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system_data_types.7: Add 'void *' 

Reported-by: Paul Eggert <eggert@cs.ucla.edu>
Reported-by: David Laight <David.Laight@ACULAB.COM>
Reported-by: Jonathan Wakely <jwakely.gcc@gmail.com>
Reported-by: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Alejandro Colomar 2020-10-02 21:28:15 +02:00 committed by Michael Kerrisk
parent c2ccd81779
commit baab22e295
1 changed files with 74 additions and 2 deletions
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76
man7/system_data_types.7
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@ -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 +1812,80 @@ 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 convert 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 typically use some mechanism to know
the real type of the data being passed via a pointer to
.IR void .
.PP
A value of this type can't be dereferenced,
as it would give a value of type
.IR void ,
which is not possible.
Likewise, pointer arithmetic is not possible with this type.
However, in GNU C, pointer 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 .
.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,