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attributes.7: New page describing POSIX safety concepts 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Copyright (c) 2014, Red Hat, Inc.
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@node POSIX Safety Concepts, Unsafe Features, , POSIX
@subsubsection POSIX Safety Concepts
@cindex POSIX Safety Concepts
This manual documents various safety properties of @glibcadj{}
functions, in lines that follow their prototypes and look like:
@sampsafety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
The properties are assessed according to the criteria set forth in the
POSIX standard for such safety contexts as Thread-, Async-Signal- and
Async-Cancel- -Safety. Intuitive definitions of these properties,
attempting to capture the meaning of the standard definitions, follow.
@itemize @bullet
@item
@cindex MT-Safe
@cindex Thread-Safe
@code{MT-Safe} or Thread-Safe functions are safe to call in the presence
of other threads. MT, in MT-Safe, stands for Multi Thread.
Being MT-Safe does not imply a function is atomic, nor that it uses any
of the memory synchronization mechanisms POSIX exposes to users. It is
even possible that calling MT-Safe functions in sequence does not yield
an MT-Safe combination. For example, having a thread call two MT-Safe
functions one right after the other does not guarantee behavior
equivalent to atomic execution of a combination of both functions, since
concurrent calls in other threads may interfere in a destructive way.
Whole-program optimizations that could inline functions across library
interfaces may expose unsafe reordering, and so performing inlining
across the @glibcadj{} interface is not recommended. The documented
MT-Safety status is not guaranteed under whole-program optimization.
However, functions defined in user-visible headers are designed to be
safe for inlining.
@item
@cindex AS-Safe
@cindex Async-Signal-Safe
@code{AS-Safe} or Async-Signal-Safe functions are safe to call from
asynchronous signal handlers. AS, in AS-Safe, stands for Asynchronous
Signal.
Many functions that are AS-Safe may set @code{errno}, or modify the
floating-point environment, because their doing so does not make them
unsuitable for use in signal handlers. However, programs could
misbehave should asynchronous signal handlers modify this thread-local
state, and the signal handling machinery cannot be counted on to
preserve it. Therefore, signal handlers that call functions that may
set @code{errno} or modify the floating-point environment @emph{must}
save their original values, and restore them before returning.
@item
@cindex AC-Safe
@cindex Async-Cancel-Safe
@code{AC-Safe} or Async-Cancel-Safe functions are safe to call when
asynchronous cancellation is enabled. AC in AC-Safe stands for
Asynchronous Cancellation.
The POSIX standard defines only three functions to be AC-Safe, namely
@code{pthread_cancel}, @code{pthread_setcancelstate}, and
@code{pthread_setcanceltype}. At present @theglibc{} provides no
guarantees beyond these three functions, but does document which
functions are presently AC-Safe. This documentation is provided for use
by @theglibc{} developers.
Just like signal handlers, cancellation cleanup routines must configure
the floating point environment they require. The routines cannot assume
a floating point environment, particularly when asynchronous
cancellation is enabled. If the configuration of the floating point
environment cannot be performed atomically then it is also possible that
the environment encountered is internally inconsistent.
@item
@cindex MT-Unsafe
@cindex Thread-Unsafe
@cindex AS-Unsafe
@cindex Async-Signal-Unsafe
@cindex AC-Unsafe
@cindex Async-Cancel-Unsafe
@code{MT-Unsafe}, @code{AS-Unsafe}, @code{AC-Unsafe} functions are not
safe to call within the safety contexts described above. Calling them
within such contexts invokes undefined behavior.
Functions not explicitly documented as safe in a safety context should
be regarded as Unsafe.
@item
@cindex Preliminary
@code{Preliminary} safety properties are documented, indicating these
properties may @emph{not} be counted on in future releases of
@theglibc{}.
Such preliminary properties are the result of an assessment of the
properties of our current implementation, rather than of what is
mandated and permitted by current and future standards.
