Hello Alex, On 10/2/20 9:28 PM, Alejandro Colomar wrote: > Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> Patch applied. Thanks, Michael > system_data_types.7: void *: Add info about generic function parameters and return value > > Reported-by: Paul Eggert <eggert@xxxxxxxxxxx> > Reported-by: David Laight <David.Laight@xxxxxxxxxx> > Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> > > system_data_types.7: void *: Add info about pointer artihmetic > > Reported-by: Paul Eggert <eggert@xxxxxxxxxxx> > Reported-by: David Laight <David.Laight@xxxxxxxxxx> > Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> > > 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@xxxxxxxxx> > Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> > > system_data_types.7: void *: wfix > > Reported-by: Jonathan Wakely <jwakely.gcc@xxxxxxxxx> > Reported-by: Paul Eggert <eggert@xxxxxxxxxxx> > Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> > --- > man7/system_data_types.7 | 76 ++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 74 insertions(+), 2 deletions(-) > > diff --git a/man7/system_data_types.7 b/man7/system_data_types.7 > index c82d3b388..7c1198802 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 +1812,80 @@ 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, > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
