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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)

COLOPHON

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