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### 2473. `basic_filebuf`'s relation to C `FILE` semantics

Section: 29.10.2.5 [filebuf.virtuals] Status: C++17 Submitter: Aaron Ballman Opened: 2015-02-09 Last modified: 2017-07-30

Priority: 2

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Discussion:

The restrictions on reading and writing a sequence controlled by an object of class `basic_filebuf<charT, traits>` are the same as for reading and writing with the Standard C library `FILE`s. One of the restrictions placed by C is on the behavior of a stream that is opened for input and output. See the C99 standard, 7.19.5.3p6 for more details, but the gist is that when opened in update mode, reads and writes must have an intervening file positioning or flushing call to not trigger UB.

29.10.2.5 [filebuf.virtuals] p13 specifies that `basic_filebuf::seekoff()` calls `std::fseek()`. However, there is no mention of `std::fseek()` in `basic_filebuf::seekpos()`, and no mention of `std::fflush()` in `basic_filebuf::sync()`, which seem like an oversight.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Change 29.10.2.5 [filebuf.virtuals] p16 as follows [Editorial note: A footnote referring to `fseek` is not needed, because this is already covered by the existing footnote 334]:

>    > -16- Alters the file position, if possible, to correspond to the position stored in `sp` (as described below). Altering the file position performs as follows:

>    > 1. if `(om & ios_base::out) != 0`, then update the output sequence and write any unshift sequence;

>    > 2. set the file position to `sp` as if by calling `std::fseek(file, sp, SEEK_SET)`;

>    > 3. if `(om & ios_base::in) != 0`, then update the input sequence;

>    > where `om` is the open mode passed to the last call to `open()`. The operation fails if `is_open()` returns false.

> 2. Change 29.10.2.5 [filebuf.virtuals] p19 as follows and add a new footnote that mimics comparable footnotes in 29.10.2.4 [filebuf.members] and 29.10.2.5 [filebuf.virtuals]:

>    > -19- Effects: If a put area exists, calls `filebuf::overflow` to write the characters to the file, then flushes the file as if by calling `std::fflush(file)` [Footnote: The function signature `fflush(FILE*)` is declared in `<cstdio>` (27.9.2).]. If a get area exists, the effect is implementation-defined.

[2015-05, Lenexa]

Aaron provides improved wording by removing the params from `std::fseek()` due to the concerns regarding the parameters on systems where `fseek` uses 32-bit parameters.

Second wording improvement, replacing the new one see below. It

1. drops the `std::`

2. drops the footnote for `fflush`

3. replaces `fseek` with `fsetpos`

Previous resolution [SUPERSEDED]:

> This wording is relative to N4431.

> 1. Change 29.10.2.5 [filebuf.virtuals] p16 as follows [Editorial note: A footnote referring to `fseek` is not needed, because this is already covered by the existing footnote 334]:

>    > -16- Alters the file position, if possible, to correspond to the position stored in `sp` (as described below). Altering the file position performs as follows:

>    > 1. if `(om & ios_base::out) != 0`, then update the output sequence and write any unshift sequence;

>    > 2. set the file position to `sp` as if by a call to `std::fseek`;

>    > 3. if `(om & ios_base::in) != 0`, then update the input sequence;

>    > where `om` is the open mode passed to the last call to `open()`. The operation fails if `is_open()` returns false.

> 2. Change 29.10.2.5 [filebuf.virtuals] p19 as follows and add a new footnote that mimics comparable footnotes in 29.10.2.4 [filebuf.members] and 29.10.2.5 [filebuf.virtuals]:

>    > -19- Effects: If a put area exists, calls `filebuf::overflow` to write the characters to the file, then flushes the file as if by calling `std::fflush(file)` [Footnote: The function signature `fflush(FILE*)` is declared in `<cstdio>` (27.9.2).]. If a get area exists, the effect is implementation-defined.

Proposed resolution:

This wording is relative to N4431.

1. Change 29.10.2.5 [filebuf.virtuals] p16 as follows:

   > -16- Alters the file position, if possible, to correspond to the position stored in `sp` (as described below). Altering the file position performs as follows:

   > 1. if `(om & ios_base::out) != 0`, then update the output sequence and write any unshift sequence;

   > 2. set the file position to `sp` as if by a call to `fsetpos`;

   > 3. if `(om & ios_base::in) != 0`, then update the input sequence;

   > where `om` is the open mode passed to the last call to `open()`. The operation fails if `is_open()` returns false.

2. Change 29.10.2.5 [filebuf.virtuals] p19 as follows:

   > -19- Effects: If a put area exists, calls `filebuf::overflow` to write the characters to the file, then flushes the file as if by calling `fflush(file)`. If a get area exists, the effect is implementation-defined.