Re: OFF-TOPIC : Confusion about basic C program behaviour

[Stephen Ray <steve@mrmighty.net>]

Just a thought, but not a particularly educated one.  Compiling with 
-Wall gives me:

test.c:4: warning: return type defaults to `int'
test.c: In function `main':
test.c:15: warning: int format, double arg (arg 3)
test.c:16: warning: int format, double arg (arg 2)
test.c:18: warning: int format, double arg (arg 2)
test.c:18: warning: int format, double arg (arg 3)
test.c:20: warning: control reaches end of non-void function

Looks like maybe the floats are automatically promoted to doubles before 
they are passed to printf.  I don't know if that's standard behaviour, 
but it seems reasonable.  So then in the first case, two 64-bit values 
are put on the stack, and two 64-bit values are taken off the stack.

In the second case, two 64-bit values are put on the stack, one 64-bit 
value is taken off the stack and presented correctly, and one 64-bit 
value has only the first 32 bits read, and misinterpreted as a signed int.

In the third case, two 64-bit values are put on the stack, and the first 
32 bits are interpreted as the first signed int, and the second next 64 
are interpreted as a double.  So case 3 is different from case 2 in that 
the double value in case 3 is made up of two halves of two different 
doubles, while in case 2 the double is made from an actual double.

Or something like that.