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authorFlorian Weimer <fweimer@redhat.com>2017-04-11 18:04:34 +0200
committerFlorian Weimer <fweimer@redhat.com>2017-04-11 18:04:34 +0200
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manual: readdir, readdir64 are thread-safe
They only modify the state in the dirstream argument, and we generally do not treat this as a reason to mark a function as not thread-safe. For an example, see random_r, which is marked as thread-safe even though the random state is not protected by a lock.
Diffstat (limited to 'manual/filesys.texi')
-rw-r--r--manual/filesys.texi29
1 files changed, 20 insertions, 9 deletions
diff --git a/manual/filesys.texi b/manual/filesys.texi
index edc7c64..a255c8f 100644
--- a/manual/filesys.texi
+++ b/manual/filesys.texi
@@ -478,7 +478,7 @@ symbols are declared in the header file @file{dirent.h}.
@comment dirent.h
@comment POSIX.1
@deftypefun {struct dirent *} readdir (DIR *@var{dirstream})
-@safety{@prelim{}@mtunsafe{@mtasurace{:dirstream}}@asunsafe{@asulock{}}@acunsafe{@aculock{}}}
+@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acunsafe{@aculock{}}}
@c This function holds dirstream's non-recursive lock, which brings
@c about the usual issues with locks and async signals and cancellation,
@c but the lock taking is not enough to make the returned value safe to
@@ -507,13 +507,24 @@ must set @code{errno} to zero before calling @code{readdir}. To avoid
entering an infinite loop, you should stop reading from the directory
after the first error.
-In POSIX.1-2008, @code{readdir} is not thread-safe. In @theglibc{}
-implementation, it is safe to call @code{readdir} concurrently on
-different @var{dirstream}s, but multiple threads accessing the same
-@var{dirstream} result in undefined behavior. @code{readdir_r} is a
-fully thread-safe alternative, but suffers from poor portability (see
-below). It is recommended that you use @code{readdir}, with external
-locking if multiple threads access the same @var{dirstream}.
+@strong{Caution:} The pointer returned by @code{readdir} points to
+a buffer within the @code{DIR} object. The data in that buffer will
+be overwritten by the next call to @code{readdir}. You must take care,
+for instance, to copy the @code{d_name} string if you need it later.
+
+Because of this, it is not safe to share a @code{DIR} object among
+multiple threads, unless you use your own locking to ensure that
+no thread calls @code{readdir} while another thread is still using the
+data from the previous call. In @theglibc{}, it is safe to call
+@code{readdir} from multiple threads as long as each thread uses
+its own @code{DIR} object. POSIX.1-2008 does not require this to
+be safe, but we are not aware of any operating systems where it
+does not work.
+
+@code{readdir_r} allows you to provide your own buffer for the
+@code{struct dirent}, but it is less portable than @code{readdir}, and
+has problems with very long filenames (see below). We recommend
+you use @code{readdir}, but do not share @code{DIR} objects.
@end deftypefun
@comment dirent.h
@@ -592,7 +603,7 @@ of the last two functions.
@comment dirent.h
@comment LFS
@deftypefun {struct dirent64 *} readdir64 (DIR *@var{dirstream})
-@safety{@prelim{}@mtunsafe{@mtasurace{:dirstream}}@asunsafe{@asulock{}}@acunsafe{@aculock{}}}
+@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acunsafe{@aculock{}}}
The @code{readdir64} function is just like the @code{readdir} function
except that it returns a pointer to a record of type @code{struct
dirent64}. Some of the members of this data type (notably @code{d_ino})