[using.linkage]

# 16 Library introduction [[library]](./#library)

## 16.4 Library-wide requirements [[requirements]](requirements#using.linkage)

### 16.4.3 Using the library [[using]](using#linkage)

#### 16.4.3.3 Linkage [using.linkage]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1724)

Entities in the C++ standard library have [external linkage](basic.link "6.7 Program and linkage [basic.link]")[.](#1.sentence-1)

Unless otherwise specified, objects and functions have the defaultextern "C++" linkage ([[dcl.link]](dcl.link "9.12 Linkage specifications"))[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1730)

Whether a name from the C standard library declared with
external linkage hasextern "C" orextern "C++" linkage is implementation-defined[.](#2.sentence-1)

It is recommended that an
implementation useextern "C++" linkage for this purpose[.](#2.sentence-2)[150](#footnote-150 "The only reliable way to declare an object or function signature from the C standard library is by including the header that declares it, notwithstanding the latitude granted in ISO/IEC 9899:2024, 7.1.4.")

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1751)

Objects and functions
defined in the library and required by a C++ program are included in
the program prior to program startup[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1757)

See also[replacement functions](replacement.functions "16.4.5.6 Replacement functions [replacement.functions]"),[runtime changes](handler.functions "16.4.5.7 Handler functions [handler.functions]")[.](#4.sentence-1)

[150)](#footnote-150)[150)](#footnoteref-150)

The only reliable way to declare an object or
function signature from the C standard library is by including the header that
declares it, notwithstanding the latitude granted in ISO/IEC 9899:2024, 7.1.4[.](#footnote-150.sentence-1)