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<chapter id="xfunc">

</para>

<para>

Obviously, the data field shown here is not long enough to hold

all possible strings; it's impossible to declare such

a structure in <acronym>C</acronym>. When manipulating

Obviously, the data field declared here is not long enough to hold

all possible strings. Since it's impossible to declare a variable-size

structure in <acronym>C</acronym>, we rely on the knowledge that the

<acronym>C</acronym> compiler won't range-check array subscripts. We

just allocate the necessary amount of space and then access the array as

if it were declared the right length. (If this isn't a familiar trick to

you, you may wish to spend some time with an introductory

<acronym>C</acronym> programming textbook before delving deeper into

<productname>Postgres</productname> server programming.)

When manipulating

variable-length types, we must be careful to allocate

the correct amount of memory andinitialize the length field.

the correct amount of memory andset the length field correctly.

For example, if we wanted to store 40 bytes in a text

structure, we might use a code fragment like this:

...

text *destination = (text *) palloc(VARHDRSZ + 40);

destination-&gt;length = VARHDRSZ + 40;

memmove(destination-&gt;data, buffer, 40);

memcpy(destination-&gt;data, buffer, 40);

...

</programlisting>

<literal>VARHDRSZ</> is the same as <literal>sizeof(int4)</>, but

it's considered good style to use the macro <literal>VARHDRSZ</>

to refer to the size of the overhead for a variable-length type.

</para>

<para>