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The routines in this section allow you to create and release
cached values. Like scalar cookies, in theory, cached values
are not necessary. You can create numbers and strings using
the functions in Constructor Functions. You can then
assign those values to variables using sym_update()
or sym_update_scalar()
, as you like.
However, you can understand the point of cached values if you remember that
every string value’s storage must come from gawk_malloc()
,
gawk_calloc()
, or gawk_realloc()
.
If you have 20 variables, all of which have the same string value, you
must create 20 identical copies of the string.108
It is clearly more efficient, if possible, to create a value once, and
then tell gawk
to reuse the value for multiple variables. That
is what the routines in this section let you do. The functions are as follows:
awk_bool_t create_value(awk_value_t *value, awk_value_cookie_t *result);
Create a cached string or numeric value from value
for
efficient later assignment. Only values of type AWK_NUMBER
, AWK_REGEX
, AWK_STRNUM
,
and AWK_STRING
are allowed. Any other type is rejected.
AWK_UNDEFINED
could be allowed, but doing so would result in
inferior performance.
awk_bool_t release_value(awk_value_cookie_t vc);
Release the memory associated with a value cookie obtained
from create_value()
.
You use value cookies in a fashion similar to the way you use scalar cookies. In the extension initialization routine, you create the value cookie:
static awk_value_cookie_t answer_cookie; /* static value cookie */ static void my_extension_init() { awk_value_t value; char *long_string; size_t long_string_len; /* code from earlier */ … /* … fill in long_string and long_string_len … */ make_malloced_string(long_string, long_string_len, & value); create_value(& value, & answer_cookie); /* create cookie */ … }
Once the value is created, you can use it as the value of any number of variables:
static awk_value_t * do_magic(int nargs, awk_value_t *result) { awk_value_t new_value; … /* as earlier */ value.val_type = AWK_VALUE_COOKIE; value.value_cookie = answer_cookie; sym_update("VAR1", & value); sym_update("VAR2", & value); … sym_update("VAR100", & value); … }
Using value cookies in this way saves considerable storage, as all of
VAR1
through VAR100
share the same value.
You might be wondering, “Is this sharing problematic?
What happens if awk
code assigns a new value to VAR1
;
are all the others changed too?”
That’s a great question. The answer is that no, it’s not a problem.
Internally, gawk
uses reference-counted strings. This means
that many variables can share the same string value, and gawk
keeps track of the usage. When a variable’s value changes, gawk
simply decrements the reference count on the old value and updates
the variable to use the new value.
Finally, as part of your cleanup action (see section Registering An Exit Callback Function)
you should release any cached values that you created, using
release_value()
.
Numeric values
are clearly less problematic, requiring only a C double
to store.
But of course, GMP and MPFR values do take up more memory.
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