projects / thirdparty / man-pages.git / blame
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| cb0f97b2 | 1 | .\" Copyright (c) 2020-2022 by Alejandro Colomar <alx@kernel.org> |
| 3e33f851 AC |
2 | .\" and Copyright (c) 2020 by Michael Kerrisk <mtk.manpages@gmail.com> |
| 3 | .\" | |
| 4 | .\" SPDX-License-Identifier: Linux-man-pages-copyleft | |
| 5 | .\" | |
| 6 | .\" | |
| 4c1c5274 | 7 | .TH void 3type (date) "Linux man-pages (unreleased)" |
| 3e33f851 AC |
8 | .SH NAME |
| 9 | void \- abstract type | |
| 10 | .SH SYNOPSIS | |
| 11 | .nf | |
| 12 | .B void * | |
| 13 | .fi | |
| 14 | .SH DESCRIPTION | |
| 728c2f35 | 15 | A pointer to any object type may be converted to a pointer to |
| 3e33f851 AC |
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). | |
| 3113c7f3 | 67 | .SH STANDARDS |
| 801a509d | 68 | C99 and later; |
| 3e33f851 AC |
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) |