A conforming standard library file math.h is not only not required to, but actually must not define M_PI by default. In this context ‘by default’ means that M_PI must only get defined through compiler-specific tricks, most often undefined behavior through the use of reserved identifiers. Just define the constant yourself (you can use the name … Read more
C89, not totally (ยง3.3.5/6). It can be either -5 or 5, because -5 / 10 can return 0 or -1 (% is defined in terms of a linear equation involving /, * and +): When integers are divided and the division is inexact, if both operands are positive the result of the / operator is … Read more
Everything before standardization is generally called “K&R C”, after the famous book (1st edition and 2nd edition), with Dennis Ritchie, the inventor of the C language, as one of the authors. This was “the C language” from 1972-1989. The first C standard was released 1989 nationally in USA, by their national standard institute ANSI. This … Read more
It compiles successfully because GCC allows the declaration of s as a GNU extension, even though it’s not part of the C89 or ANSI standard. If you want to adhere strictly to those standards, you must pass the -pedantic flag. The declaration of c at the start of a { } block is part of … Read more
MSVC is not a C99 compiler, and does not support variable length arrays. At https://docs.microsoft.com/en-us/cpp/c-language/ansi-conformance MSVC is documented as conforming to C90.
It depends on what you mean by ‘truly standards compliant’. However, the short answer is “it is a good idea to ensure that all functions have a prototype in scope before being used”. A more qualified answer notes that if the function accepts variable arguments (notably the printf() family of functions), then a prototype must … Read more
As the C FAQ says: It’s not clear if it’s legal or portable, but it is rather popular. and: … an official interpretation has deemed that it is not strictly conforming with the C Standard, although it does seem to work under all known implementations. (Compilers which check array bounds carefully might issue warnings.) The … Read more
The strncpy() function was designed with a very particular problem in mind: manipulating strings stored in the manner of original UNIX directory entries. These used a short fixed-sized array (14 bytes), and a nul-terminator was only used if the filename was shorter than the array. That’s what’s behind the two oddities of strncpy(): It doesn’t … Read more
Will result always be the floor of the division? What is the defined behavior? Not quite. It rounds toward 0, rather than flooring. 6.5.5 Multiplicative operators 6 When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded.88) If the quotient a/b is representable, the expression (a/b)*b … Read more