I'm working on the mysql client lib for Julia. I'm using the C api that
support statement mode. To retrieve data after executing the query, it
needs C structs (MYSQL_BIND) to be created and passed to C. Each field of
the row retrieved from db will be stored in the corresponding struct and I
should read the value from the struct.
It is unfortunate that I did not notice the line in Julia documentation
stating "Currently, it is not possible to pass structs and other
non-primitive types from Julia to C libraries." and spent an inordinate
amount of time trying to debug what I did wrong.
This particular struct is defined in mysql/mysql.h. What are the options I
have?
1. Is there a julia call that can read the header files and create the
struct or an equivalent feature?
2. The non-statement mode doesn't seem to have complex structures. Is
that the better approach?
For reference, to test passing structs, I created the following files
*teststruct.jl *
somenum= uint64(1)
type MyStruct
somefield :: Ptr {Uint64}
end
function check(my :: MyStruct)
ccall((:check, "structtest.so"), Cint, (Ptr{Void},) ,
pointer_from_objref(my))end
newstruct = MyStruct(pointer_from_objref(somenum))
print("The pointer address stored in somefield as seen in julia: ")
println(newstruct.somefield)
print("The pointer address of newstruct as seen in julia: ")
println(pointer_from_objref(newstruct))
check(newstruct)
*structtest.c *
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
typedef struct st_mystruct
{
unsigned long *somefield;
} MyStruct;
int check(MyStruct *my)
{
printf("The pointer address of struct as seen in c: %p\n", my);
printf("The pointer address stored in somefield as seen in c: %p\n",
my->somefield);
}
The output was
*The pointer address stored in somefield as seen in julia: Ptr{Uint64}
@0x00000000020c1560The pointer address of newstruct as seen in julia:
Ptr{Void} @0x00000000022e5540The pointer address of struct as seen in c:
0x22e5540The pointer address stored in somefield as seen in c: 0x206ee90*
As we can see, the value of 'somefield' ends up garbled.
