Gbase 8s UDR memory management_ 02_ mi_ dalloc

         function mi_dalloc The role of is ： Allocate the specified amount of memory within the specified memory duration , And return a pointer to the allocated block .

1. grammar

        void *mi_dalloc (size, duration)

        mi_integer size;

        MI_MEMORY_DURATION duration;

        size Number of bytes to allocate .

        duration

Specifies the value of the memory duration of the user memory to be allocated . The valid values of duration are as follows ：

value	meaning
PER_ROUTINE	stay UDR During an iteration of
PER_COMMAND	The duration of executing the current subquery
PER_STATEMENT (Deprecated)	At present SQL The duration of the statement
PER_STMT_EXEC	At present SQL During the execution of the statement
PER_STMT_PREP	In the current preparation SQL The duration of the statement
PER_TRANSACTION (Advanced)	During a transaction
PER_SESSION (Advanced)	During the current client session
PER_SYSTEM (Advanced)	During database server execution

On the client side LIBMI Valid in application ？	Valid in user-defined routines ？
yes （ however , Application ignores memory duration ）	yes

2. usage

        mi_dalloc() The function is a DataBlade API The module allocates size bytes of shared memory .mi_dalloc() Function is a constructor for user memory . The behavior of this function is similar to mi_alloc() Exactly the same , It's just mi_dalloc() Allows you to explicitly specify the memory duration when allocating .

The server

about C UDR Most memory allocations in ,duration The parameter must be one of the following public memory duration constants :

 PER_ROUTINE( or PER_FUNCTION)

Appoint PER_ROUTINE, The database server will be in UDR Free allocated memory on return .

 PER_COMMAND

Appoint PER_COMMAND, The database server will free memory when the subquery is completed .

 PER_STMT_EXEC

Appoint PER_STMT_EXEC, The database server will be in SQL Release memory when the statement is completed .

 PER_STMT_PREP

Appoint PER_STMT_PREP, The database server will be ready SQL Release memory when the statement terminates .

         Tips : Only when the regular memory duration cannot safely perform the tasks that need to be completed , Only then C UDR Advanced memory duration used in . These advanced memories last a long time , It will increase the possibility of memory leakage .

         stay C UDR in , When an exception is thrown , The database server is automatically released through mi_dalloc() Allocated memory .

         a key : stay C UDR in , Use DataBlade API Memory management function to allocate memory . Use DataBlade API The memory management function can ensure that the database server can automatically release memory , Especially in the case of return value or exception , Routine cannot free memory .

         When there is no need for mi_dalloc() When allocating memory ,UDR have access to mi_free() Explicitly release mi_dalloc() Allocated memory .

client

         On the client side LIBMI In the application ,mi_dalloc() How it works and malloc() Exactly the same :

         The storage is allocated on the heap of the client process . however , The database server will not automatically free this memory . therefore , client LIBMI Application must use mi_free() Function explicitly releases mi_dalloc() All assignments made . The client LIBMI Application ignores memory duration .

         In the callback function , call mi_dalloc() request PER_COMMAND The duration of will return NULL. therefore , The callback function must call mi_dalloc(), Its duration is PER_ROUTINE.

        mi_dalloc() Function returns a pointer to the newly allocated memory . Convert this pointer to match the structure of the user-defined buffer or structure you allocated .

3. return value

         Pointer to newly allocated memory . Cast this pointer to match the user-defined buffer or structure for which memory is allocated .

NULL Function cannot allocate memory .

        mi_dalloc() The function will not throw when encountering a runtime error MI_Exception event . therefore , This function does not cause the callback to be called .