(gawk.info.gz) Library Names

 
 Naming Library Function Global Variables
 ========================================
 
    Due to the way the `awk' language evolved, variables are either
 "global" (usable by the entire program) or "local" (usable just by a
 specific function).  There is no intermediate state analogous to
 `static' variables in C.
 
    Library functions often need to have global variables that they can
 use to preserve state information between calls to the function--for
 example, `getopt''s variable `_opti' ( Processing Command-Line
 Options Getopt Function.).  Such variables are called "private", since
 the only functions that need to use them are the ones in the library.
 
    When writing a library function, you should try to choose names for
 your private variables that will not conflict with any variables used by
 either another library function or a user's main program.  For example,
 a name like `i' or `j' is not a good choice, because user programs
 often use variable names like these for their own purposes.
 
    The example programs shown in this major node all start the names of
 their private variables with an underscore (`_').  Users generally
 don't use leading underscores in their variable names, so this
 convention immediately decreases the chances that the variable name
 will be accidentally shared with the user's program.
 
    In addition, several of the library functions use a prefix that helps
 indicate what function or set of functions use the variables--for
 example, `_pw_byname' in the user database routines ( Reading the
 User Database Passwd Functions.).  This convention is recommended,
 since it even further decreases the chance of inadvertent conflict
 among variable names.  Note that this convention is used equally well
 for variable names and for private function names as well.(1)
 
    As a final note on variable naming, if a function makes global
 variables available for use by a main program, it is a good convention
 to start that variable's name with a capital letter--for example,
 `getopt''s `Opterr' and `Optind' variables ( Processing
 Command-Line Options Getopt Function.).  The leading capital letter
 indicates that it is global, while the fact that the variable name is
 not all capital letters indicates that the variable is not one of
 `awk''s built-in variables, such as `FS'.
 
    It is also important that _all_ variables in library functions that
 do not need to save state are, in fact, declared local.(2) If this is
 not done, the variable could accidentally be used in the user's
 program, leading to bugs that are very difficult to track down:
 
      function lib_func(x, y,    l1, l2)
      {
          ...
          USE VARIABLE some_var   # some_var should be local
          ...                   # but is not by oversight
      }
 
    A different convention, common in the Tcl community, is to use a
 single associative array to hold the values needed by the library
 function(s), or "package."  This significantly decreases the number of
 actual global names in use.  For example, the functions described in
  Reading the User Database Passwd Functions, might have used
 array elements `PW_data["inited"]', `PW_data["total"]',
 `PW_data["count"]', and `PW_data["awklib"]', instead of `_pw_inited',
 `_pw_awklib', `_pw_total', and `_pw_count'.
 
    The conventions presented in this minor node are exactly that:
 conventions. You are not required to write your programs this way--we
 merely recommend that you do so.
 
    ---------- Footnotes ----------
 
    (1) While all the library routines could have been rewritten to use
 this convention, this was not done, in order to show how my own `awk'
 programming style has evolved and to provide some basis for this
 discussion.
 
    (2) `gawk''s `--dump-variables' command-line option is useful for
 verifying this.
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