Re: Packing byte fields and an array object into struct
On Wednesday, November 13, 2013 1:31:49 PM UTC-5, krishna...@gmail.com wrote:
Hello,
I am trying to build a structure to be passed down to an I2C device driver.
The driver expects a struct that has a data array of size 512 bytes among
other things. This is my code -
rd_wr = 0x0 # Read operation
i2c_addr= addr
mux = mux_sel
multi_len = count
cmd = 0x0
i2c_inst_type = CHEL_I2C_AUTO_RD_TYPE_5
flag= CHEL_I2C_AUTO_VALID
status = 0x0
data= array.array('B', (0 for x in range(0,512)))
os_inst_bytes = (struct.pack('B', rd_wr) +
struct.pack('B', i2c_addr) +
struct.pack('B', mux)+
struct.pack('B', multi_len) +
struct.pack('B', cmd)+
struct.pack('B', i2c_inst_type) +
struct.pack('B', flag) +
struct.pack('I', status) +
struct.pack('512B', data))
#Convert to byte array
os_inst = bytearray(os_inst_bytes)
ret = fcntl.ioctl(self._dev_fd,
self.__IOWR(FXCB_FPGAIO_I2C_AUTO_OS_INST),
os_inst, 1)
I get an error like this -
591 struct.pack('B', flag) +
592 struct.pack('I', status) +
-- 593 struct.pack('512B', data))
error: pack requires exactly 512 arguments
In [1]:
Even though data is a 512 element array, it is not treat as such in this
struct.pack. The data field is used to return data from the driver. I should
be able to unpack the struct os_inst and read the data buffer after the IOCTL
call. How can I achieve this ?
Thanks in advance!
To make a 512-byte field in struct, use '512s' with a data value of ''. It
will zero-pad the result and give you 512 zero bytes. Also, you can use
multiple format specifiers in one struct.pack call:
os_inst_bytes = struct.pack('7BI512s', rd_wr, i2c_addr, muc, multi_len, cmd,
i2c_inst_type, flag, status, '')
Lastly, I suspect the value returned from struct.pack is byte-string enough for
the ioctl call, no need to use bytearray.
--Ned.
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Re: Packing byte fields and an array object into struct
Correction in the last input line...
In [16]: result = struct.unpack('5p', os_inst[11:16])
---
error Traceback (most recent call last)
ipython-input-16-42b59e00d5af in module()
1 result = struct.unpack('5p', os_inst[11:16])
error: unpack requires a bytes object of length 5
In [17]:
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Re: Packing byte fields and an array object into struct
Thanks for your reply Ned!
I tried this your suggestion and this is what it complains...
os_inst_bytes = struct.pack('7BI512s', 0, 0x51, 0x10, 5, 0, 0xD, 0x80, 0, '')
---
error
Traceback (most recent call last)
ipython-input-6-d36f45a8d3e6 in module()
1 os_inst_bytes = struct.pack('7BI512s', 0, 0x51, 0x10, 5, 0, 0xD, 0x80,
0, )
error: argument for 's' must be a bytes object
In [7]:
And about the bytearray() call, I want to pass a mutable object to the IOCTL to
be able to get the data back from the driver. Without bytearray(), the ioctl
with mutable flag set to 1 would complain.
I tried to use the p format specifier with pack after converting the array
object to byte stream. Packing seems fine. However, I cant seem to unpack.
In [1]: import array
In [2]: import struct
In [3]: data= array.array('B', (1 for x in range(5)))
In [4]: data_bytes = data.tobytes()
In [5]: os_inst_bytes = struct.pack('7BIp', 0, 0x51, 0x10, 5, 0, 0xD, 0x80, 0,
data_bytes)
In [6]:
In [6]: os_inst = bytearray(os_inst_bytes)
In [7]: result = struct.unpack('7B', os_inst[0:7])
In [8]: print(result)
(0, 81, 16, 5, 0, 13, 128)
In [9]: result = struct.unpack('I', os_inst[7:11])
In [10]: print(result)
(0,)
In [11]: result = struct.unpack('5s', os_inst[11:16])
---
error Traceback (most recent call last)
ipython-input-11-da14a6693435 in module()
1 result = struct.unpack('5s', os_inst[11:16])
error: unpack requires a bytes object of length 5
In [12]:
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Re: Packing byte fields and an array object into struct
On Wednesday, November 13, 2013 3:41:03 PM UTC-5, krishna...@gmail.com wrote:
Thanks for your reply Ned!
I tried this your suggestion and this is what it complains...
os_inst_bytes = struct.pack('7BI512s', 0, 0x51, 0x10, 5, 0, 0xD, 0x80, 0, '')
---
error
Traceback (most recent call last)
ipython-input-6-d36f45a8d3e6 in module()
1 os_inst_bytes = struct.pack('7BI512s', 0, 0x51, 0x10, 5, 0, 0xD,
0x80, 0, )
error: argument for 's' must be a bytes object
OK, looks like you are using Python 3 (it helps to specify these things
up-front). Use b to get an empty byte-string.
In [7]:
And about the bytearray() call, I want to pass a mutable object to the IOCTL
to be able to get the data back from the driver. Without bytearray(), the
ioctl with mutable flag set to 1 would complain.
OK, keep the bytearray call if it helps.
--Ned.
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