Path: csiph.com!news.swapon.de!eternal-september.org!feeder.eternal-september.org!reader01.eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Protable way of reading str[4] as uint32_t
Date: Mon, 30 Mar 2020 19:04:32 -0700
Organization: None to speak of
Lines: 38
Message-ID: <875zeluvnz.fsf@nosuchdomain.example.com>
References: <40d1f926-8737-4332-93f2-c37c46f731f9@googlegroups.com> <yOvgG.106450$Gy2.9178@fx30.iad>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: reader02.eternal-september.org; posting-host="dbc073e31c3669f15dacaa958203e3d4"; logging-data="29477"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19DxnbyQ7KUleQ+IKBpZVRB"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)
Cancel-Lock: sha1:zyvorG+07Qkc28xDiOhXSVYpfQk= sha1:QPfICFaGpyJchahWbtpa84ky3Qk=
Xref: csiph.com comp.lang.c:150919

Richard Damon <Richard@Damon-Family.org> writes:
> On 3/30/20 2:42 PM, wyniijj@gmail.com wrote:
>> I'd like to read the value a.value in the following program as 0x78563412 in a 
>> portable way, none of hton or ntoh is useful. How should I do? 
>> 
>> #include <stdio.h>
>> #include <arpa/inet.h>
>> 
>> int main()
>> {
>>  union {
>>    char str[4]; 
>>    uint32_t value;
>>  } a;
>> 
>>  a.str[0]=0x12;
>>  a.str[1]=0x34;
>>  a.str[2]=0x56;
>>  a.str[3]=0x78;
[...]
> Network order is Big Endian, your problem statement defines decoding
> little endian, so the network order functions won't work.
>
> In my mind the simplest is:
>
> a.ser[0] | (a.str[1] << 8) | (a.str[2] << 16) | (a.str[3] << 24)
>
> i.e., be explicit and access by bytes in the order you want.

And definitely use unsigned char rather than char, and test with values
exceeding 0x7f.

[...]

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */