Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> 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> --- 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
