mirror of https://github.com/mkerrisk/man-pages
884 lines
34 KiB
Groff
884 lines
34 KiB
Groff
.\" Copyright (c) 2014, Red Hat, Inc
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.\" Written by Alexandre Oliva <aoliva@redhat.com>
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.TH ATTRIBUTES 7 2015-03-02 "Linux" "Linux Programmer's Manual"
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.SH NAME
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attributes \- POSIX safety concepts
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.SH DESCRIPTION
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.\"
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.\"
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.IR Note :
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the text of this man page is based on the material taken from
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the "POSIX Safety Concepts" section of the GNU C Library manual.
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Further details on the topics described here can be found in that
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manual.
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.PP
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Various function manual pages include a section ATTRIBUTES
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that describes the safety of calling the function in various contexts.
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This section annotates functions with the following safety markings:
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.TP
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.I MT-Safe
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.I MT-Safe
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or
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Thread-Safe functions are safe to call in the presence
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of other threads.
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MT, in MT-Safe, stands for Multi Thread.
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.IP
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Being MT-Safe does not imply a function is atomic, nor that it uses any
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of the memory synchronization mechanisms POSIX exposes to users.
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It is even possible that calling MT-Safe functions in sequence
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does not yield an MT-Safe combination.
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For example, having a thread call two MT-Safe
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functions one right after the other does not guarantee behavior
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equivalent to atomic execution of a combination of both functions,
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since concurrent calls in other threads may interfere in a destructive way.
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.IP
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Whole-program optimizations that could inline functions across library
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interfaces may expose unsafe reordering, and so performing inlining
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across the GNU C Library interface is not recommended.
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The documented
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MT-Safety status is not guaranteed under whole-program optimization.
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However, functions defined in user-visible headers are designed to be
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safe for inlining.
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.\" .TP
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.\" .I AS-Safe
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.\" .I AS-Safe
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.\" or Async-Signal-Safe functions are safe to call from
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.\" asynchronous signal handlers.
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.\" AS, in AS-Safe, stands for Asynchronous Signal.
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.\"
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.\" Many functions that are AS-Safe may set
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.\" .IR errno ,
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.\" or modify the floating-point environment,
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.\" because their doing so does not make them
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.\" unsuitable for use in signal handlers.
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.\" However, programs could misbehave should asynchronous signal handlers
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.\" modify this thread-local state,
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.\" and the signal handling machinery cannot be counted on to
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.\" preserve it.
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.\" Therefore, signal handlers that call functions that may set
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.\" .I errno
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.\" or modify the floating-point environment
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.\" .I must
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.\" save their original values, and restore them before returning.
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.\" .TP
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.\" .I AC-Safe
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.\" .I AC-Safe
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.\" or Async-Cancel-Safe functions are safe to call when
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.\" asynchronous cancellation is enabled.
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.\" AC in AC-Safe stands for Asynchronous Cancellation.
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.\"
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.\" The POSIX standard defines only three functions to be AC-Safe, namely
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.\" .BR pthread_cancel (3),
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.\" .BR pthread_setcancelstate (3),
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.\" and
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.\" .BR pthread_setcanceltype (3).
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.\" At present the GNU C Library provides no
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.\" guarantees beyond these three functions,
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.\" but does document which functions are presently AC-Safe.
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.\" This documentation is provided for use
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.\" by the GNU C Library developers.
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.\"
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.\" Just like signal handlers, cancellation cleanup routines must configure
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.\" the floating point environment they require.
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.\" The routines cannot assume a floating point environment,
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.\" particularly when asynchronous cancellation is enabled.
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.\" If the configuration of the floating point
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.\" environment cannot be performed atomically then it is also possible that
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.\" the environment encountered is internally inconsistent.
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.TP
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.IR MT-Unsafe \" ", " AS-Unsafe ", " AC-Unsafe
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.IR MT-Unsafe \" ", " AS-Unsafe ", " AC-Unsafe
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functions are not safe to call in a multithreaded programs.
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.\" functions are not
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.\" safe to call within the safety contexts described above.
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.\" Calling them
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.\" within such contexts invokes undefined behavior.
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.\"
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.\" Functions not explicitly documented as safe in a safety context should
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.\" be regarded as Unsafe.
