man3type/void.3type

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   7 .TH VOID 3type 2022-07-20 "Linux man-pages (unreleased)"
   8 .SH NAME
   9 void \- abstract type
  10 .SH SYNOPSIS
  11 .nf
  12 .B void *
  13 .fi
  14 .SH DESCRIPTION
  15 A pointer to any object type may be converted to a pointer to
  16 .I void
  17 and back.
  18 POSIX further requires that any pointer,
  19 including pointers to functions,
  20 may be converted to a pointer to
  21 .I void
  22 and back.
  23 .PP
  24 Conversions from and to any other pointer type are done implicitly,
  25 not requiring casts at all.
  26 Note that this feature prevents any kind of type checking:
  27 the programmer should be careful not to convert a
  28 .I void *
  29 value to a type incompatible to that of the underlying data,
  30 because that would result in undefined behavior.
  31 .PP
  32 This type is useful in function parameters and return value
  33 to allow passing values of any type.
  34 The function will typically use some mechanism to know
  35 the real type of the data being passed via a pointer to
  36 .IR void .
  37 .PP
  38 A value of this type can't be dereferenced,
  39 as it would give a value of type
  40 .IR void ,
  41 which is not possible.
  42 Likewise, pointer arithmetic is not possible with this type.
  43 However, in GNU C, pointer arithmetic is allowed
  44 as an extension to the standard;
  45 this is done by treating the size of a
  46 .I void
  47 or of a function as 1.
  48 A consequence of this is that
  49 .I sizeof
  50 is also allowed on
  51 .I void
  52 and on function types, and returns 1.
  53 .SS Use with printf(3) and scanf(3)
  54 The conversion specifier for
  55 .I void *
  56 for the
  57 .BR printf (3)
  58 and the
  59 .BR scanf (3)
  60 families of functions is
  61 .BR p .
  62 .SH VERSIONS
  63 The POSIX requirement about compatibility between
  64 .I void *
  65 and function pointers was added in
  66 POSIX.1-2008 Technical Corrigendum 1 (2013).
  67 .SH STANDARDS
  68 C99 and later;
  69 POSIX.1-2001 and later.
  70 .SH SEE ALSO
  71 .BR malloc (3),
  72 .BR memcmp (3),
  73 .BR memcpy (3),
  74 .BR memset (3),
  75 .BR intptr_t (3type)