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NAME
attributes - POSIX safety concepts
DESCRIPTION
Note: the text of this man page is based on the material taken from the "POSIX Safety
Concepts" section of the GNU C Library manual. Further details on the topics described
here can be found in that manual.
Various function manual pages include a section ATTRIBUTES that describes the safety of
calling the function in various contexts. This section annotates functions with the
following safety markings:
MT-Safe
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 GNU C Library
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.
MT-Unsafe
MT-Unsafe functions are not safe to call in a multithreaded programs.
Other keywords that appear in safety notes are defined in subsequent sections.
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 indicates 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.
init Functions marked with 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.
race Functions annotated with 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.
const Functions marked with const as an MT-Safety issue non-atomically modify internal
objects that are better regarded as constant, because a substantial portion of the
GNU C Library accesses them without synchronization. Unlike race, which causes
both readers and writers of internal objects to be regarded as MT-Unsafe, this mark
is applied to writers only. Writers remain MT-Unsafe to call, but the then-
mandatory constness of objects they modify enables readers to be regarded as MT-
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 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-recursive read-write lock associated with the identifier,
and guard all calls to functions marked with const followed by the identifier with
a write lock, and all calls to functions marked with the identifier by itself with
a read lock.
sig Functions marked with sig as a MT-Safety issue 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.
term Functions marked with term as an MT-Safety issue may change the terminal settings
in the recommended way, namely: call tcgetattr(3), modify some flags, and then call
tcsetattr(3), this creates a window in which changes made by other threads are
lost. Thus, functions marked with term are MT-Unsafe.
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 race:tcattr(fd), where 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.
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:
locale Functions annotated with locale as an MT-Safety issue read from the locale object
without any form of synchronization. Functions annotated with 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-Unsafe, however, because functions that modify
the locale object are marked with 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.
env Functions marked with env as an MT-Safety issue access the environment with
getenv(3) or similar, without any guards to ensure safety in the presence of
concurrent modifications.
We do not mark these functions as MT-Unsafe, however, because functions that modify
the environment are all marked with 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.
hostid The function marked with 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 (gethostid(3)) is
regarded as safe, whereas the function that modifies it (sethostid(3)) is marked
with const:hostid, 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.
sigintr
Functions marked with sigintr as an MT-Safety issue access the GNU C Library
_sigintr internal data structure without any guards to ensure safety in the
presence of concurrent modifications.
We do not mark these functions as MT-Unsafe, however, because functions that modify
this data structure are all marked with 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.
cwd Functions marked with 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-Unsafe, but when
this behavior is optional (e.g., nftw(3) with FTW_CHDIR), avoiding the option may
be a good alternative to using full pathnames or file descriptor-relative (e.g.,
openat(2)) system calls.
:identifier
Annotations may sometimes be followed by identifiers, intended to group several
functions that, for example, access the data structures in an unsafe way, as in
race and const, or to provide more specific information, such as naming a signal in
a function marked with sig. It is envisioned that it may be applied to lock and
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, for example, :buf(arg) to denote a
buffer associated with the argument arg, or :tcattr(fd) to denote the terminal
attributes of a file descriptor 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.
/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 to true. For example, /!ps and /one_per_line indicate the preceding
marker only applies when argument ps is NULL, or global variable 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.
SEE ALSO
pthreads(7), signal-safety(7)
COLOPHON
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