Unsigned to signed conversion in C

Question

Is the following guaranteed to work or implementation defined?

unsigned int a = 4294967294;
signed int b = a;

The value of b is -2 on gcc.

From C99 (§6.3.1.3/3) Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

Answer 1

The conversion of a value to signed int is implementation-defined (as you correctly mentioned because of 6.3.1.3p3) . On some systems for example it can be INT_MAX (saturating conversion).

For gcc the implementation behavior is defined here:

The result of, or the signal raised by, converting an integer to a signed integer type when the value cannot be represented in an object of that type (C90 6.2.1.2, C99 6.3.1.3).

For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type; no signal is raised.

http://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html