How do I treat an integer as an array of bytes in Python?

Question

I'm trying to decode the result of the Python os.wait() function. This returns, according to the Python docs:

a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.

How do I decode the exit status indication (which is an integer) to obtain the high and low byte? To be specific, how do I implement the decode function used in the following code snippet:

(pid,status) = os.wait()
(exitstatus, signum) = decode(status)

Mark Harrison · Accepted Answer · 2008-08-13 17:56:34Z

14

This will do what you want:

signum = status & 0xff
exitstatus = (status & 0xff00) >> 8

answered Aug 13, 2008 at 17:56

Mark Harrison

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1 Comment

Scott Griffiths Over a year ago

Although the & 0xff00 is redundant if status really is only 16 bits.

bad_coder · Accepted Answer · 2020-06-24 21:33:17Z

13

To answer your general question, you can use bit manipulation

pid, status = os.wait()
exitstatus, signum = status & 0xFF, (status & 0xFF00) >> 8

However, there are also built-in functions for interpreting exit status values:

pid, status = os.wait()
exitstatus, signum = os.WEXITSTATUS( status ), os.WTERMSIG( status )

Comments

RobM · Accepted Answer · 2009-03-26 14:52:42Z

2

You can get break your int into a string of unsigned bytes with the struct module:

import struct
i = 3235830701  # 0xC0DEDBAD
s = struct.pack(">L", i)  # ">" = Big-endian, "<" = Little-endian
print s         # '\xc0\xde\xdb\xad'
print s[0]      # '\xc0'
print ord(s[0]) # 192 (which is 0xC0)

If you couple this with the array module you can do this more conveniently:

import struct
i = 3235830701  # 0xC0DEDBAD
s = struct.pack(">L", i)  # ">" = Big-endian, "<" = Little-endian

import array
a = array.array("B")  # B: Unsigned bytes
a.fromstring(s)
print a   # array('B', [192, 222, 219, 173])

answered Mar 26, 2009 at 14:52

RobM

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Comments

tzot · Accepted Answer · 2009-03-26 23:48:20Z

2

exitstatus, signum= divmod(status, 256)

answered Mar 26, 2009 at 23:48

tzot

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Comments

ArtOfWarfare · Accepted Answer · 2020-06-26 18:17:28Z

1

You can unpack the status using bit-shifting and masking operators.

low = status & 0x00FF
high = (status & 0xFF00) >> 8

I'm not a Python programmer, so I hope got the syntax correct.

edited Jun 26, 2020 at 18:17

ArtOfWarfare

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answered Aug 13, 2008 at 17:58

Derek Park

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Comments

Patrick · Accepted Answer · 2008-08-13 18:26:37Z

0

The folks before me've nailed it, but if you really want it on one line, you can do this:

(signum, exitstatus) = (status & 0xFF, (status >> 8) & 0xFF)

EDIT: Had it backwards.

edited Aug 13, 2008 at 18:26

answered Aug 13, 2008 at 18:14

Patrick

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Comments

Barish Ahsen · Accepted Answer · 2018-04-28 07:50:13Z

0

import amp as amp
import status
signum = status &amp; 0xff
exitstatus = (status &amp; 0xff00) &gt;&gt; 8

answered Apr 28, 2018 at 7:50

Barish Ahsen

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2 Comments

Ray Over a year ago

This isn't much more useful than the existing answers. Perhaps you could try answering the new questions with no answers?

Ray Over a year ago

No problem. Your best is what this community would be honoured to have :)

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