attributes - POSIX safety concepts
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.
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.
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.
pthreads(7)