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.\" .TP
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.\" .I Preliminary
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.\" .I Preliminary
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.\" safety properties are documented, indicating these
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.\" properties may
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.\" .I not
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.\" be counted on in future releases of
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.\" the GNU C Library.
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.\"
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.\" Such preliminary properties are the result of an assessment of the
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.\" properties of our current implementation,
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.\" rather than of what is mandated and permitted
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.\" by current and future standards.
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.\"
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.\" Although we strive to abide by the standards, in some cases our
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.\" implementation is safe even when the standard does not demand safety,
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.\" and in other cases our implementation does not meet the standard safety
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.\" requirements.
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.\" The latter are most likely bugs; the former, when marked
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.\" as
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.\" .IR Preliminary ,
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.\" should not be counted on: future standards may
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.\" require changes that are not compatible with the additional safety
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.\" properties afforded by the current implementation.
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.\"
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.\" Furthermore,
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.\" the POSIX standard does not offer a detailed definition of safety.
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.\" We assume that, by "safe to call", POSIX means that,
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.\" as long as the program does not invoke undefined behavior,
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.\" the "safe to call" function behaves as specified,
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.\" and does not cause other functions to deviate from their specified behavior.
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.\" We have chosen to use its loose
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.\" definitions of safety, not because they are the best definitions to use,
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.\" but because choosing them harmonizes this manual with POSIX.
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.\"
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.\" Please keep in mind that these are preliminary definitions and annotations,
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.\" and certain aspects of the definitions are still under
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.\" discussion and might be subject to clarification or change.
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.\"
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.\" Over time,
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.\" we envision evolving the preliminary safety notes into stable commitments,
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.\" as stable as those of our interfaces.
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.\" As we do, we will remove the
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.\" .I Preliminary
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.\" keyword from safety notes.
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.\" As long as the keyword remains, however,
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.\" they are not to be regarded as a promise of future behavior.
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.PP
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Other keywords that appear in safety notes are defined in subsequent sections.
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.\"
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.\"
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.\" .SS Unsafe features
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.\" Functions that are unsafe to call in certain contexts are annotated with
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.\" keywords that document their features that make them unsafe to call.
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.\" AS-Unsafe features in this section indicate the functions are never safe
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.\" to call when asynchronous signals are enabled.
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.\" AC-Unsafe features
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.\" indicate they are never safe to call when asynchronous cancellation is
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.\" .\" enabled.
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.\" There are no MT-Unsafe marks in this section.
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.\" .TP
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.\" .\" .I code
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.\" Functions marked with
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.\" .I lock
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.\" as an AS-Unsafe feature may be
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.\" .\" interrupted by a signal while holding a non-recursive lock.
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.\" If the signal handler calls another such function that takes the same lock,
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.\" the result is a deadlock.
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.\"
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.\" Functions annotated with
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.\" .I lock
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.\" as an AC-Unsafe feature may, if canceled asynchronously,
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.\" fail to release a lock that would have been released if their execution
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.\" had not been interrupted by asynchronous thread cancellation.
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.\" Once a lock is left taken,
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.\" attempts to take that lock will block indefinitely.
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.\" .TP
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.\" .I corrupt
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.\" Functions marked with
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.\" .\" .I corrupt
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.\" as an AS-Unsafe feature may corrupt
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.\" data structures and misbehave when they interrupt,
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.\" or are interrupted by, another such function.
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.\" Unlike functions marked with
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.\" .IR lock ,
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.\" these take recursive locks to avoid MT-Safety problems,
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.\" but this is not enough to stop a signal handler from observing
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.\" a partially-updated data structure.
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.\" Further corruption may arise from the interrupted function's
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.\" failure to notice updates made by signal handlers.
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.\"
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.\" Functions marked with
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.\" .I corrupt
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.\" as an AC-Unsafe feature may leave
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.\" data structures in a corrupt, partially updated state.
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.\" Subsequent uses of the data structure may misbehave.
