Re: [PATCH v2 1/2] system_data_types.7: Add 'void *'

[Alejandro Colomar <colomar.6.4.3@gmail.com>]

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.</p>
functions/dlsym.html:138:<p>Note that conversion from a <b>void *</b> 
pointer to a function pointer as in:</p>
functions/regcomp.html:530:&quot;undefined&quot; means that the action 
by the application is an error, of similar severity to passing a bad 
pointer to a function.</p>
functions/regexec.html:530:&quot;undefined&quot; means that the action 
by the application is an error, of similar severity to passing a bad 
pointer to a function.</p>
functions/V2_chap02.html:3039:<p>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:&quot;undefined&quot; means that the action 
by the application is an error, of similar severity to passing a bad 
pointer to a function.</p>
user@debian:~/Desktop/src/Standards/susv4-2018$ grep -rn "function pointer"
basedefs/glob.h.html:165:"../functions/glob.html"><i>glob</i>()</a> 
prototype definition by removing the <b>restrict</b> 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.</p>
functions/dlsym.html:138:<p>Note that conversion from a <b>void *</b> 
pointer to a function pointer as in:</p>
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 <b>void</b>; otherwise, <i>dlsym</i>() 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.
]]