Previous: Variable Scope, Up: Function Caveats


9.2.3.3 Passing Function Arguments By Value Or By Reference

In awk, when you declare a function, there is no way to declare explicitly whether the arguments are passed by value or by reference.

Instead the passing convention is determined at runtime when the function is called according to the following rule:

Passing an argument by value means that when a function is called, it is given a copy of the value of this argument. 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 termed 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]
     }

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 (see Options), gawk reports calls to undefined functions.

Some awk implementations generate a runtime error if you use either the next statement or the nextfile statement (see Next Statement, also see Nextfile Statement) inside a user-defined function. gawk does not have this limitation.