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.\"
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.\" .\" A special case, probably not worth documenting separately, involves
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.\" .\" reallocing, or even freeing pointers. Any case involving free could
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.\" .\" be easily turned into an ac-safe leak by resetting the pointer before
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.\" .\" releasing it; I don't think we have any case that calls for this sort
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.\" .\" of fixing. Fixing the realloc cases would require a new interface:
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.\" .\" instead of @code{ptr=realloc(ptr,size)} we'd have to introduce
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.\" .\" @code{acsafe_realloc(&ptr,size)} that would modify ptr before
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.\" .\" releasing the old memory. The ac-unsafe realloc could be implemented
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.\" .\" in terms of an internal interface with this semantics (say
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.\" .\" __acsafe_realloc), but since realloc can be overridden, the function
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.\" .\" we call to implement realloc should not be this internal interface,
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.\" .\" but another internal interface that calls __acsafe_realloc if realloc
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.\" .\" was not overridden, and calls the overridden realloc with async
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.\" .\" cancel disabled. --lxoliva
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.\" .TP
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.\" .I heap
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.\" Functions marked with
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.\" .I heap
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.\" may call heap memory management functions from the
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.\" .BR malloc (3)/ free (3)
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.\" family of functions and are only as safe as those functions.
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.\" This note is thus equivalent to:
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.\"
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.\" | AS-Unsafe lock | AC-Unsafe lock fd mem |
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.\" .\" @sampsafety{@asunsafe{@asulock{}}@acunsafe{@aculock{} @acsfd{} @acsmem{}}}
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.\" .\"
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.\" .\" Check for cases that should have used plugin instead of or in
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.\" .\" addition to this. Then, after rechecking gettext, adjust i18n if
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.\" .\" needed.
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.\" .TP
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.\" .I dlopen
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.\" Functions marked with
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.\" .I dlopen
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.\" use the dynamic loader to load
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.\" shared libraries into the current execution image.
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.\" This involves opening files, mapping them into memory,
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.\" allocating additional memory, resolving symbols,
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.\" applying relocations and more,
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.\" all of this while holding internal dynamic loader locks.
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.\"
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.\" The locks are enough for these functions to be AS-Unsafe and AC-Unsafe,
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.\" but other issues may arise.
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.\" At present this is a placeholder for all
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.\" potential safety issues raised by
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.\" .BR dlopen (3).
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.\"
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.\" .\" dlopen runs init and fini sections of the module; does this mean
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.\" .\" dlopen always implies plugin?
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.\" .TP
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.\" .I plugin
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.\" Functions annotated with
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.\" .I plugin
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.\" may run code from plugins that
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.\" may be external to the GNU C Library.
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.\" Such plugin functions are assumed to be
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.\" MT-Safe, AS-Unsafe and AC-Unsafe.
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.\" Examples of such plugins are stack unwinding libraries,
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.\" name service switch (NSS) and character set conversion (iconv) back-ends.
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.\"
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.\" Although the plugins mentioned as examples are all brought in by means
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.\" of dlopen, the
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.\" .I plugin
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.\" keyword does not imply any direct
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.\" involvement of the dynamic loader or the
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.\" .I libdl
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.\" interfaces,
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.\" those are covered by
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.\" .IR dlopen .
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.\" For example, if one function loads a module and finds the addresses
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.\" of some of its functions,
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.\" while another just calls those already-resolved functions,
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.\" the former will be marked with
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.\" .IR dlopen ,
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.\" whereas the latter will get the
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.\" .IR plugin .
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.\" When a single function takes all of these actions, then it gets both marks.
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.\" .TP
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.\" .I i18n
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.\" Functions marked with
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.\" .I i18n
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.\" may call internationalization
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.\" functions of the
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.\" .BR gettext (3)
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.\" family and will be only as safe as those
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.\" functions.
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.\" This note is thus equivalent to:
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.\"
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.\" | MT-Safe env | AS-Unsafe corrupt heap dlopen | AC-Unsafe corrupt |
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.\"
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.\" .\" @sampsafety{@mtsafe{@mtsenv{}}@asunsafe{@asucorrupt{} @ascuheap{} @ascudlopen{}}@acunsafe{@acucorrupt{}}}
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.\" .TP
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.\" .I timer
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.\" Functions marked with
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.\" .I timer
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.\" use the
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.\" .BR alarm (3)
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.\" function or
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.\" similar to set a time-out for a system call or a long-running operation.
