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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 cancelation 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)
Linux man-pages 6.9.1 2024-05-02 attributes(7)