Although we strive to abide by the standards, in some cases our
implementation is safe even when the standard does not demand safety,
and in other cases our implementation does not meet the standard safety
requirements. The latter are most likely bugs; the former, when marked
as @code{Preliminary}, should not be counted on: future standards may
require changes that are not compatible with the additional safety
properties afforded by the current implementation.
Furthermore, the POSIX standard does not offer a detailed definition of
safety. We assume that, by ``safe to call'', POSIX means that, as long
as the program does not invoke undefined behavior, the ``safe to call''
function behaves as specified, and does not cause other functions to
deviate from their specified behavior. We have chosen to use its loose
definitions of safety, not because they are the best definitions to use,
but because choosing them harmonizes this manual with POSIX.
Please keep in mind that these are preliminary definitions and
annotations, and certain aspects of the definitions are still under
discussion and might be subject to clarification or change.
Over time, we envision evolving the preliminary safety notes into stable
commitments, as stable as those of our interfaces. As we do, we will
remove the @code{Preliminary} keyword from safety notes. As long as the
keyword remains, however, they are not to be regarded as a promise of
future behavior.
@end itemize
Other keywords that appear in safety notes are defined in subsequent
sections.
@node Unsafe Features, Conditionally Safe Features, POSIX Safety Concepts, POSIX
@subsubsection Unsafe Features
@cindex Unsafe Features
Functions that are unsafe to call in certain contexts are annotated with
keywords that document their features that make them unsafe to call.
AS-Unsafe features in this section indicate the functions are never safe
to call when asynchronous signals are enabled. AC-Unsafe features
indicate they are never safe to call when asynchronous cancellation is
enabled. There are no MT-Unsafe marks in this section.
@itemize @bullet
@item @code{lock}
@cindex lock
Functions marked with @code{lock} as an AS-Unsafe feature may be
interrupted by a signal while holding a non-recursive lock. If the
signal handler calls another such function that takes the same lock, the
result is a deadlock.
Functions annotated with @code{lock} as an AC-Unsafe feature may, if
cancelled asynchronously, fail to release a lock that would have been
released if their execution had not been interrupted by asynchronous
thread cancellation. Once a lock is left taken, attempts to take that
lock will block indefinitely.
@item @code{corrupt}
@cindex corrupt
Functions marked with @code{corrupt} as an AS-Unsafe feature may corrupt
data structures and misbehave when they interrupt, or are interrupted
by, another such function. Unlike functions marked with @code{lock},
these take recursive locks to avoid MT-Safety problems, but this is not
enough to stop a signal handler from observing a partially-updated data
structure. Further corruption may arise from the interrupted function's
failure to notice updates made by signal handlers.
Functions marked with @code{corrupt} as an AC-Unsafe feature may leave
data structures in a corrupt, partially updated state. Subsequent uses
of the data structure may misbehave.
@c A special case, probably not worth documenting separately, involves
@c reallocing, or even freeing pointers. Any case involving free could
@c be easily turned into an ac-safe leak by resetting the pointer before
@c releasing it; I don't think we have any case that calls for this sort
@c of fixing. Fixing the realloc cases would require a new interface:
@c instead of @code{ptr=realloc(ptr,size)} we'd have to introduce
@c @code{acsafe_realloc(&ptr,size)} that would modify ptr before
@c releasing the old memory. The ac-unsafe realloc could be implemented
@c in terms of an internal interface with this semantics (say
@c __acsafe_realloc), but since realloc can be overridden, the function
@c we call to implement realloc should not be this internal interface,
@c but another internal interface that calls __acsafe_realloc if realloc
@c was not overridden, and calls the overridden realloc with async
@c cancel disabled. --lxoliva
@item @code{heap}
@cindex heap
Functions marked with @code{heap} may call heap memory management
functions from the @code{malloc}/@code{free} family of functions and are
only as safe as those functions. This note is thus equivalent to:
@sampsafety{@asunsafe{@asulock{}}@acunsafe{@aculock{} @acsfd{} @acsmem{}}}
@c Check for cases that should have used plugin instead of or in
@c addition to this. Then, after rechecking gettext, adjust i18n if
@c needed.
@item @code{dlopen}
@cindex dlopen
Functions marked with @code{dlopen} use the dynamic loader to load
shared libraries into the current execution image. This involves
opening files, mapping them into memory, allocating additional memory,
resolving symbols, applying relocations and more, all of this while
holding internal dynamic loader locks.
The locks are enough for these functions to be AS- and AC-Unsafe, but
other issues may arise. At present this is a placeholder for all
potential safety issues raised by @code{dlopen}.
@c dlopen runs init and fini sections of the module; does this mean
@c dlopen always implies plugin?
@item @code{plugin}
@cindex plugin
Functions annotated with @code{plugin} may run code from plugins that
may be external to @theglibc{}. Such plugin functions are assumed to be
MT-Safe, AS-Unsafe and AC-Unsafe. Examples of such plugins are stack
@cindex NSS
unwinding libraries, name service switch (NSS) and character set
@cindex iconv
conversion (iconv) back-ends.
Although the plugins mentioned as examples are all brought in by means
of dlopen, the @code{plugin} keyword does not imply any direct
involvement of the dynamic loader or the @code{libdl} interfaces, those
are covered by @code{dlopen}. For example, if one function loads a
module and finds the addresses of some of its functions, while another
just calls those already-resolved functions, the former will be marked
with @code{dlopen}, whereas the latter will get the @code{plugin}. When
a single function takes all of these actions, then it gets both marks.
@item @code{i18n}
@cindex i18n
Functions marked with @code{i18n} may call internationalization
functions of the @code{gettext} family and will be only as safe as those
functions. This note is thus equivalent to:
@sampsafety{@mtsafe{@mtsenv{}}@asunsafe{@asucorrupt{} @ascuheap{} @ascudlopen{}}@acunsafe{@acucorrupt{}}}
@item @code{timer}
@cindex timer
Functions marked with @code{timer} use the @code{alarm} function or
similar to set a time-out for a system call or a long-running operation.
In a multi-threaded program, there is a risk that the time-out signal
will be delivered to a different thread, thus failing to interrupt the
intended thread. Besides being MT-Unsafe, such functions are always
AS-Unsafe, because calling them in signal handlers may interfere with
timers set in the interrupted code, and AC-Unsafe, because there is no
safe way to guarantee an earlier timer will be reset in case of
asynchronous cancellation.
@end itemize
@node Conditionally Safe Features, Other Safety Remarks, Unsafe Features, POSIX
@subsubsection Conditionally Safe Features
@cindex Conditionally Safe Features
For some features that make functions unsafe to call in certain
contexts, there are known ways to avoid the safety problem other than
refraining from calling the function altogether. The keywords that
follow refer to such features, and each of their definitions indicate
how the whole program needs to be constrained in order to remove the
safety problem indicated by the keyword. Only when all the reasons that
make a function unsafe are observed and addressed, by applying the
documented constraints, does the function become safe to call in a
context.
@itemize @bullet
@item @code{init}
@cindex init
Functions marked with @code{init} as an MT-Unsafe feature perform
MT-Unsafe initialization when they are first called.
Calling such a function at least once in single-threaded mode removes
this specific cause for the function to be regarded as MT-Unsafe. If no
other cause for that remains, the function can then be safely called
after other threads are started.
Functions marked with @code{init} as an AS- or AC-Unsafe feature use the
internal @code{libc_once} machinery or similar to initialize internal
data structures.
If a signal handler interrupts such an initializer, and calls any
function that also performs @code{libc_once} initialization, it will
deadlock if the thread library has been loaded.
Furthermore, if an initializer is partially complete before it is
canceled or interrupted by a signal whose handler requires the same
initialization, some or all of the initialization may be performed more
than once, leaking resources or even resulting in corrupt internal data.
Applications that need to call functions marked with @code{init} as an
AS- or AC-Unsafe feature should ensure the initialization is performed
before configuring signal handlers or enabling cancellation, so that the
AS- and AC-Safety issues related with @code{libc_once} do not arise.
