使用sprintf打印六进制值的便携式方法

Question

I've had some trouble with binary-to-(printable)hexa conversions. I've reached a functional (for my system) way of writing the code, but I need to know if it is portable on all systems (OS & hardware).

So this is my function (trying to construct a UUID from a piece of binary text):

int extractInfo( unsigned char * text )
{
    char h[3];
    int i;

    this->str.append( "urn:uuid:" );
    for( i = 56; i < 72; i++ )
    {
        ret = snprintf( h, 3, "%02x", text[i] );
        if( ret != 2 )
            return 1;
        this->str.append( h );
        if( i == 59 || i == 61 || i == 63 || i == 65 )
            this->str.append( "-" );
    }

    return 0;
}

I understood that because of the sign extension my values are not printed well if I use char instead of unsigned char ( C++ read binary file and convert to hex ). Accepted and modified respectively.

But I've encountered more variants of doing this: Conversion from binary file to hex in C , and I am really lost. In unwind's piece of code:

sprintf(hex, "%02x", (unsigned int) buffer[0] & 0xff);

I did not understood why, although the array is unsigned char (as defined in the original posted code, by the one who asked the question), a cast to an unsigned int is needed, and also a bitwise AND on the byte to be converted...

So, as I did not understood very well the sign-extension thing, can you tell me at least if the piece of code I wrote will work on all systems?

Answer 1

since printf is not typesafe it expects for each formatting specifier a special sized argument. thatswhy you have to cast your character argument to unsigned int if you use some formatting character that expects an int-sized type.

The "%x" specifier requires an unsigned int.