identifier in c and c++

In C, identifiers may be of any length. However, not all characters will necessarily be significant. C defines two kinds of identifiers: external and internal. An external identifier will be involved in an external link process. These identifiers, called external names, include function names and global variable names that are shared between source files. If the identifier is not used in an external link process, then it is internal. This type of identifier is called an internal name and includes the names of local variables, for example. In C89, at least the first 6 characters of an external identifier and at least the first 31 characters of an internal identifier will be significant. C99 has increased these values. In C99, an external identifier has at least 31 significant characters, and an internal identifier has at least 63 significant characters. As a point of interest, in C++, at least the first 1,024 characters of an identifier are significant. These differences may be important if you are converting a program from C89 to C99, or from C to C++.