@c We may have to extend the annotations to cover conditions in which
@c initialization may or may not occur, since an initial call in a safe
@c context is no use if the initialization doesn't take place at that
@c time: it doesn't remove the risk for later calls.
@item @code{race}
@cindex race
Functions annotated with @code{race} as an MT-Safety issue operate on
objects in ways that may cause data races or similar forms of
destructive interference out of concurrent execution. In some cases,
the objects are passed to the functions by users; in others, they are
used by the functions to return values to users; in others, they are not
even exposed to users.
We consider access to objects passed as (indirect) arguments to
functions to be data race free. The assurance of data race free objects
is the caller's responsibility. We will not mark a function as
MT-Unsafe or AS-Unsafe if it misbehaves when users fail to take the
measures required by POSIX to avoid data races when dealing with such
objects. As a general rule, if a function is documented as reading from
an object passed (by reference) to it, or modifying it, users ought to
use memory synchronization primitives to avoid data races just as they
would should they perform the accesses themselves rather than by calling
the library function. @code{FILE} streams are the exception to the
general rule, in that POSIX mandates the library to guard against data
races in many functions that manipulate objects of this specific opaque
type. We regard this as a convenience provided to users, rather than as
a general requirement whose expectations should extend to other types.
In order to remind users that guarding certain arguments is their
responsibility, we will annotate functions that take objects of certain
types as arguments. We draw the line for objects passed by users as
follows: objects whose types are exposed to users, and that users are
expected to access directly, such as memory buffers, strings, and
various user-visible @code{struct} types, do @emph{not} give reason for
functions to be annotated with @code{race}. It would be noisy and
redundant with the general requirement, and not many would be surprised
by the library's lack of internal guards when accessing objects that can
be accessed directly by users.
As for objects that are opaque or opaque-like, in that they are to be
manipulated only by passing them to library functions (e.g.,
@code{FILE}, @code{DIR}, @code{obstack}, @code{iconv_t}), there might be
additional expectations as to internal coordination of access by the
library. We will annotate, with @code{race} followed by a colon and the
argument name, functions that take such objects but that do not take
care of synchronizing access to them by default. For example,
@code{FILE} stream @code{unlocked} functions will be annotated, but
those that perform implicit locking on @code{FILE} streams by default
will not, even though the implicit locking may be disabled on a
per-stream basis.
In either case, we will not regard as MT-Unsafe functions that may
access user-supplied objects in unsafe ways should users fail to ensure
the accesses are well defined. The notion prevails that users are
expected to safeguard against data races any user-supplied objects that
the library accesses on their behalf.
@c The above describes @mtsrace; @mtasurace is described below.
This user responsibility does not apply, however, to objects controlled
by the library itself, such as internal objects and static buffers used
to return values from certain calls. When the library doesn't guard
them against concurrent uses, these cases are regarded as MT-Unsafe and
AS-Unsafe (although the @code{race} mark under AS-Unsafe will be omitted
as redundant with the one under MT-Unsafe). As in the case of
user-exposed objects, the mark may be followed by a colon and an
identifier. The identifier groups all functions that operate on a
certain unguarded object; users may avoid the MT-Safety issues related
with unguarded concurrent access to such internal objects by creating a
non-recursive mutex related with the identifier, and always holding the
mutex when calling any function marked as racy on that identifier, as
they would have to should the identifier be an object under user
control. The non-recursive mutex avoids the MT-Safety issue, but it
trades one AS-Safety issue for another, so use in asynchronous signals
remains undefined.
When the identifier relates to a static buffer used to hold return
values, the mutex must be held for as long as the buffer remains in use
by the caller. Many functions that return pointers to static buffers
offer reentrant variants that store return values in caller-supplied
buffers instead. In some cases, such as @code{tmpname}, the variant is
chosen not by calling an alternate entry point, but by passing a
non-@code{NULL} pointer to the buffer in which the returned values are
to be stored. These variants are generally preferable in multi-threaded
programs, although some of them are not MT-Safe because of other
internal buffers, also documented with @code{race} notes.
