File: gawk.info,  Node: Library Names,  Next: General Functions,  Up: Library Functions

10.1 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' (*note Getopt Function::).  Such
variables are called "private", as 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 names 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 (*note Passwd
Functions::).  This convention is recommended, as 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.(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 those variables' names with a capital letter--for example,
'getopt()''s 'Opterr' and 'Optind' variables (*note 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 predefined 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 variables 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)
     {
         ...
         # some_var should be local but by oversight is not
         USE VARIABLE some_var
         ...
     }

   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
*note 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.

   Beginning with version 5.0, 'gawk' provides a powerful mechanism for
solving the problems described in this section: "namespaces".
Namespaces and their use are described in detail in *note Namespaces::.

   ---------- Footnotes ----------

   (1) Although all the library routines could have been rewritten to
use this convention, this was not done, in order to show how our 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.