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std::FILE(3) C++ Standard Libary std::FILE(3)


std::FILE - std::FILE


Defined in header <cstdio>
typedef /* unspecified */ FILE;

Each std::FILE object denotes a C stream.

C standard (referenced by C++ standard) does not specify whether std::FILE is a
complete object type. While it may be possible to copy a valid std::FILE, using a
pointer to such a copy as an argument for an I/O function invokes unspecified
behavior. In other words, std::FILE may be semantically non-copyable.

I/O streams can be used for both unformatted and formatted input and output.
Furthermore, the functions that handle input and output can also be
locale-sensitive, such that wide/multibyte conversions are performed as necessary.

Stream state

Besides the system-specific information necessary to access the device (e.g., a
POSIX file descriptor), each std::FILE object directly or indirectly holds the

1. Character width: unset, narrow, or wide.
2. Parse state for conversions between multibyte and wide characters (an object of
type std::mbstate_t)
3. Buffering state: unbuffered, line-buffered, fully buffered.
4. The buffer, which may be replaced by an external, user-provided buffer.
5. I/O mode: input, output, or update (both input and output).
6. Binary/text mode indicator.
7. End-of-file status indicator.
8. Error status indicator.
9. File position indicator, accessible as an object of type std::fpos_t, which, for
wide streams, includes parse state.
10. (C++17) Reentrant lock used to prevent data races when multiple threads read,
write, position, or query the position of a stream.

Narrow and wide orientation

A newly opened stream has no orientation. The first call to std::fwide or to any I/O
function establishes the orientation: a wide I/O function makes the stream
wide-oriented; a narrow I/O function makes the stream narrow-oriented. Once set, the
orientation can be changed with only std::freopen. Narrow I/O functions cannot be
called on a wide-oriented stream; wide I/O functions cannot be called on a
narrow-oriented stream. Wide I/O functions convert between wide and multibyte
characters as if by calling std::mbrtowc or std::wcrtomb with the coversion state as
described by the stream. Unlike the multibyte character strings that are valid in a
program, multibyte character sequences in the file may contain embedded nulls and do
not have to begin or end in the initial shift state.

The conversion state of a stream with wide orientation is established by the C
locale that is installed at the time the stream's orientation is set.

Binary and text modes

A text stream is an ordered sequence of characters that can be composed into lines;
a line can be decomposed into zero or more characters plus a terminating '\n'
(“newline”) character. Whether the last line requires a terminating '\n' is
implementation-defined. Furthermore, characters may have to be added, altered, or
deleted on input and output to conform to the conventions for representing text in
the OS (in particular, C streams on Windows OS convert '\n' to '\r\n' on output, and
convert '\r\n' to '\n' on input).

Data read in from a text stream is guaranteed to compare equal to the data that were
earlier written out to that stream only if each of the following is true:

* The data consist of only printing characters and/or the control characters '\t'
and '\n' (in particular, on Windows OS, the character '\0x1A' terminates input).
* No '\n' character is immediately preceded by space characters (such space
characters may disappear when such output is later read as input).
* The last character is '\n'.

A binary stream is an ordered sequence of characters that can transparently record
internal data. Data read in from a binary stream always equal the data that were
earlier written out to that stream, except that an implementation is allowed to
append an indeterminate number of null characters to the end of the stream. A wide
binary stream doesn't need to end in the initial shift state.


POSIX explicitly requires that the LC_CTYPE facet of the currently installed C
locale be stored within the FILE object the moment the stream's orientation becomes
wide; POSIX requires that this LC_CTYPE facet be used for all future I/O on this
stream until the orientation is changed, regardless of any subsequent call to

It is intended that each line of text be composed of data that are essentially
human-readable. POSIX implementations do not distinguish between text and binary
streams (there is no special mapping for '\n' or any other characters).

See also

basic_streambuf abstracts a raw device
(class template)
basic_filebuf implements raw file device
(class template)
stdin expression of type FILE* associated with the input stream
stdout expression of type FILE* associated with the output stream
stderr expression of type FILE* associated with the error output stream
(macro constant)