C library functions that are not thread-safe
4.60 C library functions that are not thread-safe
The following table shows the C library functions that are not thread-safe.
Table 4-2 Functions that are not thread-safe
asctime(), localtime(), strtok()
These functions are all
thread-unsafe. Each contains a static buffer that might be overwritten
by another thread between a call to the function and the subsequent
use of its return value.
supplies reentrant versions,
_strtok_r(). ARM recommends that you use these functions instead to
NoteThese reentrant versions take additional parameters.
_asctime_r() takes an additional parameter that is a
pointer to a buffer that the output string is written into.
_localtime_r() takes an additional parameter that is a
pointer to a
struct tm, that the result is
_strtok_r() takes an additional
parameter that is a pointer to a
char pointer to the next token.
Do not call
a multithreaded program even if you have provided an implementation
of the underlying
_sys_exit() that actually
terminates all threads.
In this case,
up before calling
disrupts other threads.
gamma(), lgamma(), lgammaf(), lgammal() a
|These extended mathlib functions
use a global variable, _signgam, so are not thread-safe.
mbrlen(), mbsrtowcs(), mbrtowc(), wcrtomb(), wcsrtombs()
The C90 multibyte conversion functions (defined in stdlib.h) are not thread-safe, for example
mbtowc(), because they
contain internal static state that is shared between all threads without
However, the extended restartable
versions (defined in wchar.h) are thread-safe,
provided you pass in a pointer to your own mbstate_t object.
You must exclusively use these functions with non-NULL mbstate_t
* parameters if you want to ensure thread-safety when
handling multibyte strings.
These functions keep internal
state that is both global and unprotected. This means that calls
rand() are never thread-safe.
recommends that you do one of the following:
Use the reentrant versions
_srand_r() supplied by
ARM. These use user-provided buffers instead of static data within the C
Use your own locking to ensure that only one thread
rand() at a time, for example, by
$Sub$$rand() if you want to avoid
changing your code.
Arrange that only one thread ever needs to generate
Supply your own random number generator that can
have multiple independent instances.
_srand_r() both take an additional parameter that
is a pointer to a buffer storing the state of the random number generator.
used for setting and reading locale settings. The locale settings
are global across all threads, and are not protected by a lock.
If two threads call
setlocale() to simultaneously
modify the locale settings, or if one thread reads the settings
while another thread is modifying them, data corruption might occur.
Also, many other functions, for example
read the current locale settings. Therefore, if one thread calls
with another thread calling such a function, there might be unexpected
Multiple threads reading the
settings simultaneously is thread-safe in simple cases and if no
other thread is simultaneously modifying those settings, but where internally
an intermediate buffer is required for more complicated returned
results, unexpected results can occur unless you use a reentrant
recommends that you either:
Choose the locale
you want and call
setlocale() once to initialize
it. Do this before creating any additional threads in your program
so that any number of threads can read the locale settings concurrently
without interfering with one another.
Use the reentrant version
supplied by ARM. This returns a string that is either a pointer to a
constant string, or a pointer to a string stored in a user-supplied buffer
that can be used for thread-local storage, rather than using memory within the
C library. The buffer must be at least
_SETLOCALE_R_BUFSIZE bytes long, including space for a trailing
Be aware that
_setlocale_r() is not fully thread-safe when accessed concurrently
to change locale settings. This access is not lock-protected.
localeconv() is not thread-safe.
Call the ARM function
_get_lconv() with a pointer
to a user-supplied buffer instead.