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Try to be a little less terse about dealing with variable-length structs

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in C, but recommend that newbies who don't recognize this trick should do
some studying ...
This commit is contained in:
Tom Lane 2001-11-14 22:14:22 +00:00
parent 74114938ad
commit a370cad990
1 changed files with 17 additions and 6 deletions
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23
doc/src/sgml/xfunc.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.42 2001/11/12 19:19:39 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.43 2001/11/14 22:14:22 tgl Exp $
-->
<chapter id="xfunc">
@ -947,11 +947,18 @@ typedef struct {
</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 and initialize the length field.
the correct amount of memory and set 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:
@ -962,9 +969,13 @@ char buffer[40]; /* our source data */
...
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>