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Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org, gcc@gcc.gnu.org,
libc-alpha@sourceware.org
Subject: Re: [PATCH v2 1/2] system_data_types.7: Add 'void *'
To: Alejandro Colomar
References: <41affebd-3354-9420-0048-bffd14535e95@gmail.com>
<20201001154946.104626-2-colomar.6.4.3@gmail.com>
<41ab7ed9-3ce0-f8ad-38f4-c4bbeca138f8@gmail.com>
From: "Michael Kerrisk (man-pages)"
Message-ID: <51fd9c8d-9030-775f-8c35-f5e41bab4c0e@gmail.com>
Date: Fri, 2 Oct 2020 13:54:03 +0200
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Hi Alex,
On 10/1/20 6:55 PM, Alejandro Colomar wrote:
>
>
> On 2020-10-01 18:38, Michael Kerrisk (man-pages) wrote:
>> Hi Alex,
>>
>>> +According to the C language standard,
>>> +a pointer to any object type may be converted to a pointer to
>>> +.I void
>>> +and back.
>>> +POSIX further requires that any pointer,
>>> +including pointers to functions,
>>> +may be converted to a pointer to
>>> +.I void
>>> +and back.
>> I know you are correct about POSIX, but which part of the
>> standard did you find this information in? The only
>> reference that I find in POSIX is the dlsym() spec. Is it
>> covered also somewhere else in the standrd?
>>
>> Thanks,
>>
>> Michael
>>
>
> Hi Michael,
>
> I've bean searching, and dlsym is the only one:
>
> ________
>
> user@debian:~/Desktop/src/Standards/susv4-2018$ grep -rn "pointer to a
> function"
> functions/regfree.html:530:"undefined" means that the action
> by the application is an error, of similar severity to passing a bad
> pointer to a function.
> functions/dlsym.html:138:Note that conversion from a void *
> pointer to a function pointer as in:
> functions/regcomp.html:530:"undefined" means that the action
> by the application is an error, of similar severity to passing a bad
> pointer to a function.
> functions/regexec.html:530:"undefined" means that the action
> by the application is an error, of similar severity to passing a bad
> pointer to a function.
> functions/V2_chap02.html:3039:There are three types of action that
> can be associated with a signal: SIG_DFL, SIG_IGN, or a pointer to a
> function. Initially,
> functions/regerror.html:530:"undefined" means that the action
> by the application is an error, of similar severity to passing a bad
> pointer to a function.
> user@debian:~/Desktop/src/Standards/susv4-2018$ grep -rn "function pointer"
> basedefs/glob.h.html:165:"../functions/glob.html">glob()
> prototype definition by removing the restrict qualifier from the
> function pointer
> xrat/V4_xsh_chap02.html:114:when the application requires it; for
> example, if its address is to be stored in a function pointer variable.
> functions/dlsym.html:138:Note that conversion from a void *
> pointer to a function pointer as in:
> user@debian:~/Desktop/src/Standards/susv4-2018$ grep -rn "pointer to
> function"
> functions/dlsym.html:73:converted from type pointer to function to type
> pointer to void; otherwise, dlsym() shall return the
> address of the
> user@debian:~/Desktop/src/Standards/susv4-2018$
>
> From those, the only one that documents this is functions/dlsym.
> The rest is noise.
>
> The most explicit paragraph in dlsym is the following:
>
> [[
> Note that conversion from a void * pointer to a function pointer as in:
>
> fptr = (int (*)(int))dlsym(handle, "my_function");
>
> is not defined by the ISO C standard.
> This standard requires this conversion to work correctly
> on conforming implementations.
> ]]
Okay -- so, one more thing for a revised (squashed) patch.
I think you better say that that POSIX requirements exists
only since POS0X.1-2008 Technical Corrigendum 1 (2013).
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/