As far as I understand, you should use my_free() for this.

Sure, that's what I could see in the libmysqld source also. The issue is that my_free() isn't exported from libmysqld, nor is this documented, so I figured it might change and if my_free() really is to be used, then make sure my_free() is exported, make sure my_free() is documented and make sure to document that my_free() should be used for this purpose.

my_free should be used, but I do not understand what should be fixed, documentation?

Two things: Documentation and, on Windows, make sure that my_free is exported from the .dll. The following samle program:
#include <stdio.h>
#include <my_global.h>
#include <mysql.h>
#include <ma_dyncol.h>

int main(int argc, char* argv[])
{
DYNAMIC_COLUMN dc;
DYNAMIC_COLUMN_VALUE dcv[1];
uint colno[] =

;

dcv[0].type = DYN_COL_INT;
dcv[0].x.long_value = 1;
mariadb_dyncol_create_many_num(&dc, 1, colno, dcv, TRUE);
my_free(dc.str);
return 0;
}
Compiles and runs nicely on Linux, but on Windows I get:
Error 2 error C3861: 'my_free': identifier not found c:\programming\src\consoleapplication1\consoleapplication1\consoleapplication1.cpp 14 1 ConsoleApplication1
Tested with MariaDB 10.0.7 on both Linux and WIndows

Shall we rather provide mariadb_dyncol_unpack_free() for this? That'd be more logical, than explaining to use my_free(). Any better name than mariadb_dyncol_unpack_free()?

mariadb_dyncol_unpack_free() looks quite logical for me...

karlsson do you think we should provide mariadb_dyncol_unpack_free() or something like that?

Yes, mariadb_dyncol_unpack_free() solves the problem and looks logical. Just make that this is exported from the DLL on Windows (my_free() is not exported on Windows, so on that platform the workaround to just call my_free() doesn't work there. This was true in 10.0.7 at least).