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.th signal-safety 7 2021-03-22 "linux" "linux programmer's manual"
.sh name
signal-safety \- async-signal-safe functions
.sh description
an
.i async-signal-safe
function is one that can be safely called from within a signal handler.
many functions are
.i not
async-signal-safe.
in particular,
nonreentrant functions are generally unsafe to call from a signal handler.
.pp
the kinds of issues that render a function
unsafe can be quickly understood when one considers
the implementation of the
.i stdio
library, all of whose functions are not async-signal-safe.
.pp
when performing buffered i/o on a file, the
.i 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
.i stdio
function such as
.br 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
.br printf (3),
then the second call to
.br printf (3)
will operate on inconsistent data, with unpredictable results.
.pp
to avoid problems with unsafe functions, there are two possible choices:
.ip 1. 3
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.
.ip 2.
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 handler.
.pp
generally, the second choice is difficult in programs of any complexity,
so the first choice is taken.
.pp
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.)
.pp
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 handler).
.pp
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 standards.
.pp
.ts
lb lb
l l.
function notes
\fbabort\fp(3) added in posix.1-2001 tc1
\fbaccept\fp(2)
\fbaccess\fp(2)
\fbaio_error\fp(3)
\fbaio_return\fp(3)
\fbaio_suspend\fp(3) see notes below
\fbalarm\fp(2)
\fbbind\fp(2)
\fbcfgetispeed\fp(3)
\fbcfgetospeed\fp(3)
\fbcfsetispeed\fp(3)
\fbcfsetospeed\fp(3)
\fbchdir\fp(2)
\fbchmod\fp(2)
\fbchown\fp(2)
\fbclock_gettime\fp(2)
\fbclose\fp(2)
\fbconnect\fp(2)
\fbcreat\fp(2)
\fbdup\fp(2)
\fbdup2\fp(2)