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.\" In a multi-threaded program, there is a risk that the time-out signal
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.\" will be delivered to a different thread,
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.\" thus failing to interrupt the intended thread.
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.\" Besides being MT-Unsafe, such functions are always
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.\" AS-Unsafe, because calling them in signal handlers may interfere with
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.\" timers set in the interrupted code, and AC-Unsafe,
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.\" because there is no safe way to guarantee an earlier timer
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.\" will be reset in case of asynchronous cancellation.
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.\"
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.\"
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.SS Conditionally safe features
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For some features that make functions unsafe to call in certain contexts,
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there are known ways to avoid the safety problem other than
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refraining from calling the function altogether.
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The keywords that follow refer to such features,
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and each of their definitions indicates
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how the whole program needs to be constrained in order to remove the
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safety problem indicated by the keyword.
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Only when all the reasons that
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make a function unsafe are observed and addressed,
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by applying the documented constraints,
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does the function become safe to call in a context.
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.TP
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.I init
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Functions marked with
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.I init
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as an MT-Unsafe feature perform
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MT-Unsafe initialization when they are first called.
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.IP
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Calling such a function at least once in single-threaded mode removes
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this specific cause for the function to be regarded as MT-Unsafe.
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If no other cause for that remains,
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the function can then be safely called after other threads are started.
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.\"
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.\" Functions marked with
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.\" .I init
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.\" as an AS-Unsafe or AC-Unsafe feature use the GNU C Library internal
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.\" .I libc_once
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.\" machinery or similar to initialize internal data structures.
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.\"
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.\" If a signal handler interrupts such an initializer,
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.\" and calls any function that also performs
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.\" .I libc_once
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.\" initialization, it will deadlock if the thread library has been loaded.
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.\"
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.\" Furthermore, if an initializer is partially complete before it is canceled
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.\" or interrupted by a signal whose handler requires the same initialization,
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.\" some or all of the initialization may be performed more than once,
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.\" leaking resources or even resulting in corrupt internal data.
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.\"
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.\" Applications that need to call functions marked with
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.\" .I init
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.\" as an AS-Safety or AC-Unsafe feature should ensure
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.\" the initialization is performed
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.\" before configuring signal handlers or enabling cancellation,
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.\" so that the AS-Safety and AC-Safety issues related with
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.\" .I libc_once
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.\" do not arise.
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.\"
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.\" .\" We may have to extend the annotations to cover conditions in which
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.\" .\" initialization may or may not occur, since an initial call in a safe
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.\" .\" context is no use if the initialization doesn't take place at that
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.\" .\" time: it doesn't remove the risk for later calls.
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.TP
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.I race
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Functions annotated with
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.I race
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as an MT-Safety issue operate on
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objects in ways that may cause data races or similar forms of
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destructive interference out of concurrent execution.
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In some cases,
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the objects are passed to the functions by users;
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in others, they are used by the functions to return values to users;
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in others, they are not even exposed to users.
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.\"
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.\" We consider access to objects passed as (indirect) arguments to
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.\" functions to be data race free.
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.\" The assurance of data race free objects
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.\" is the caller's responsibility.
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.\" We will not mark a function as MT-Unsafe or AS-Unsafe
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.\" if it misbehaves when users fail to take the measures required by
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.\" POSIX to avoid data races when dealing with such objects.
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.\" As a general rule, if a function is documented as reading from
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.\" an object passed (by reference) to it, or modifying it,
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.\" users ought to use memory synchronization primitives
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.\" to avoid data races just as they would should they perform
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.\" the accesses themselves rather than by calling the library function.
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.\" Standard I/O
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.\" .RI ( "FILE *" )
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.\" streams are the exception to the general rule,
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.\" in that POSIX mandates the library to guard against data races
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.\" in many functions that manipulate objects of this specific opaque type.
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.\" We regard this as a convenience provided to users,
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.\" rather than as a general requirement whose expectations
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.\" should extend to other types.
