sqlite mode

A mode() aggregate function for sqlite.

Warning

Not for production use.

See license

Build

linux:

clang -g -shared -fPIC -Wall -Wl,-lbsd,-lsqlite3,-lc -O2 -Werror -o mode.dylib mode.c modemath.c

mac osx:

clang -g -fPIC -Wall -dynamiclib -Wl,-lsqlite3,-lc,-L$HOME/opt/anaconda3/lib/ -I $HOME/opt/anaconda3/include -arch x86_64 -O2 -Werror -o mode.dylib mode.c modemath.c

Example usage

Load module & create test table

sqlite3 -cmd ".load $(pwd)/mode" \
 -cmd "create table people (name varchar(10), age numeric); insert into people values ('me', 1), ('you', 2), ('them', 2);" \
 :memory:

Use the mode function

sqlite> select mode(age) from people
2.0

Debugging

1. In terminal A: Run interactive sqlite3 interactive shell and load module as described above

2. In terminal B: start lldb & attach

   lldb -n sqlite3
   b &modeDataInit
   c
   

3. In terminal A: run mode function in sql statement

4. In terminal B: go back to lldb and continue the deubugging session...

(lldb) frame variable
(ModeData *) data = 0x00007fbeb0016838
(lldb) frame variable *data
(ModeData) *data = {
  size = 0
  capacity = 0
  values = 0x0000000000000000
}
(lldb) po ((ModeData*) 0x00007fbeb0016838).size
0
(lldb) dis -F intel
mode.dylib`modeDataInit:
    0x10ada1de0 <+0>:  push   rbp
    0x10ada1de1 <+1>:  mov    rbp, rsp
->  0x10ada1de4 <+4>:  xorps  xmm0, xmm0
    0x10ada1de7 <+7>:  movups xmmword ptr [rdi], xmm0
    0x10ada1dea <+10>: mov    qword ptr [rdi + 0x10], 0x0
    0x10ada1df2 <+18>: xor    eax, eax
    0x10ada1df4 <+20>: pop    rbp
    0x10ada1df5 <+21>: ret    

(lldb) register read
General Purpose Registers:
       rax = 0x00007fbeb0016830
       rbx = 0x00007fbeb0016830
       rcx = 0x00007fbeb0016850
       rdx = 0x0000000000000000
       rdi = 0x00007fbeb0016838
       rsi = 0x0000000000000000
       rbp = 0x00000003087a46f0
       rsp = 0x00000003087a46f0
        r8 = 0x000000000000003b
        r9 = 0x0000000000000003
       r10 = 0x0000000000000000
       r11 = 0x0000000000000000
       r12 = 0x00007fbeb0013730
       r13 = 0x00007fbeb0013aa0
       r14 = 0x00007fbeb00167b0
       r15 = 0x00007fbeb00167e0
       rip = 0x000000010ada1de4  mode.dylib`modeDataInit + 4 at modemath.c:75:5
    rflags = 0x0000000000000246
        cs = 0x000000000000002b
        fs = 0x0000000000000000
        gs = 0x0000000000000000

first and last two instructions

PIC function with zero variables start: save base pointer register to stack, move rsp to rpb. The next instruction would be to increment rbp, which we didnt do because this function has zero variables 0x10ada1de0 <+0>: push rbp 0x10ada1de1 <+1>: mov rbp, rsp

Compare with another funciton:

objdump -M intel --disassemble-symbols=_modeDataFinish mode.@(so|dynlib)

0000000000003e00 <_modeDataFinish>:
    // return address currently saved in [rsp+8]
    // save the previous frame pointer
    3e00: 55                           	push	rbp
    3e01: 48 89 e5                     	mov	rbp, rsp
    // push 2 register in the stack
    3e04: 53                           	push	rbx
    3e05: 50                           	push	rax
    // worth noting other 128 bytes can be accessed down the stack in the "red zone"
    // ... stuff happens ...
    // pop stack & throw away rax (belongs to callee)
    3e20: 48 83 c4 08                  	add	rsp, 8
    // pop stack & restore rbx (belongs to caller)
    3e24: 5b                           	pop	rbx
    // pop stack & restore rbp (belongs to caller)
    3e25: 5d                           	pop	rbp
    // this will do something like mov rsp, [rsp+8] \n jmp [rsp]
    3e26: c3                           	ret
    3e27: 66 0f 1f 84 00 00 00 00 00   	nop	word ptr [rax + rax]

other intstrcutions

First parameter to function (pointer to data) is passed in rdi

Return value is in eax

xor operations with the same register are used to zero the register

Other useful commands

objdump -t -M intel mode.@(so|dynlib) 

objdump --disassemble-symbols=_modeDataInit -M intel mode.@(so|dynlib) 

otool -Iv mode.@(so|dynlib)