semantics

In computer science, semantics is an approach of formalizing the meanings of programming languages by constructing mathematical objects that describe the meanings of expressions from the languages.

Semantics is about the meaning of the sentence. It answers the questions: is this sentence valid? If so, what does the sentence mean? For example:

x++;                  // increment
foo(xyz, --b, &qrs);  // call foo

are syntactically valid C statements. But what do they mean? Is it even valid to attempt to transform these statements into an executable sequence of instructions? These questions are at the heart of semantics.

Consider the ++ operator in the first statement:

• If x is a float data type, this statement has no meaning (according to the C language rules) and thus it is an error even though the statement is syntactically correct.

• If x is a pointer to some data type, the meaning of the statement is to “add size of(some data type) to the value at address x and store the result into the location at address x”.

• If x is a scalar, the meaning of the statement is “add one to the value at address x and store the result into the location at address x”.

Finally, note that some semantics cannot be determined at compile-time and must therefore must be evaluated at run-time. In the ++ operator example, if x is already at the maximum value for its data type, the semantic error happens when you try to add 1 to it.

In summary, syntax is the concept that concerns itself only whether or not the sentence is valid for the grammar of the language. Semantics is about whether or not the sentence has a valid meaning.

So two programs written in different languages could do the same thing (semantics) but the symbols used to write the program would be different (syntax).

A compiler will check your syntax for you (compile-time errors), and derive the semantics from the language rules (mapping the syntax to machine instructions say), but won’t find all the semantic errors (run-time errors, e.g. calculating the wrong result because the code says add 1 instead of add 2).

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