What is PORTABLE and UN-PORTABLE CODE

Unportable Code:
implementation-defined— The compiler-writer chooses what happens, and has to document it.
Example: whether the sign bit is propagated, when shifting an int right.
unspecified— The behavior for something correct, on which the standard does not impose any requirements.
Example: the order of argument evaluation.
Bad Code:
undefined— The behavior for something incorrect, on which the standard does not impose any requirements. Anything is allowed to happen, from nothing, to a warning message to program termination, to CPU meltdown, to launching nuclear missiles (assuming you have the correct hardware option installed).
Example: what happens when a signed integer overflows.
a constraint— This is a restriction or requirement that must be obeyed. If you don't, your program behavior becomes undefined in the sense above. Now here's an amazing thing: it's easy to tell if something is a constraint or not, because each topic in the standard has a subparagraph labelled "Constraints" that lists them all. Now here's an even more amazing thing: the standard specifies [5] that compilers only have to produce error messages for violations of syntax and constraints! This means that any semantic rule that's not in a constraints subsection can be broken, and since the behavior is undefined, the compiler is free to do anything and doesn't even have to warn you about it!

Example: the operands of the % operator must have integral type. So using a non-integral type with % must cause a diagnostic.
Example of a rule that is not a constraint: all identifiers declared in the C standard header files are reserved for the implementation, so you may not declare a function called malloc() because a standard header file already has a function of that name. But since this is not a constraint, the rule can be broken, and the compiler doesn't have to warn you

Portable Code:

strictly-conforming— A strictly-conforming program is one that:
• only uses specified features.
• doesn't exceed any implementation-defined limit.
• has no output that depends on implementation-defined, unspecified, or undefined features.

This was intended to describe maximally portable programs, which will always produce the identical output whatever they are run on. In fact, it is not a very interesting class because it is so small compared to the universe of conforming programs. For example, the following program is not strictly conforming:

#include
#include
int main() { (void) printf("biggest int is %d", INT_MAX);
return 0;}
/* not strictly conforming: implementation-defined output! */


conforming— A conforming program can depend on the nonportable features of an implementation. So a program is conforming with respect to a specific implementation, and the same program may be nonconforming using a different compiler. It can have extensions, but not extensions that alter the
behavior of a strictly-conforming program. This rule is not a constraint, however, so don't expect the compiler to warn you about violations that render your program nonconforming!
The program example above is conforming.