@item @code{const}
@cindex const
Functions marked with @code{const} as an MT-Safety issue non-atomically
modify internal objects that are better regarded as constant, because a
substantial portion of @theglibc{} accesses them without
synchronization. Unlike @code{race}, that causes both readers and
writers of internal objects to be regarded as MT-Unsafe and AS-Unsafe,
this mark is applied to writers only. Writers remain equally MT- and
AS-Unsafe to call, but the then-mandatory constness of objects they
modify enables readers to be regarded as MT-Safe and AS-Safe (as long as
no other reasons for them to be unsafe remain), since the lack of
synchronization is not a problem when the objects are effectively
constant.
The identifier that follows the @code{const} mark will appear by itself
as a safety note in readers. Programs that wish to work around this
safety issue, so as to call writers, may use a non-recursve
@code{rwlock} associated with the identifier, and guard @emph{all} calls
to functions marked with @code{const} followed by the identifier with a
write lock, and @emph{all} calls to functions marked with the identifier
by itself with a read lock. The non-recursive locking removes the
MT-Safety problem, but it trades one AS-Safety problem for another, so
use in asynchronous signals remains undefined.
@c But what if, instead of marking modifiers with const:id and readers
@c with just id, we marked writers with race:id and readers with ro:id?
@c Instead of having to define each instance of “id”, we'd have a
@c general pattern governing all such “id”s, wherein race:id would
@c suggest the need for an exclusive/write lock to make the function
@c safe, whereas ro:id would indicate “id” is expected to be read-only,
@c but if any modifiers are called (while holding an exclusive lock),
@c then ro:id-marked functions ought to be guarded with a read lock for
@c safe operation. ro:env or ro:locale, for example, seems to convey
@c more clearly the expectations and the meaning, than just env or
@c locale.
@item @code{sig}
@cindex sig
Functions marked with @code{sig} as a MT-Safety issue (that implies an
identical AS-Safety issue, omitted for brevity) may temporarily install
a signal handler for internal purposes, which may interfere with other
uses of the signal, identified after a colon.
This safety problem can be worked around by ensuring that no other uses
of the signal will take place for the duration of the call. Holding a
non-recursive mutex while calling all functions that use the same
temporary signal; blocking that signal before the call and resetting its
handler afterwards is recommended.
There is no safe way to guarantee the original signal handler is
restored in case of asynchronous cancellation, therefore so-marked
functions are also AC-Unsafe.
@c fixme: at least deferred cancellation should get it right, and would
@c obviate the restoring bit below, and the qualifier above.
Besides the measures recommended to work around the MT- and AS-Safety
problem, in order to avert the cancellation problem, disabling
asynchronous cancellation @emph{and} installing a cleanup handler to
restore the signal to the desired state and to release the mutex are
recommended.
@item @code{term}
@cindex term
Functions marked with @code{term} as an MT-Safety issue may change the
terminal settings in the recommended way, namely: call @code{tcgetattr},
modify some flags, and then call @code{tcsetattr}; this creates a window
in which changes made by other threads are lost. Thus, functions marked
with @code{term} are MT-Unsafe. The same window enables changes made by
asynchronous signals to be lost. These functions are also AS-Unsafe,
but the corresponding mark is omitted as redundant.
It is thus advisable for applications using the terminal to avoid
concurrent and reentrant interactions with it, by not using it in signal
handlers or blocking signals that might use it, and holding a lock while
calling these functions and interacting with the terminal. This lock
should also be used for mutual exclusion with functions marked with
@code{@mtasurace{:tcattr(fd)}}, where @var{fd} is a file descriptor for
the controlling terminal. The caller may use a single mutex for
simplicity, or use one mutex per terminal, even if referenced by
different file descriptors.
Functions marked with @code{term} as an AC-Safety issue are supposed to
restore terminal settings to their original state, after temporarily
changing them, but they may fail to do so if cancelled.
@c fixme: at least deferred cancellation should get it right, and would
@c obviate the restoring bit below, and the qualifier above.
