signal-safety - async-signal-safe functions
function is one that can be safely called from
within a signal handler. Many functions are not
particular, nonreentrant functions are generally unsafe to call from a signal
The kinds of issues that render a function unsafe can be quickly understood when
one considers the implementation of the stdio
library, all of whose
functions are not async-signal-safe.
When performing buffered I/O on a file, the stdio
functions must maintain
a statically allocated data buffer along with associated counters and indexes
(or pointers) that record the amount of data and the current position in the
buffer. Suppose that the main program is in the middle of a call to a
function such as printf(3)
where the buffer and associated
variables have been partially updated. If, at that moment, the program is
interrupted by a signal handler that also calls printf(3)
, then the
second call to printf(3)
will operate on inconsistent data, with
To avoid problems with unsafe functions, there are two possible choices:
- Ensure that (a) the signal handler calls only async-signal-safe functions,
and (b) the signal handler itself is reentrant with respect to global
variables in the main program.
- Block signal delivery in the main program when calling functions that are
unsafe or operating on global data that is also accessed by the signal
Generally, the second choice is difficult in programs of any complexity, so the
first choice is taken.
POSIX.1 specifies a set of functions that an implementation must make
async-signal-safe. (An implementation may provide safe implementations of
additional functions, but this is not required by the standard and other
implementations may not provide the same guarantees.) In general, a function
is async-signal-safe either because it is reentrant or because it is atomic
with respect to signals (i.e., its execution can't be interrupted by a signal
The set of functions required to be async-signal-safe by POSIX.1 is shown in the
following table. The functions not otherwise noted were required to be
async-signal-safe in POSIX.1-2001; the table details changes in the subsequent
- POSIX.1-2001 and POSIX.1-2004 required the functions fpathconf(3),
pathconf(3), and sysconf(3) to be async-signal-safe, but
this requirement was removed in POSIX.1-2008.
- If a signal handler interrupts the execution of an unsafe function, and
the handler terminates via a call to longjmp(3) or
siglongjmp(3) and the program subsequently calls an unsafe
function, then the behavior of the program is undefined.
- POSIX.1-2003 clarified that if an application calls fork(2) from a
signal handler and any of the fork handlers registered by
pthread_atfork(3) calls a function that is not async-signal-safe,
the behavior is undefined. A future revision of the standard is likely to
remove fork(2) from the list of async-signal-safe functions.
Deviations in the GNU C library¶
The following known deviations from the standard occur in the GNU C library:
- Before glibc 2.24, execl(3) and execle(3) employed
realloc(3) internally and were consequently not async-signal-safe.
This was fixed in glibc 2.24.
- The glibc implementation of aio_suspend(3) is not async-signal-safe
because it uses pthread_mutex_lock(3) internally.
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