C and C++ programs often use low-level features of the underlying system, and therefore are often more difficult to make portable to other platforms.
Several standards have been developed to help make your programs more portable. If you write programs with these standards in mind, you can have greater confidence that your programs work on a wide variety of systems. Language Standards Supported by GCC for a list of C-related standards. Many programs also assume the Posix standard.
The first widely used C variant was K&R C, which predates any C standard. K&R C compilers are no longer of practical interest, though, and Autoconf assumes at least C89, the first C standard, which is sometimes called “C90” due to a delay in standardization. C has since gone through the standards C99, C11, C17, and C23, and Autoconf is compatible with all these standards.
Program portability is a huge topic, and this section can only briefly introduce common pitfalls. See Portability between System Types in The GNU Coding Standards, for more information.