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   attributes - POSIX safety concepts

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   _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)](../man3/tcgetattr.3.html), modify some flags, and then call
          [tcsetattr(3)](../man3/tcsetattr.3.html), 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)](../man3/getenv.3.html) 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_ Functions marked with _hostid_ as an MT-Safety issue read
          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)](../man3/gethostid.3.html)) is regarded as safe, whereas the
          function that modifies it ([sethostid(3)](../man3/sethostid.3.html)) 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)](../man3/nftw.3.html) with **FTW_CHDIR**), avoiding the option may be a good
          alternative to using full pathnames or file descriptor-
          relative (e.g., [openat(2)](../man2/openat.2.html)) 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 _/oneperline_ indicate the
          preceding marker only applies when argument _ps_ is NULL, or
          global variable _oneperline_ 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.

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Linux man-pages 6.10 2024-11-17 attributes(7)

attributes(7) - Linux manual page (original) (raw)

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