Besides the measures recommended to work around the MT- and AS-Safety
problem, in order to avert the cancellation problem, disabling
asynchronous cancellation @emph{and} installing a cleanup handler to
restore the terminal settings to the original state and to release the
mutex are recommended.
@end itemize
@node Other Safety Remarks, , Conditionally Safe Features, POSIX
@subsubsection Other Safety Remarks
@cindex Other Safety Remarks
Additional keywords may be attached to functions, indicating features
that do not make a function unsafe to call, but that may need to be
taken into account in certain classes of programs:
@itemize @bullet
@item @code{locale}
@cindex locale
Functions annotated with @code{locale} as an MT-Safety issue read from
the locale object without any form of synchronization. Functions
annotated with @code{locale} called concurrently with locale changes may
behave in ways that do not correspond to any of the locales active
during their execution, but an unpredictable mix thereof.
We do not mark these functions as MT- or AS-Unsafe, however, because
functions that modify the locale object are marked with
@code{const:locale} and regarded as unsafe. Being unsafe, the latter
are not to be called when multiple threads are running or asynchronous
signals are enabled, and so the locale can be considered effectively
constant in these contexts, which makes the former safe.
@c Should the locking strategy suggested under @code{const} be used,
@c failure to guard locale uses is not as fatal as data races in
@c general: unguarded uses will @emph{not} follow dangling pointers or
@c access uninitialized, unmapped or recycled memory. Each access will
@c read from a consistent locale object that is or was active at some
@c point during its execution. Without synchronization, however, it
@c cannot even be assumed that, after a change in locale, earlier
@c locales will no longer be used, even after the newly-chosen one is
@c used in the thread. Nevertheless, even though unguarded reads from
@c the locale will not violate type safety, functions that access the
@c locale multiple times may invoke all sorts of undefined behavior
@c because of the unexpected locale changes.
@item @code{env}
@cindex env
Functions marked with @code{env} as an MT-Safety issue access the
environment with @code{getenv} or similar, without any guards to ensure
safety in the presence of concurrent modifications.
We do not mark these functions as MT- or AS-Unsafe, however, because
functions that modify the environment are all marked with
@code{const:env} and regarded as unsafe. Being unsafe, the latter are
not to be called when multiple threads are running or asynchronous
signals are enabled, and so the environment can be considered
effectively constant in these contexts, which makes the former safe.
@item @code{hostid}
@cindex hostid
The function marked with @code{hostid} as an MT-Safety issue reads from
the system-wide data structures that hold the ``host ID'' of the
machine. These data structures cannot generally be modified atomically.
Since it is expected that the ``host ID'' will not normally change, the
function that reads from it (@code{gethostid}) is regarded as safe,
whereas the function that modifies it (@code{sethostid}) is marked with
@code{@mtasuconst{:@mtshostid{}}}, indicating it may require special
care if it is to be called. In this specific case, the special care
amounts to system-wide (not merely intra-process) coordination.
@item @code{sigintr}
@cindex sigintr
Functions marked with @code{sigintr} as an MT-Safety issue access the
@code{_sigintr} internal data structure without any guards to ensure
safety in the presence of concurrent modifications.
We do not mark these functions as MT- or AS-Unsafe, however, because
functions that modify the this data structure are all marked with
@code{const:sigintr} and regarded as unsafe. Being unsafe, the latter
are not to be called when multiple threads are running or asynchronous
signals are enabled, and so the data structure can be considered
effectively constant in these contexts, which makes the former safe.
@item @code{fd}
@cindex fd
Functions annotated with @code{fd} as an AC-Safety issue may leak file
descriptors if asynchronous thread cancellation interrupts their
execution.
Functions that allocate or deallocate file descriptors will generally be
marked as such. Even if they attempted to protect the file descriptor
allocation and deallocation with cleanup regions, allocating a new
descriptor and storing its number where the cleanup region could release
it cannot be performed as a single atomic operation. Similarly,
releasing the descriptor and taking it out of the data structure
normally responsible for releasing it cannot be performed atomically.