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.\"
|
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.\" In order to remind users that guarding certain arguments is their
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.\" responsibility, we will annotate functions that take objects of certain
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.\" types as arguments.
|
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.\" We draw the line for objects passed by users as follows:
|
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.\" objects whose types are exposed to users,
|
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.\" and that users are expected to access directly,
|
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.\" such as memory buffers, strings,
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.\" and various user-visible structured types, do
|
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.\" .I not
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.\" give reason for functions to be annotated with
|
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.\" .IR race .
|
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.\" It would be noisy and redundant with the general requirement,
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.\" and not many would be surprised by the library's lack of internal
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.\" guards when accessing objects that can be accessed directly by users.
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.\"
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|
.\" As for objects that are opaque or opaque-like,
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.\" in that they are to be manipulated only by passing them
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.\" to library functions (e.g.,
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.\" .IR FILE ,
|
|
.\" .IR DIR ,
|
|
.\" .IR obstack ,
|
|
.\" .IR iconv_t ),
|
|
.\" there might be additional expectations as to internal coordination
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.\" of access by the library.
|
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.\" We will annotate, with
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.\" .I race
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|
.\" followed by a colon and the argument name,
|
|
.\" functions that take such objects but that do not take
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.\" care of synchronizing access to them by default.
|
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.\" For example,
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.\" .I FILE
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.\" stream
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|
.\" .I unlocked
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|
.\" functions
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|
.\" .RB ( unlocked_stdio (3))
|
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.\" will be annotated,
|
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.\" but those that perform implicit locking on
|
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.\" .I FILE
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.\" streams by default will not,
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.\" even though the implicit locking may be disabled on a per-stream basis.
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.\"
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|
.\" In either case, we will not regard as MT-Unsafe functions that may
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.\" access user-supplied objects in unsafe ways should users fail to ensure
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.\" the accesses are well defined.
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.\" The notion prevails that users are expected to safeguard against
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.\" data races any user-supplied objects that the library accesses
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.\" on their behalf.
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.\"
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.\" .\" The above describes @mtsrace; @mtasurace is described below.
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.\"
|
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.\" This user responsibility does not apply, however,
|
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.\" to objects controlled by the library itself,
|
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.\" such as internal objects and static buffers used
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.\" to return values from certain calls.
|
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.\" When the library doesn't guard them against concurrent uses,
|
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.\" these cases are regarded as MT-Unsafe and AS-Unsafe (although the
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.\" .I race
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.\" mark under AS-Unsafe will be omitted
|
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.\" as redundant with the one under MT-Unsafe).
|
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.\" As in the case of user-exposed objects,
|
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.\" the mark may be followed by a colon and an identifier.
|
|
.\" The identifier groups all functions that operate on a
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.\" certain unguarded object; users may avoid the MT-Safety issues related
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.\" with unguarded concurrent access to such internal objects by creating a
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.\" non-recursive mutex related with the identifier,
|
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.\" and always holding the mutex when calling any function marked
|
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.\" as racy on that identifier,
|
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.\" as they would have to should the identifier be
|
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.\" an object under user control.
|
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.\" The non-recursive mutex avoids the MT-Safety issue,
|
|
.\" but it trades one AS-Safety issue for another,
|
|
.\" so use in asynchronous signals remains undefined.
|
|
.\"
|
|
.\" When the identifier relates to a static buffer used to hold return values,
|
|
.\" the mutex must be held for as long as the buffer remains in use
|
|
.\" by the caller.
|
|
.\" Many functions that return pointers to static buffers offer reentrant
|
|
.\" variants that store return values in caller-supplied buffers instead.
|
|
.\" In some cases, such as
|
|
.\" .BR tmpname (3),
|
|
.\" the variant is chosen not by calling an alternate entry point,
|
|
.\" but by passing a non-NULL pointer to the buffer in which the
|
|
.\" returned values are to be stored.
|
|
.\" These variants are generally preferable in multi-threaded programs,
|
|
.\" although some of them are not MT-Safe because of other internal buffers,
|
|
.\" also documented with
|
|
.\" .I race
|
|
.\" notes.
|
|
.TP
|
|
.I const
|
|
Functions marked with
|
|
.I 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
|
|
.IR race ,
|
|
which causes both readers and
|
|
writers of internal objects to be regarded as MT-Unsafe, \" and AS-Unsafe,
|
|
this mark is applied to writers only.
