access byte by byte in python ctypes class(Structure)

Question

This is a followup question to my question , that was left unanswered
EDIT and organized

from my module:

class t_data_block(Structure):
_fields_ = [("current_readings",c_ulong * PORTS_NUM),
            ("power_reading",c_ulong)]

class t_sys_flags(Structure):
"System flags"
_fields_ = [("Alarm1",c_ulong,1),
            ("Alarm2",c_ulong,1),
            ("reserved",c_ulong,30)]

class t_input_buff(Structure):
"Input buffer from the board"
    _fields_ = [("header",c_ulong),
                ("sys_flags", t_sys_flags),# t_sys_flags is another structure
                ("data_block", t_data_block * CHIP_NUM)] # t_data_block is another structure

I need to go over each byte in buff and i tried the following:

from pc_memory import* 
def calc_formula(buff,len):
    sum = 0
    for curChar in buff:
        numericByteValue = ord(curChar)
        sum += numericByteValue
    return sum

def main:
    input_buff = t_input_buff()

    calc_formula(input_buff,len)

and i get "error:TypeError: 't_input_buff' object is not iterable" upon executing the for command

i also tried use str(buff) with no luck
Any suggestions?

please, give also description of "data_block" structure so I can help — Sergey Karpukhin
– Sergey Karpukhin, Commented Dec 4, 2011 at 9:23
I update my question with data_block and sys_flags strucutres definitions — Gil.I
– Gil.I, Commented Dec 4, 2011 at 9:34

Mark Tolonen · Accepted Answer · 2018-10-04 01:21:52Z

4

Check out buffer:

from binascii import hexlify
x = t_input_buff()

x.header = 1
x.sys_flags.Alarm1 = 1
x.sys_flags.Alarm2 = 0
x.sys_flags.reserved = 0x15555555
x.data_block[0].current_readings[0] = 2
x.data_block[0].current_readings[1] = 3
x.data_block[0].power_reading = 4
x.data_block[1].current_readings[0] = 5
x.data_block[1].current_readings[1] = 6
x.data_block[1].power_reading = 7

b = buffer(x)
print hexlify(b[:])

Output:

0100000055555555020000000300000004000000050000000600000007000000

You can also use ctypes casting:

p=cast(byref(x),POINTER(c_byte*sizeof(x)))
print p.contents[:]

Output:

[1, 0, 0, 0, 85, 85, 85, 85, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0]

Note: In Python 3, buffer no longer exists. bytes, and list(bytes(...)) give the equivalent of buffer and the ctypes casting example.

edited Oct 4, 2018 at 1:21

answered Dec 4, 2011 at 19:55

Mark Tolonen

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2 Comments

Gil.I Over a year ago

I checked out the 'buffer' and it seems to work. however i prefer the 'union' solution (maybe because i'm used to write in C) . maybe i will use the 'buffer' option to take slice of the source by using offset parameter

drlolly Over a year ago

V good exposition. It might be useful to explicitly say that the 3 fields Alarm1, Alarm2 and reserved form a 32 bit word, with Alarm1 as bit 0, Alarm2 as bit 1 and reserved as bits 2 to 31. Hence the data in this word will appear as 5 repeated 8 times.

Sergey Karpukhin · Accepted Answer · 2011-12-04 08:28:35Z

0

I don't know what is the actual type of your "buff" variable, but you can try to explicitly convert it to string in the loop head

for curChar in str(buff):

answered Dec 4, 2011 at 8:28

Sergey Karpukhin

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Comments

Gil.I · Accepted Answer · 2011-12-05 08:09:25Z

I found the solution:(i shortened the original structures for easier understanding)

from ctypes import*  

class t_mem_fields(Structure):
    _fields_ = [("header",c_ulong),
                ("log", c_byte *  10)]             

class t_mem(Union):
    _fields_ = [("st_field",t_mem_fields),
                ("byte_index",c_byte * 14)]  

def calc(buff,len):
    sum = 0
    print(type(buff))
    for cur_byte in buff.byte_index:
         print "cur_byte = %x" %cur_byte
         sum += cur_byte
    print "sum = %x" %sum
    return sum     

def main():
    mem1 = t_mem()
    mem1.st_field.header = 0x12345678
    mem1.byte_index[4] = 1
    mem1.byte_index[5] = 2
    mem1.byte_index[6] = 3
    mem1.byte_index[7] = 4

   calc(mem1,14)

main()

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