It's impossible to send chars in background because I usually need to send more than one string to the serial port

Exemple :
buffer's length = 255

foo()
{
nbrToSend = sprintf(buffer, [string of 255 chars]);
flush();
nbrToSend = sprintf(buffer, [string of 255 chars]);
flush();
}

if the function flush don't block in infinity loop while all chars are sent, I'll have a problem when I'll call the second sprintf which will rewrite the buffer with new chars.