|
|
Writers remain \" equally
|
|
MT-Unsafe \" and AS-Unsafe
|
|
to call,
|
|
but the then-mandatory constness of objects they
|
|
modify enables readers to be regarded as MT-Safe \" and AS-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.
|
|
.IP
|
|
The identifier that follows the
|
|
.I 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
|
|
.I all
|
|
calls to functions marked with
|
|
.I const
|
|
followed by the identifier with a write lock, and
|
|
.I all
|
|
calls to functions marked with the identifier
|
|
by itself with a read lock.
|
|
.\" The non-recursive locking removes the MT-Safety problem,
|
|
.\" but it trades one AS-Safety problem for another,
|
|
.\" so use in asynchronous signals remains undefined.
|
|
.\"
|
|
.\" .\" But what if, instead of marking modifiers with const:id and readers
|
|
.\" .\" with just id, we marked writers with race:id and readers with ro:id?
|
|
.\" .\" Instead of having to define each instance of 'id', we'd have a
|
|
.\" .\" general pattern governing all such 'id's, wherein race:id would
|
|
.\" .\" suggest the need for an exclusive/write lock to make the function
|
|
.\" .\" safe, whereas ro:id would indicate 'id' is expected to be read-only,
|
|
.\" .\" but if any modifiers are called (while holding an exclusive lock),
|
|
.\" .\" then ro:id-marked functions ought to be guarded with a read lock for
|
|
.\" .\" safe operation. ro:env or ro:locale, for example, seems to convey
|
|
.\" .\" more clearly the expectations and the meaning, than just env or
|
|
.\" .\" locale.
|
|
.TP
|
|
.I sig
|
|
Functions marked with
|
|
.I sig
|
|
as a MT-Safety issue
|
|
.\" (that implies an identical AS-Safety issue, omitted for brevity)
|
|
may temporarily install a signal handler for internal purposes,
|
|
which may interfere with other uses of the signal,
|
|
identified after a colon.
|
|
.IP
|
|
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.
|
|
.\"
|
|
.\" There is no safe way to guarantee the original signal handler is
|
|
.\" restored in case of asynchronous cancellation,
|
|
.\" therefore so-marked functions are also AC-Unsafe.
|
|
.\"
|
|
.\" .\" fixme: at least deferred cancellation should get it right, and would
|
|
.\" .\" obviate the restoring bit below, and the qualifier above.
|
|
.\"
|
|
.\" Besides the measures recommended to work around the
|
|
.\" MT-Safety and AS-Safety problem,
|
|
.\" in order to avert the cancellation problem,
|
|
.\" disabling asynchronous cancellation
|
|
.\" .I and
|
|
.\" installing a cleanup handler to restore the signal to the desired state
|
|
.\" and to release the mutex are recommended.
|
|
.TP
|
|
.I term
|
|
Functions marked with
|
|
.I term
|
|
as an MT-Safety issue may change the
|
|
terminal settings in the recommended way, namely: call
|
|
.BR tcgetattr (3),
|
|
modify some flags, and then call
|
|
.BR tcsetattr (3),
|
|
this creates a window in which changes made by other threads are lost.
|
|
Thus, functions marked with
|
|
.I term
|
|
are MT-Unsafe.
|
|
.\" The same window enables changes made by asynchronous signals to be lost.
|
|
.\" These functions are also AS-Unsafe,
|
|
.\" but the corresponding mark is omitted as redundant.
|
|
.IP
|
|
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
|
|
.IR race:tcattr(fd) ,
|
|
where
|
|
.I 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.
|
|
.\"
|
|
.\" Functions marked with
|
|
.\" .I term
|
|
.\" as an AC-Safety issue are supposed to
|
|
.\" restore terminal settings to their original state,
|
|
.\" after temporarily changing them, but they may fail to do so if canceled.
|
|
.\"
|
|
.\" .\" fixme: at least deferred cancellation should get it right, and would
|
|
.\" .\" obviate the restoring bit below, and the qualifier above.
|
|
.\"
|
|
.\" Besides the measures recommended to work around the
|
|
.\" MT-Safety and AS-Safety problem,
|
|
.\" in order to avert the cancellation problem,
|
|
.\" disabling asynchronous cancellation
|
|
.\" .I and
|
|
.\" installing a cleanup handler to
|
|
.\" restore the terminal settings to the original state and to release the
|
|
.\" mutex are recommended.
