|
NAME | DESCRIPTION | SEE ALSO | COLOPHON |
ATTRIBUTES(7) Linux Programmer's Manual ATTRIBUTES(7)
attributes - POSIX safety concepts
Note: the text of this man page is based on the material taken from
the "POSIX Safety Concepts" section of the GNU C Library manual.
Further details on the topics described here can be found in that
manual.
Various function manual pages include a section ATTRIBUTES that
describes the safety of calling the function in various contexts.
This section annotates functions with the following safety markings:
MT-Safe
MT-Safe or Thread-Safe functions are safe to call in the
presence of other threads. MT, in MT-Safe, stands for Multi
Thread.
Being MT-Safe does not imply a function is atomic, nor that it
uses any of the memory synchronization mechanisms POSIX
exposes to users. It is even possible that calling MT-Safe
functions in sequence does not yield an MT-Safe combination.
For example, having a thread call two MT-Safe functions one
right after the other does not guarantee behavior equivalent
to atomic execution of a combination of both functions, since
concurrent calls in other threads may interfere in a
destructive way.
Whole-program optimizations that could inline functions across
library interfaces may expose unsafe reordering, and so
performing inlining across the GNU C Library interface is not
recommended. The documented MT-Safety status is not
guaranteed under whole-program optimization. However,
functions defined in user-visible headers are designed to be
safe for inlining.
MT-Unsafe
MT-Unsafe functions are not safe to call in a multithreaded
programs.
Other keywords that appear in safety notes are defined in subsequent
sections.
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.
pthreads(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2015-03-02 ATTRIBUTES(7)
Pages that refer to this page: adjtimex(2), clock_getres(2), eventfd(2), getrlimit(2), getrusage(2), mmap(2), sigaltstack(2), utimensat(2), a64l(3), abort(3), abs(3), acos(3), acosh(3), addseverity(3), adjtime(3), aio_cancel(3), aio_error(3), aio_fsync(3), aio_read(3), aio_return(3), aio_suspend(3), aio_write(3), alloca(3), argz_add(3), asin(3), asinh(3), asprintf(3), assert(3), assert_perror(3), atan2(3), atan(3), atanh(3), atexit(3), atof(3), atoi(3), backtrace(3), basename(3), bcmp(3), bcopy(3), bindresvport(3), bsd_signal(3), bsearch(3), btowc(3), byteorder(3), bzero(3), cabs(3), cacos(3), cacosh(3), canonicalize_file_name(3), carg(3), casin(3), casinh(3), catan(3), catanh(3), catgets(3), catopen(3), cbrt(3), ccos(3), ceil(3), cexp(3), cfree(3), cimag(3), clearenv(3), clock(3), clock_getcpuclockid(3), clog10(3), clog(3), closedir(3), confstr(3), conj(3), copysign(3), cos(3), cosh(3), cpow(3), cproj(3), creal(3), crypt(3), csin(3), csinh(3), csqrt(3), ctan(3), ctanh(3), ctermid(3), ctime(3), daemon(3), des_crypt(3), difftime(3), dirfd(3), div(3), dladdr(3), dlerror(3), dlinfo(3), dl_iterate_phdr(3), dlopen(3), dlsym(3), drand48(3), drand48_r(3), dysize(3), ecvt(3), ecvt_r(3), encrypt(3), envz_add(3), erf(3), erfc(3), err(3), error(3), ether_aton(3), euidaccess(3), exec(3), exit(3), exp10(3), exp2(3), exp(3), expm1(3), fabs(3), fclose(3), fcloseall(3), fdim(3), fenv(3), ferror(3), fexecve(3), fflush(3), ffs(3), fgetc(3), fgetgrent(3), fgetwc(3), fgetws(3), finite(3), flockfile(3), floor(3), fma(3), fmax(3), fmemopen(3), fmin(3), fmod(3), fmtmsg(3), fnmatch(3), fopen(3), fopencookie(3), fpathconf(3), fpclassify(3), fpurge(3), fputwc(3), fputws(3), fread(3), frexp(3), fseek(3), fseeko(3), ftime(3), ftok(3), fts(3), ftw(3), futimes(3), gamma(3), gcvt(3), getaddrinfo(3), getaddrinfo_a(3), getauxval(3), getcontext(3), getcwd(3), getdate(3), getdirentries(3), getdtablesize(3), getenv(3), getfsent(3), getgrent(3), getgrent_r(3), getgrnam(3), getgrouplist(3), gethostbyname(3), gethostid(3), getifaddrs(3), getline(3), getloadavg(3), getlogin(3), getmntent(3), getnameinfo(3), getnetent(3), getnetent_r(3), get_nprocs_conf(3), getopt(3), getpass(3), getprotoent(3), getprotoent_r(3), getpt(3), getpw(3), getpwent(3), getpwent_r(3), getpwnam(3), getrpcent(3), getrpcent_r(3), getrpcport(3), gets(3), getservent(3), getservent_r(3), getspnam(3), getsubopt(3), getttyent(3), getusershell(3), getutent(3), getutmp(3), getw(3), getwchar(3), glob(3), gnu_get_libc_version(3), grantpt(3), gsignal(3), hsearch(3), hypot(3), iconv(3), iconv_close(3), iconv_open(3), if_nameindex(3), if_nametoindex(3), ilogb(3), index(3), inet(3), inet_ntop(3), inet_pton(3), initgroups(3), insque(3), isalpha(3), isatty(3), isgreater(3), iswalnum(3), iswalpha(3), iswblank(3), iswcntrl(3), iswctype(3), iswdigit(3), iswgraph(3), iswlower(3), iswprint(3), iswpunct(3), iswspace(3), iswupper(3), iswxdigit(3), j0(3), key_setsecret(3), ldexp(3), lio_listio(3), localeconv(3), lockf(3), log10(3), log1p(3), log2(3), log(3), logb(3), login(3), lrint(3), lround(3), lsearch(3), lseek64(3), makecontext(3), makedev(3), mallinfo(3), malloc(3), malloc_get_state(3), malloc_info(3), malloc_stats(3), malloc_trim(3), malloc_usable_size(3), matherr(3), mblen(3), mbrlen(3), mbrtowc(3), mbsinit(3), mbsnrtowcs(3), mbsrtowcs(3), mbstowcs(3), mbtowc(3), mcheck(3), memccpy(3), memchr(3), memcmp(3), memcpy(3), memfrob(3), memmem(3), memmove(3), mempcpy(3), memset(3), mkdtemp(3), mkfifo(3), mkstemp(3), mktemp(3), modf(3), mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mtrace(3), nan(3), nextafter(3), nextup(3), nl_langinfo(3), ntp_gettime(3), on_exit(3), opendir(3), open_memstream(3), openpty(3), perror(3), popen(3), posix_fallocate(3), posix_memalign(3), posix_openpt(3), pow10(3), pow(3), __ppc_set_ppr_med(3), __ppc_yield(3), printf(3), profil(3), psignal(3), pthread_attr_init(3), pthread_attr_setaffinity_np(3), pthread_attr_setdetachstate(3), pthread_attr_setguardsize(3), pthread_attr_setinheritsched(3), pthread_attr_setschedparam(3), pthread_attr_setschedpolicy(3), pthread_attr_setscope(3), pthread_attr_setstack(3), pthread_attr_setstackaddr(3), pthread_attr_setstacksize(3), pthread_cancel(3), pthread_cleanup_push(3), pthread_create(3), pthread_detach(3), pthread_equal(3), pthread_exit(3), pthread_getattr_default_np(3), pthread_getattr_np(3), pthread_getcpuclockid(3), pthread_join(3), pthread_kill(3), pthread_kill_other_threads_np(3), pthread_self(3), pthread_setaffinity_np(3), pthread_setcancelstate(3), pthread_setconcurrency(3), pthread_setname_np(3), pthread_setschedparam(3), pthread_setschedprio(3), pthread_sigmask(3), pthread_sigqueue(3), pthread_testcancel(3), pthread_tryjoin_np(3), pthread_yield(3), ptsname(3), putenv(3), putgrent(3), putpwent(3), puts(3), putwchar(3), qecvt(3), qsort(3), raise(3), rand(3), random(3), random_r(3), rcmd(3), readdir(3), readdir_r(3), realpath(3), re_comp(3), regex(3), remainder(3), remove(3), remquo(3), resolver(3), rewinddir(3), rexec(3), rint(3), round(3), rpc(3), rpmatch(3), rtime(3), scalb(3), scalbln(3), scandir(3), scanf(3), sched_getcpu(3), seekdir(3), sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), setaliasent(3), setbuf(3), setenv(3), setjmp(3), setlocale(3), setlogmask(3), setnetgrent(3), shm_open(3), siginterrupt(3), signbit(3), significand(3), sigpause(3), sigqueue(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3), sin(3), sincos(3), sinh(3), sleep(3), sockatmark(3), sqrt(3), statvfs(3), stdarg(3), stdio_ext(3), stpcpy(3), stpncpy(3), strcasecmp(3), strcat(3), strchr(3), strcmp(3), strcoll(3), strcpy(3), strdup(3), strerror(3), strfmon(3), strfromd(3), strfry(3), strftime(3), strlen(3), strnlen(3), strpbrk(3), strptime(3), strsep(3), strsignal(3), strspn(3), strstr(3), strtod(3), strtoimax(3), strtok(3), strtol(3), strtoul(3), strverscmp(3), strxfrm(3), swab(3), sysconf(3), syslog(3), system(3), sysv_signal(3), tan(3), tanh(3), tcgetpgrp(3), tcgetsid(3), telldir(3), tempnam(3), termios(3), tgamma(3), timegm(3), tmpfile(3), tmpnam(3), toascii(3), toupper(3), towctrans(3), towlower(3), towupper(3), trunc(3), tsearch(3), ttyname(3), ttyslot(3), tzset(3), ualarm(3), ulimit(3), ungetwc(3), unlocked_stdio(3), unlockpt(3), updwtmp(3), usleep(3), wcpcpy(3), wcpncpy(3), wcrtomb(3), wcscasecmp(3), wcscat(3), wcschr(3), wcscmp(3), wcscpy(3), wcscspn(3), wcsdup(3), wcslen(3), wcsncasecmp(3), wcsncat(3), wcsncmp(3), wcsncpy(3), wcsnlen(3), wcsnrtombs(3), wcspbrk(3), wcsrchr(3), wcsrtombs(3), wcsspn(3), wcsstr(3), wcstoimax(3), wcstok(3), wcstombs(3), wcswidth(3), wctob(3), wctomb(3), wctrans(3), wctype(3), wcwidth(3), wmemchr(3), wmemcmp(3), wmemcpy(3), wmemmove(3), wmemset(3), wordexp(3), wprintf(3), xcrypt(3), xdr(3), y0(3), man-pages(7), pthreads(7), standards(7)