There will always be a window in which the descriptor cannot be released
because it was not stored in the cleanup handler argument yet, or it was
already taken out before releasing it. It cannot be taken out after
release: an open descriptor could mean either that the descriptor still
has to be closed, or that it already did so but the descriptor was
reallocated by another thread or signal handler.
Such leaks could be internally avoided, with some performance penalty,
by temporarily disabling asynchronous thread cancellation. However,
since callers of allocation or deallocation functions would have to do
this themselves, to avoid the same sort of leak in their own layer, it
makes more sense for the library to assume they are taking care of it
than to impose a performance penalty that is redundant when the problem
is solved in upper layers, and insufficient when it is not.
This remark by itself does not cause a function to be regarded as
AC-Unsafe. However, cumulative effects of such leaks may pose a
problem for some programs. If this is the case, suspending asynchronous
cancellation for the duration of calls to such functions is recommended.
@item @code{mem}
@cindex mem
Functions annotated with @code{mem} as an AC-Safety issue may leak
memory if asynchronous thread cancellation interrupts their execution.
The problem is similar to that of file descriptors: there is no atomic
interface to allocate memory and store its address in the argument to a
cleanup handler, or to release it and remove its address from that
argument, without at least temporarily disabling asynchronous
cancellation, which these functions do not do.
This remark does not by itself cause a function to be regarded as
generally AC-Unsafe. However, cumulative effects of such leaks may be
severe enough for some programs that disabling asynchronous cancellation
for the duration of calls to such functions may be required.
@item @code{cwd}
@cindex cwd
Functions marked with @code{cwd} as an MT-Safety issue may temporarily
change the current working directory during their execution, which may
cause relative pathnames to be resolved in unexpected ways in other
threads or within asynchronous signal or cancellation handlers.
This is not enough of a reason to mark so-marked functions as MT- or
AS-Unsafe, but when this behavior is optional (e.g., @code{nftw} with
@code{FTW_CHDIR}), avoiding the option may be a good alternative to
using full pathnames or file descriptor-relative (e.g. @code{openat})
system calls.
@item @code{!posix}
@cindex !posix
This remark, as an MT-, AS- or AC-Safety note to a function, indicates
the safety status of the function is known to differ from the specified
status in the POSIX standard. For example, POSIX does not require a
function to be Safe, but our implementation is, or vice-versa.
For the time being, the absence of this remark does not imply the safety
properties we documented are identical to those mandated by POSIX for
the corresponding functions.
@item @code{:identifier}
@cindex :identifier
Annotations may sometimes be followed by identifiers, intended to group
several functions that e.g. access the data structures in an unsafe way,
as in @code{race} and @code{const}, or to provide more specific
information, such as naming a signal in a function marked with
@code{sig}. It is envisioned that it may be applied to @code{lock} and
@code{corrupt} as well in the future.
In most cases, the identifier will name a set of functions, but it may
name global objects or function arguments, or identifiable properties or
logical components associated with them, with a notation such as
e.g. @code{:buf(arg)} to denote a buffer associated with the argument
@var{arg}, or @code{:tcattr(fd)} to denote the terminal attributes of a
file descriptor @var{fd}.
The most common use for identifiers is to provide logical groups of
functions and arguments that need to be protected by the same
synchronization primitive in order to ensure safe operation in a given
context.
@item @code{/condition}
@cindex /condition
Some safety annotations may be conditional, in that they only apply if a
boolean expression involving arguments, global variables or even the
underlying kernel evaluates evaluates to true. Such conditions as
@code{/hurd} or @code{/!linux!bsd} indicate the preceding marker only
applies when the underlying kernel is the HURD, or when it is neither
Linux nor a BSD kernel, respectively. @code{/!ps} and
@code{/one_per_line} indicate the preceding marker only applies when
argument @var{ps} is NULL, or global variable @var{one_per_line} is
nonzero.
When all marks that render a function unsafe are adorned with such
conditions, and none of the named conditions hold, then the function can
be regarded as safe.
@end itemize