|
|
.\"
|
|
.\"
|
|
.SS 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:
|
|
.TP
|
|
.I locale
|
|
Functions annotated with
|
|
.I locale
|
|
as an MT-Safety issue read from
|
|
the locale object without any form of synchronization.
|
|
Functions
|
|
annotated with
|
|
.I 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.
|
|
.IP
|
|
We do not mark these functions as MT-Unsafe, \" or AS-Unsafe,
|
|
however,
|
|
because functions that modify the locale object are marked with
|
|
.I 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.
|
|
.\" Should the locking strategy suggested under @code{const} be used,
|
|
.\" failure to guard locale uses is not as fatal as data races in
|
|
.\" general: unguarded uses will @emph{not} follow dangling pointers or
|
|
.\" access uninitialized, unmapped or recycled memory. Each access will
|
|
.\" read from a consistent locale object that is or was active at some
|
|
.\" point during its execution. Without synchronization, however, it
|
|
.\" cannot even be assumed that, after a change in locale, earlier
|
|
.\" locales will no longer be used, even after the newly-chosen one is
|
|
.\" used in the thread. Nevertheless, even though unguarded reads from
|
|
.\" the locale will not violate type safety, functions that access the
|
|
.\" locale multiple times may invoke all sorts of undefined behavior
|
|
.\" because of the unexpected locale changes.
|
|
.TP
|
|
.I env
|
|
Functions marked with
|
|
.I env
|
|
as an MT-Safety issue access the
|
|
environment with
|
|
.BR getenv (3)
|
|
or similar, without any guards to ensure
|
|
safety in the presence of concurrent modifications.
|
|
.IP
|
|
We do not mark these functions as MT-Unsafe, \" or AS-Unsafe,
|
|
however,
|
|
because functions that modify the environment are all marked with
|
|
.I 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.
|
|
.TP
|
|
.I hostid
|
|
The function marked with
|
|
.I 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
|
|
.RB ( gethostid (3))
|
|
is regarded as safe,
|
|
whereas the function that modifies it
|
|
.RB ( sethostid (3))
|
|
is marked with
|
|
.IR 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.
|
|
.TP
|
|
.I sigintr
|
|
Functions marked with
|
|
.I sigintr
|
|
as an MT-Safety issue access the
|
|
GNU C Library
|
|
.I _sigintr
|
|
internal data structure without any guards to ensure
|
|
safety in the presence of concurrent modifications.
|
|
.IP
|
|
We do not mark these functions as MT-Unsafe, \" or AS-Unsafe,
|
|
however,
|
|
because functions that modify this data structure are all marked with
|
|
.I 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.
|
|
.\" .TP
|
|
.\" .I fd
|
|
.\" Functions annotated with
|
|
.\" .I fd
|
|
.\" as an AC-Safety issue may leak file
|
|
.\" descriptors if asynchronous thread cancellation interrupts their
|
|
.\" execution.
|
|
.\"
|
|
.\" Functions that allocate or deallocate file descriptors will generally be
|
|
.\" marked as such.
|
|
.\" Even if they attempted to protect the file descriptor
|
|
.\" allocation and deallocation with cleanup regions,
|
|
.\" allocating a new descriptor and storing its number where the cleanup region
|
|
.\" could release it cannot be performed as a single atomic operation.
|
|
.\" Similarly,
|
|
.\" releasing the descriptor and taking it out of the data structure
|
|
.\" normally responsible for releasing it cannot be performed atomically.
|
|
.\" There will always be a window in which the descriptor cannot be released
|
|
.\" because it was not stored in the cleanup handler argument yet,
|
|
.\" or it was already taken out before releasing it.
|
|
.\" .\" It cannot be taken out after release:
|
|
.\" an open descriptor could mean either that the descriptor still
|
|
.\" has to be closed,
|
|
.\" or that it already did so but the descriptor was
|
|
.\" reallocated by another thread or signal handler.
|
|
.\"
|
|
.\" Such leaks could be internally avoided, with some performance penalty,
|
|
.\" by temporarily disabling asynchronous thread cancellation.
