You are passing a float for the %d format string, but printf is expecting an int. printf is a variable argument list function: printf(const char *format_string,…); This means that the rest of the arguments after the format string can be of any type and number, and the compiler doesn’t know what they should be. It … Read more
Technically, the value of an uninitialized non static local variable is Indeterminate[Ref 1]. In short it can be anything. Accessing such a uninitialized variable leads to an Undefined Behavior.[Ref 2] [Ref 1] C99 section 6.7.8 Initialization: If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. [Ref 2] C99 … Read more
In C++, such code has undefined behaviour: [stmt.return]/2 … Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function. … Most compilers will produce a warning for code similar to that in the question. The C++ standard does not require this … Read more
As others have noted, this is Undefined Behavior (UB). In practice, it will (probably) actually (kind of) work. Reading from an uninitialized register on x86[-64] architectures will indeed produce garbage results, and probably won’t do anything bad (as opposed to e.g. Itanium, where registers can be flagged as invalid, so that reads propagate errors like … Read more