CODEWITHSUNDEEP
pythonendiannesssign
Niklas R
Niklas R

Reputation: 16900

Read 32-bit signed value from an "unsigned" bytestream

I want to extract data from a file whoose information is stored in big-endian and always unsigned. How does the "cast" from unsigned int to int affect the actual decimal value? Am I correct that the most left bit decides about the whether the value is positive or negative?

I want to parse that file-format with python, and reading and unsigned value is easy:

def toU32(bits):
    return ord(bits[0]) << 24 | ord(bits[1]) << 16 | ord(bits[2]) << 8  | ord(bits[3])

but how would the corresponding toS32 function look like?

Thanks for the info about the struct-module. But I am still interested in the solution about my actual question.

Upvotes: 4

Views: 5145

Answers (3)

Pablo Sole
Pablo Sole

Reputation: 138

The non-conditional version of toS32(bits) could be something like:

def toS32(bits):
    decoded = toU32(bits)
    return -(decoded & 0x80000000) + (decoded & 0x7fffffff)

You can pre-compute the mask for any other bit size too of course.

Upvotes: 0

grieve
grieve

Reputation: 13508

I would use struct.

import struct

def toU32(bits):
    return struct.unpack_from(">I", bits)[0]

def toS32(bits):
    return struct.unpack_from(">i", bits)[0]

The format string, ">I", means read a big endian, ">", unsigned integer, "I", from the string bits. For signed integers you can use ">i".

EDIT

Had to look at another StackOverflow answer to remember how to "convert" a signed integer from an unsigned integer in python. Though it is less of a conversion and more of reinterpreting the bits.

import struct

def toU32(bits):
        return ord(bits[0]) << 24 | ord(bits[1]) << 16 | ord(bits[2]) << 8  | ord(bits[3])

def toS32(bits):
    candidate = toU32(bits);
    if (candidate >> 31): # is the sign bit set?
        return (-0x80000000 + (candidate & 0x7fffffff)) # "cast" it to signed
    return candidate


for x in range(-5,5):
    bits = struct.pack(">i", x)
    print toU32(bits)
    print toS32(bits)

Upvotes: 10

Nick Haddad
Nick Haddad

Reputation: 8937

I would use the struct module's pack and unpack methods.

See Endianness of integers in Python for some examples.

Upvotes: 0

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