diff options
| author | Jonathan Wakely <jwakely@redhat.com> | 2014-01-10 14:30:27 +0000 |
|---|---|---|
| committer | Jonathan Wakely <jwakely@redhat.com> | 2014-01-10 14:30:27 +0000 |
| commit | f272ebc9041a375d2f545d0e4f606eebfe7b3aa6 (patch) | |
| tree | 57b376f27a4b81c5ba9e0891ca99a6f2dbfe3e93 /libstdc++-v3/doc/xml | |
| parent | c60d121ef56173990789bbf95d4befc00f93baa5 (diff) | |
PR libstdc++/59687
* doc/xml/manual/backwards_compatibility.xml
(backwards.third.nocreate_noreplace): Correct and expand.
git-svn-id: https://gcc.gnu.org/svn/gcc/trunk@206525 138bc75d-0d04-0410-961f-82ee72b054a4
Diffstat (limited to 'libstdc++-v3/doc/xml')
| -rw-r--r-- | libstdc++-v3/doc/xml/manual/backwards_compatibility.xml | 27 |
1 files changed, 17 insertions, 10 deletions
diff --git a/libstdc++-v3/doc/xml/manual/backwards_compatibility.xml b/libstdc++-v3/doc/xml/manual/backwards_compatibility.xml index 869b63f1992..2a2f77d216a 100644 --- a/libstdc++-v3/doc/xml/manual/backwards_compatibility.xml +++ b/libstdc++-v3/doc/xml/manual/backwards_compatibility.xml @@ -782,18 +782,25 @@ AC_DEFUN([AC_HEADER_EXT_HASH_SET], [ </title></info> -<para> The existence of <code>ios::nocreate</code> being used for -input-streams has been confirmed, most probably because the author -thought it would be more correct to specify nocreate explicitly. So -it can be left out for input-streams. +<para>Historically these flags were used with iostreams to control whether +new files are created or not when opening a file stream, similar to the +<code>O_CREAT</code> and <code>O_EXCL</code> flags for the +<function>open(2)</function> system call. Because iostream modes correspond +to <function>fopen(3)</function> modes these flags are not supported. +For input streams a new file will not be created anyway, so +<code>ios::nocreate</code> is not needed. +For output streams, a new file will be created if it does not exist, which is +consistent with the behaviour of <function>fopen</function>. </para> -<para>For output streams, <quote>nocreate</quote> is probably the default, -unless you specify <code>std::ios::trunc</code> ? To be safe, you can -open the file for reading, check if it has been opened, and then -decide whether you want to create/replace or not. To my knowledge, -even older implementations support <code>app</code>, <code>ate</code> -and <code>trunc</code> (except for <code>app</code> ?). +<para>When one of these flags is needed a possible alternative is to attempt +to open the file using <type>std::ifstream</type> first to determine whether +the file already exists or not. This may not be reliable however, because +whether the file exists or not could change between opening the +<type>std::istream</type> and re-opening with an output stream. If you need +to check for existence and open a file as a single operation then you will +need to use OS-specific facilities outside the C++ standard library, such +as <function>open(2)</function>. </para> </section> |