Thread: CurrentMemoryContext and MemoryContextStrdup

Yessica Brinkmann <yessica.brinkmann@gmail.com> writes:
> I have written in the general list on this subject, but since I do not get
> an answer according to my little knowledge on the subject, I write on this
> list, since perhaps my question is very basic.
> My query is the following: can you please give me some reference (link on
> the Internet) that explains how CurrentMemoryContext and
> MemoryContextStrdup is used, and also some example of source code of use?

I think you'd feel a lot better informed after reading

src/backend/utils/mmgr/README

            regards, tom lane

Yessica Brinkmann <yessica.brinkmann@gmail.com> writes:
> Thank you very much for the reply!
> Not really, I don't feel better informed because MemoryContextStrdup is not
> even mentioned once in the README.

The next thing to do would be to look at that function's header comment
(find it in src/backend/utils/mmgr/mcxt.c):

/*
 * MemoryContextStrdup
 *        Like strdup(), but allocate from the specified context
 */
char *
MemoryContextStrdup(MemoryContext context, const char *string)


As I recall your original problem, people were suggesting that
you make a longer-lived copy of some transiently-allocated
string.  Copying it into a different context would be the
way to do that, as I hope you now understand from the README
discussion, and this function is the easiest way to do that.

Or at least the second easiest; the very easiest is pstrdup,
which is just

char *
pstrdup(const char *in)
{
    return MemoryContextStrdup(CurrentMemoryContext, in);
}

but I don't remember whether the current context was a suitable
target for your use-case.

            regards, tom lane

Yessica Brinkmann <yessica.brinkmann@gmail.com> writes:
> Now, as I understand it (I don't know if I'm right), what I would have to
> do would be this:
> 1. I save the CurrentMemoryContext, for example as follows:
> MemoryContext oldcontext = CurrentMemoryContext;
> 2. I make the call to SPI, which was what caused the context problem.
> 3. I copy my variable in a different context, for example as follows:
> MemoryContext newcontext;
> char * copy = MemoryContextStrdup (newcontext, data);
> 4. Then, at the end of my call to SPI, after SPI_finish () I would have in
> the copy variable, the copy of the data variable, to use it as I want.
> Is that so? I am correct at least to try to modify my program or is it
> totally something else what should I do?

IIRC, the real issue here is that SPI_connect creates and switches into
a temporary context that SPI_finish will destroy; and the string you want
is in that short-lived context.  So what you want is to copy it out of
that context, probably into the context that was current before you
call SPI_connect.  So what you want is to do this before calling
SPI_connect:

    MemoryContext outer_context = CurrentMemoryContext;

and then this, inside the SPI operation, will save your string safely:

    copy = MemoryContextStrdup(outer_context, data);

            regards, tom lane