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From:   Alejandro Colomar <colomar.6.4.3@gmail.com>
To:     mtk.manpages@gmail.com
Cc:     Alejandro Colomar <colomar.6.4.3@gmail.com>,
        linux-man@vger.kernel.org, gcc-patches@gcc.gnu.org,
        libc-alpha@sourceware.org, linux-kernel@vger.kernel.org,
        eggert@cs.ucla.edu, jwakely.gcc@gmail.com, David.Laight@ACULAB.COM
Subject: [PATCH v4 1/2] system_data_types.7: Add 'void *'
Date:   Fri,  2 Oct 2020 17:14:19 +0200
Message-Id: <20201002151419.32053-2-colomar.6.4.3@gmail.com>
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Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>

system_data_types.7: void *: Add info about generic function parameters and return value

Reported-by: Paul Eggert <eggert@cs.ucla.edu>
Reported-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>

system_data_types.7: void *: Add info about pointer artihmetic

Reported-by: Paul Eggert <eggert@cs.ucla.edu>
Reported-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>

system_data_types.7: void *: Add Versions notes

Compatibility between function pointers and void * hasn't always been so.
Document when that was added to POSIX.

Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
---
 man7/system_data_types.7 | 80 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 78 insertions(+), 2 deletions(-)

diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
index c82d3b388..277e05b12 100644
--- a/man7/system_data_types.7
+++ b/man7/system_data_types.7
@@ -679,7 +679,6 @@ See also the
 .I uintptr_t
 and
 .I void *
-.\" TODO: Document void *
 types in this page.
 .RE
 .\"------------------------------------- lconv ------------------------/
@@ -1780,7 +1779,6 @@ See also the
 .I intptr_t
 and
 .I void *
-.\" TODO: Document void *
 types in this page.
 .RE
 .\"------------------------------------- va_list ----------------------/
@@ -1814,6 +1812,84 @@ See also:
 .BR va_copy (3),
 .BR va_end (3)
 .RE
+.\"------------------------------------- void * -----------------------/
+.TP
+.I void *
+.RS
+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.
+.PP
+Conversions from and to any other pointer type are done implicitly,
+not requiring casts at all.
+Note that this feature prevents any kind of type checking:
+the programmer should be careful not to cast a
+.I void *
+value to a type incompatible to that of the underlying data,
+because that would result in undefined behavior.
+.PP
+This type is useful in function parameters and return value
+to allow passing values of any type.
+The function will usually use some mechanism to know
+of which type the underlying data passed to the function really is.
+.PP
+A value of this type can't be dereferenced,
+as it would give a value of type
+.I void
+which is not possible.
+Likewise, pointer arithmetic is not possible with this type.
+However, in GNU C, poitner arithmetic is allowed
+as an extension to the standard;
+this is done by treating the size of a
+.I void
+or of a function as 1.
+A consequence of this is that
+.I sizeof
+is also allowed on
+.I void
+and on function types, and returns 1.
+.PP
+The conversion specifier for
+.I void *
+for the
+.BR printf (3)
+and the
+.BR scanf (3)
+families of functions is
+.BR p ;
+resulting commonly in
+.B %p
+for printing
+.I void *
+values.
+.PP
+Versions:
+The POSIX requirement about compatibility between
+.I void *
+and function pointers was added in
+POSIX.1-2008 Technical Corrigendum 1 (2013).
+.PP
+Conforming to:
+C99 and later; POSIX.1-2001 and later.
+.PP
+See also:
+.BR malloc (3),
+.BR memcmp (3),
+.BR memcpy (3),
+.BR memset (3)
+.PP
+See also the
+.I intptr_t
+and
+.I uintptr_t
+types in this page.
+.RE
 .\"--------------------------------------------------------------------/
 .SH NOTES
 The structures described in this manual page shall contain,
-- 
2.28.0