Packing byte fields and an array object into struct
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!
--
https://mail.python.org/mailman/listinfo/python-list
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]:
--
https://mail.python.org/mailman/listinfo/python-list
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]:
--
https://mail.python.org/mailman/listinfo/python-list
Illegal seek error with seek() and os.lseek()
I am trying to use os.open() and os.lseek() methods to operate on a device file
in Linux. My code goes something like this -
# first, open the file as a plain binary
try:
self.file = open(/dev/relpcfpga, r+b, buffering=0)
except IOError:
raise IOError ('Failed to open.')
# Figure out file size
self.file.seek(0, 2)
self.file_size = self.file.tell()
The method seek() complains OSError: [Errno 29] Illegal seek
The device relpcfpga is a char device.
The same code works with a normal text file.
I have tried to use os.open() and os.lseek() methods, but see the same error.
Is there a different method to operate on device files?
Thanks!
--
http://mail.python.org/mailman/listinfo/python-list
