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

Question

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?

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

Answer 1

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)