Re: Memory for BYTEA returned by C function is not released until connection is dropped

2015-09-21 4:31 GMT+02:00 John Leiseboer <JL(at)quintessencelabs(dot)com>:

> I have written a number of functions in C that return BYTEA type. I have
> compiled and run on both Windows and Linux, 32-bit and 64-bit, PostgreSQL
> versions 9.3 and 9.4.
>
> My functions return BYTEA data to the caller. The problem is that memory
> usage grows until there is no memory left on the host, at which point an
> error is returned. If I drop the connection (e.g. by quitting from pqsql),
> the memory is returned.
>
> I wrote the following minimal function to test palloc() and BYTEA return
> behaviour, and found that this minimal program also exhibits the unbounded
> memory growth problem.
>
>
> C source code:
>
> PG_FUNCTION_INFO_V1(test_palloc);
> Datum test_palloc()
> {
> bytea *test_ret;
> int test_len = 1024;
>
> test_ret = (bytea *)palloc(test_len + VARHDRSZ);
> SET_VARSIZE(test_ret, test_len + VARHDRSZ);
> PG_RETURN_BYTEA_P(test_ret);
> }
>
> Function definition:
>
> CREATE OR REPLACE FUNCTION test_palloc() RETURNS BYTEA
> AS E'<path to shared library>', test_palloc' LANGUAGE C IMMUTABLE STRICT;
>
> psql commands to reproduce the problem:
>
> \o out.txt
> SELECT ids.*, test_palloc() FROM GENERATE_SERIES(1, 1000000) ids;
>
> At the completion of the above command, host memory will have been
> consumed but not released back to the system. After quitting psql (\q),
> memory is released.
>
> Is this expected behaviour or a bug? Am I doing something wrong? How can I
> return a BYTEA type from a C library function without having to drop the
> connection in order to recover the allocated memory that is returned to the
> caller?
>
>
This memory is release, when memory context is dropped.

http://www.neilconway.org/talks/hacking/hack_slides.pdf look on slide 15

Regards

Pavel

> Regards,
> John
>
>
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