(libc.info.gz) Other Safety Remarks
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(libc.info.gz) Conditionally Safe Features
(libc.info.gz) POSIX
1.2.2.4 Other Safety Remarks
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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- or AS-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' or similar, without any guards to ensure
safety in the presence of concurrent modifications.
We do not mark these functions as MT- or AS-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') is
regarded as safe, whereas the function that modifies it
('sethostid') 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
'_sigintr' internal data structure without any guards to ensure
safety in the presence of concurrent modifications.
We do not mark these functions as MT- or AS-Unsafe, however,
because functions that modify the 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.
* 'fd'
Functions annotated with '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.
* 'mem'
Functions annotated with '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.
* '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-
or AS-Unsafe, but when this behavior is optional (e.g., 'nftw' with
'FTW_CHDIR'), avoiding the option may be a good alternative to
using full pathnames or file descriptor-relative (e.g. 'openat')
system calls.
* '!posix'
This remark, as an MT-, AS- 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.
* ':identifier'
Annotations may sometimes be followed by identifiers, intended to
group several functions that e.g. 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 e.g. ':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 evaluates to true. Such
conditions as '/hurd' or '/!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. '/!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.
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(libc.info.gz) Conditionally Safe Features
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