diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/xfunc.sgml | 23 |
1 files changed, 17 insertions, 6 deletions
diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml index 59dcea35510..fed2036236b 100644 --- a/doc/src/sgml/xfunc.sgml +++ b/doc/src/sgml/xfunc.sgml @@ -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->length = VARHDRSZ + 40; -memmove(destination->data, buffer, 40); +memcpy(destination->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> |