I'm trying to understand implicit datatype conversions in C. I thought that I had understood this topic, but yet the following code example is still confusing me.
Specifically, I have read about Usual Arithmetic Conversions and Integer Promotion previously from drafts of the C Standard.
unsigned short int a = 0;
printf("\n%lld", (signed int)a - 1);
I am compiling using GCC.
unsigned short int is 2 bytes.
int is 4 bytes.
When I run this code, I get the following result: 4294967295
I expected the result -1.
This is what I expected to happen:
Typecast takes precedence, and LHS of - becomes signed int.
- operation is carried out. No integer promotion or implicit conversions occur here, as LHS and RHS are already both signed int. The result of the operation is -1 with datatype signed int.
Within printf statement, value -1 is retained within the conversion to long long int, and -1 is displayed as the result.
Can someone please explain where the flaw in my logic is?
It's undefined behaviour due to %lld being the inappropriate format specifier for an int type.
Yes indeed (signed int)a - 1 is an int type with value -1, but the printf call is the undefined part. There's nothing in the C standard to suggest that a conversion to long long occurs.