|
|
.\" However,
|
|
.\" since callers of allocation or deallocation functions would have to do
|
|
.\" this themselves, to avoid the same sort of leak in their own layer,
|
|
.\" it makes more sense for the library to assume they are taking care of it
|
|
.\" than to impose a performance penalty that is redundant when the problem
|
|
.\" is solved in upper layers, and insufficient when it is not.
|
|
.\"
|
|
.\" This remark by itself does not cause a function to be regarded as
|
|
.\" AC-Unsafe.
|
|
.\" However, cumulative effects of such leaks may pose a
|
|
.\" problem for some programs.
|
|
.\" If this is the case,
|
|
.\" suspending asynchronous cancellation for the duration of calls
|
|
.\" to such functions is recommended.
|
|
.\" .TP
|
|
.\" .I mem
|
|
.\" Functions annotated with
|
|
.\" .I mem
|
|
.\" as an AC-Safety issue may leak
|
|
.\" memory if asynchronous thread cancellation interrupts their execution.
|
|
.\"
|
|
.\" The problem is similar to that of file descriptors: there is no atomic
|
|
.\" interface to allocate memory and store its address in the argument to a
|
|
.\" cleanup handler,
|
|
.\" or to release it and remove its address from that argument,
|
|
.\" without at least temporarily disabling asynchronous cancellation,
|
|
.\" which these functions do not do.
|
|
.\"
|
|
.\" This remark does not by itself cause a function to be regarded as
|
|
.\" generally AC-Unsafe.
|
|
.\" However, cumulative effects of such leaks may be
|
|
.\" severe enough for some programs that disabling asynchronous cancellation
|
|
.\" for the duration of calls to such functions may be required.
|
|
.TP
|
|
.I cwd
|
|
Functions marked with
|
|
.I 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.
|
|
.IP
|
|
This is not enough of a reason to mark so-marked functions as MT-Unsafe,
|
|
.\" or AS-Unsafe,
|
|
but when this behavior is optional (e.g.,
|
|
.BR nftw (3)
|
|
with
|
|
.BR FTW_CHDIR ),
|
|
avoiding the option may be a good alternative to
|
|
using full pathnames or file descriptor-relative (e.g.,
|
|
.BR openat (2))
|
|
system calls.
|
|
.\" .TP
|
|
.\" .I !posix
|
|
.\" This remark, as an MT-Safety, AS-Safety or AC-Safety
|
|
.\" note to a function,
|
|
.\" indicates the safety status of the function is known to differ
|
|
.\" from the specified status in the POSIX standard.
|
|
.\" For example, POSIX does not require a function to be Safe,
|
|
.\" but our implementation is, or vice-versa.
|
|
.\"
|
|
.\" For the time being, the absence of this remark does not imply the safety
|
|
.\" properties we documented are identical to those mandated by POSIX for
|
|
.\" the corresponding functions.
|
|
.TP
|
|
.I :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
|
|
.I race
|
|
and
|
|
.IR const ,
|
|
or to provide more specific information,
|
|
such as naming a signal in a function marked with
|
|
.IR sig .
|
|
It is envisioned that it may be applied to
|
|
.I lock
|
|
and
|
|
.I corrupt
|
|
as well in the future.
|
|
.IP
|
|
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,
|
|
.I :buf(arg)
|
|
to denote a buffer associated with the argument
|
|
.IR arg ,
|
|
or
|
|
.I :tcattr(fd)
|
|
to denote the terminal attributes of a file descriptor
|
|
.IR fd .
|
|
.IP
|
|
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.
|
|
.TP
|
|
.I /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.
|
|
.\" Such conditions as
|
|
.\" .I /hurd
|
|
.\" or
|
|
.\" .I /!linux!bsd
|
|
.\" indicate the preceding marker only
|
|
.\" applies when the underlying kernel is the HURD,
|
|
.\" or when it is neither Linux nor a BSD kernel, respectively.
|
|
For example,
|
|
.I /!ps
|
|
and
|
|
.I /one_per_line
|
|
indicate the preceding marker only applies when argument
|
|
.I ps
|
|
is NULL, or global variable
|
|
.I one_per_line
|
|
is nonzero.
|
|
.IP
|
|
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.
|
|
.SH SEE ALSO
|
|
.BR pthreads (7)
|