Definitions of functions can appear anywhere between the rules of an
awk program. Thus, the general form of an awk program is
extended to include sequences of rules and user-defined function
definitions.
There is no need in awk to put the definition of a function
before all uses of the function. This is because awk reads the
entire program before starting to execute any of it.
The definition of a function named name looks like this:
function name(parameter-list)
{
body-of-function
}
name is the name of the function to be defined. A valid function
name is like a valid variable name: a sequence of letters, digits and
underscores, not starting with a digit.
Within a single awk program, any particular name can only be
used as a variable, array or function.
parameter-list is a list of the function's arguments and local variable names, separated by commas. When the function is called, the argument names are used to hold the argument values given in the call. The local variables are initialized to the empty string. A function cannot have two parameters with the same name.
The body-of-function consists of awk statements. It is the
most important part of the definition, because it says what the function
should actually do. The argument names exist to give the body a
way to talk about the arguments; local variables, to give the body
places to keep temporary values.
Argument names are not distinguished syntactically from local variable names; instead, the number of arguments supplied when the function is called determines how many argument variables there are. Thus, if three argument values are given, the first three names in parameter-list are arguments, and the rest are local variables.
It follows that if the number of arguments is not the same in all calls to the function, some of the names in parameter-list may be arguments on some occasions and local variables on others. Another way to think of this is that omitted arguments default to the null string.
Usually when you write a function you know how many names you intend to use for arguments and how many you intend to use as local variables. It is conventional to place some extra space between the arguments and the local variables, to document how your function is supposed to be used.
During execution of the function body, the arguments and local variable
values hide or shadow any variables of the same names used in the
rest of the program. The shadowed variables are not accessible in the
function definition, because there is no way to name them while their
names have been taken away for the local variables. All other variables
used in the awk program can be referenced or set normally in the
function's body.
The arguments and local variables last only as long as the function body is executing. Once the body finishes, you can once again access the variables that were shadowed while the function was running.
The function body can contain expressions which call functions. They can even call this function, either directly or by way of another function. When this happens, we say the function is recursive.
In many awk implementations, including gawk,
the keyword function may be
abbreviated func. However, POSIX only specifies the use of
the keyword function. This actually has some practical implications.
If gawk is in POSIX-compatibility mode
(see section Command Line Options), then the following
statement will not define a function:
func foo() { a = sqrt($1) ; print a }
Instead it defines a rule that, for each record, concatenates the value
of the variable `func' with the return value of the function `foo'.
If the resulting string is non-null, the action is executed.
This is probably not what was desired. (awk accepts this input as
syntactically valid, since functions may be used before they are defined
in awk programs.)
To ensure that your awk programs are portable, always use the
keyword function when defining a function.
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