[iostreams.requirements]

# 31 Input/output library [[input.output]](./#input.output)

## 31.2 Iostreams requirements [iostreams.requirements]

### [31.2.1](#iostream.limits.imbue) Imbue limitations [[iostream.limits.imbue]](iostream.limits.imbue)

[1](#iostream.limits.imbue-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L44)

No function described in [[input.output]](input.output "31 Input/output library") except forios_base​::​imbue and basic_filebuf​::​pubimbue causes any instance ofbasic_ios​::​imbue orbasic_streambuf​::​imbue to be called[.](#iostream.limits.imbue-1.sentence-1)

If any user function called from a function declared in [[input.output]](input.output "31 Input/output library") or
as an overriding virtual function of any class declared in [[input.output]](input.output "31 Input/output library") callsimbue,
the behavior is undefined[.](#iostream.limits.imbue-1.sentence-2)

### [31.2.2](#stream.types) Types [[stream.types]](stream.types)

[🔗](#lib:streamoff)

`using streamoff = implementation-defined;
`

[1](#stream.types-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L67)

The type streamoff is a synonym for one of the signed basic integral types of
sufficient size to represent the maximum possible file size for the operating system[.](#stream.types-1.sentence-1)[255](#footnote-255 "Typically long long.")

[🔗](#lib:streamsize)

`using streamsize = implementation-defined;
`

[2](#stream.types-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L81)

The typestreamsize is a synonym for one of the signed basic
integral types[.](#stream.types-2.sentence-1)

It is used to represent the number of characters transferred in an I/O
operation, or the size of I/O buffers[.](#stream.types-2.sentence-2)[256](#footnote-256 "Most places where streamsize is used would use size_­t in C, or ssize_­t in POSIX.")

[255)](#footnote-255)[255)](#footnoteref-255)

Typically long long[.](#footnote-255.sentence-1)

[256)](#footnote-256)[256)](#footnoteref-256)

Most places where streamsize is used would use size_t in C,
or ssize_t in POSIX[.](#footnote-256.sentence-1)

### [31.2.3](#iostreams.limits.pos) Positioning type limitations [[iostreams.limits.pos]](iostreams.limits.pos)

[1](#iostreams.limits.pos-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L97)

The classes of [[input.output]](input.output "31 Input/output library") with template argumentscharT andtraits behave as described iftraits​::​pos_type andtraits​::​off_type arestreampos andstreamoff respectively[.](#iostreams.limits.pos-1.sentence-1)

Except as noted explicitly below, their behavior whentraits​::​pos_type andtraits​::​off_type are other types isimplementation-defined[.](#iostreams.limits.pos-1.sentence-2)

[2](#iostreams.limits.pos-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L119)

[*Note [1](#iostreams.limits.pos-note-1)*:

For each of the specializations of char_traits defined in[[char.traits.specializations]](char.traits.specializations "27.2.4 char_­traits specializations"),state_type denotes mbstate_t,pos_type denotes fpos<mbstate_t>,
and off_type denotes streamoff[.](#iostreams.limits.pos-2.sentence-1)

— *end note*]

[3](#iostreams.limits.pos-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L128)

In the classes of [[input.output]](input.output "31 Input/output library"), a template parameter with namecharT represents a member of the set of types containing char, wchar_t,
and any other implementation-defined
character container types ([[defns.character.container]](defns.character.container "3.10 character container type"))
that meet the requirements for a character on which any of
the iostream components can be instantiated[.](#iostreams.limits.pos-3.sentence-1)

### [31.2.4](#iostreams.threadsafety) Thread safety [[iostreams.threadsafety]](iostreams.threadsafety)

[1](#iostreams.threadsafety-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L138)

Concurrent access to a stream object ([[string.streams]](string.streams "31.8 String-based streams"), [[file.streams]](file.streams "31.10 File-based streams")), stream buffer
object ([[stream.buffers]](stream.buffers "31.6 Stream buffers")), or C Library stream ([[c.files]](c.files "31.13 C library files")) by multiple threads may result in
a data race ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races")) unless otherwise specified ([[iostream.objects]](iostream.objects "31.4 Standard iostream objects"))[.](#iostreams.threadsafety-1.sentence-1)

[*Note [1](#iostreams.threadsafety-note-1)*:

Data races result in undefined behavior ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races"))[.](#iostreams.threadsafety-1.sentence-2)

— *end note*]

[2](#iostreams.threadsafety-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L146)

If one thread makes a library call *a* that writes a value to a stream
and, as a result, another thread reads this value from the stream through a
library call *b* such that this does not result in a data race, then*a*'s write synchronizes with*b*'s read[.](#iostreams.threadsafety-2.sentence-1)