GNU Info

(gawk.info)Function Caveats

---

Next: Return Statement Prev: Function Example Up: User-defined

---

Enter node , (file) or (file)node

Calling User-Defined Functions
------------------------------

   "Calling a function" means causing the function to run and do its
job.  A function call is an expression and its value is the value
returned by the function.

   A function call consists of the function name followed by the
arguments in parentheses.  `awk' expressions are what you write in the
call for the arguments.  Each time the call is executed, these
expressions are evaluated, and the values are the actual arguments.  For
example, here is a call to `foo' with three arguments (the first being
a string concatenation):

     foo(x y, "lose", 4 * z)

   *Caution:* Whitespace characters (spaces and tabs) are not allowed
between the function name and the open-parenthesis of the argument list.
If you write whitespace by mistake, `awk' might think that you mean to
concatenate a variable with an expression in parentheses.  However, it
notices that you used a function name and not a variable name, and
reports an error.

   When a function is called, it is given a _copy_ of the values of its
arguments.  This is known as "call by value".  The caller may use a
variable as the expression for the argument, but the called function
does not know this--it only knows what value the argument had.  For
example, if you write the following code:

     foo = "bar"
     z = myfunc(foo)

then you should not think of the argument to `myfunc' as being "the
variable `foo'."  Instead, think of the argument as the string value
`"bar"'.  If the function `myfunc' alters the values of its local
variables, this has no effect on any other variables.  Thus, if `myfunc'
does this:

     function myfunc(str)
     {
       print str
       str = "zzz"
       print str
     }

to change its first argument variable `str', it _does not_ change the
value of `foo' in the caller.  The role of `foo' in calling `myfunc'
ended when its value (`"bar"') was computed.  If `str' also exists
outside of `myfunc', the function body cannot alter this outer value,
because it is shadowed during the execution of `myfunc' and cannot be
seen or changed from there.

   However, when arrays are the parameters to functions, they are _not_
copied.  Instead, the array itself is made available for direct
manipulation by the function.  This is usually called "call by
reference".  Changes made to an array parameter inside the body of a
function _are_ visible outside that function.

   *Note:* Changing an array parameter inside a function can be very
dangerous if you do not watch what you are doing.  For example:

     function changeit(array, ind, nvalue)
     {
          array[ind] = nvalue
     }
     
     BEGIN {
         a[1] = 1; a[2] = 2; a[3] = 3
         changeit(a, 2, "two")
         printf "a[1] = %s, a[2] = %s, a[3] = %s\n",
                 a[1], a[2], a[3]
     }

This program prints `a[1] = 1, a[2] = two, a[3] = 3', because
`changeit' stores `"two"' in the second element of `a'.

   Some `awk' implementations allow you to call a function that has not
been defined. They only report a problem at runtime when the program
actually tries to call the function. For example:

     BEGIN {
         if (0)
             foo()
         else
             bar()
     }
     function bar() { ... }
     # note that `foo' is not defined

Because the `if' statement will never be true, it is not really a
problem that `foo' has not been defined.  Usually though, it is a
problem if a program calls an undefined function.

   If `--lint' is specified (Note: Command-Line Options.),
`gawk' reports calls to undefined functions.

   Some `awk' implementations generate a runtime error if you use the
`next' statement (Note: The `next' Statement.)  inside
a user-defined function.  `gawk' does not have this limitation.