.s = 1;
     Tcl_SetVar2(interp, "tcl_platform", "byteOrder",
             ((order.c[0] == 1) ? "littleEndian" : "bigEndian"),

I extracted part of the code for verification.

union {
         char c[sizeof(short)];
         short s;
} order;

int main(void)
         printf("%s\n",order.c[0]==1 ? "littleEndian" : "bigEndian");

On my Intel notebook, it is littleEndian and on the SPARC it is bigEndian. Of course it tallies with the Tcl global variable tcl_platform(byteOrder). In case you a Perl guy, this is how you can do it:

$is_big = unpack("h*", pack("s", 1)) =~ /01/;
$is_little = unpack("h*", pack("s", 1)) =~ /^1/;

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