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In this noncompliant example, the C standard library functions fprintffputs() and free() are called from the signal handler via the function log_message(). Neither function is asynchronous-safe.

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The following table from the POSIX standard [IEEE Std 1003.1:2013] defines a set of functions that are asynchronous-signal-safe. Applications may invoke these functions, without restriction, from a signal handler.

`_Exit()`	`fexecve()`	`posix_trace_event()`	`sigprocmask()`
`_exit()`	`fork()`	`pselect()`	`sigqueue()`
`abort()`	`fstat()`	`pthread_kill()`	`sigset()`
`accept()`	`fstatat()`	`pthread_self()`	`sigsuspend()`
`access()`	`fsync()`	`pthread_sigmask()`	`sleep()`
`aio_error()`	`ftruncate()`	`raise()`	`sockatmark()`
`aio_return()`	`futimens()`	`read()`	`socket()`
`aio_suspend()`	`getegid()`	`readlink()`	`socketpair()`
`alarm()`	`geteuid()`	`readlinkat()`	`stat()`
`bind()`	`getgid()`	`recv()`	`symlink()`
`cfgetispeed()`	`getgroups()`	`recvfrom()`	`symlinkat()`
`cfgetospeed()`	`getpeername()`	`recvmsg()`	`tcdrain()`
`cfsetispeed()`	`getpgrp()`	`rename()`	`tcflow()`
`cfsetospeed()`	`getpid()`	`renameat()`	`tcflush()`
`chdir()`	`getppid()`	`rmdir()`	`tcgetattr()`
`chmod()`	`getsockname()`	`select()`	`tcgetpgrp()`
`chown()`	`getsockopt()`	`sem_post()`	`tcsendbreak()`
`clock_gettime()`	`getuid()`	`send()`	`tcsetattr()`
`close()`	`kill()`	`sendmsg()`	`tcsetpgrp()`
`connect()`	`link()`	`sendto()`	`time()`
`creat()`	`linkat()`	`setgid()`	`timer_getoverrun()`
`dup()`	`listen()`	`setpgid()`	`timer_gettime()`
`dup2()`	`lseek()`	`setsid()`	`timer_settime()`
`execl()`	`lstat()`	`setsockopt()`	`times()`
`execle()`	`mkdir()`	`setuid()`	`umask()`
`execv()`	`mkdirat()`	`shutdown()`	`uname()`
`execve()`	`mkfifo()`	`sigaction()`	`unlink()`
`faccessat()`	`mkfifoat()`	`sigaddset()`	`unlinkat()`
`fchdir()`	`mknod()`	`sigdelset()`	`utime()`
`fchmod()`	`mknodat()`	`sigemptyset()`	`utimensat()`
`fchmodat()`	`open()`	`sigfillset()`	`utimes()`
`fchown()`	`openat()`	`sigismember()`	`wait()`
`fchownat()`	`pause()`	`signal()`	`waitpid()`
`fcntl()`	`pipe()`	`sigpause()`	`write()`
`fdatasync()`	`poll()`	`sigpending()`

All functions not listed in this table are considered to be unsafe with respect to signals. In the presence of signals, all POSIX functions behave as defined when called from or interrupted by a signal handler, with a single exception: when a signal interrupts an unsafe function and the signal handler calls an unsafe function, the behavior is undefined.

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Invoking functions that are not asynchronous-safe from within a signal handler is undefined behavior.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
SIG30-C	High	Likely	Medium	P18	L1

Automated Detection

Tool

Version

Checker

Description

Astrée

Include Page

	Astrée_V
	Astrée_V

	Supported, but no explicit checker
Compass/ROSE

Can detect violations of the rule for single-file programs

LDRA tool suite

Include Page

	LDRA_V
	LDRA_V

88 D, 89 D

Partially implemented

Splint

Include Page

	Splint_V
	Splint_V

Related Vulnerabilities

For an overview of software vulnerabilities resulting from improper signal handling, see Michal Zalewski's paper "Delivering Signals for Fun and Profit" [Zalewski 2001].

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Key here (explains table format and definitions)

Taxonomy	Taxonomy item	Relationship
ISO/IEC TS 17961:2013	Calling functions in the C Standard Library other than `abort`, `_Exit`, and `signal` from within a signal handler [asyncsig]	Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11	CWE-479, Signal Handler Use of a Non-reentrant Function	2017-07-10: CERT: Exact

Bibliography

[C99 Rationale 2003]	Subclause 5.2.3, "Signals and Interrupts" Subclause 7.14.1.1, "The `signal` Function"
[Dowd 2006]	Chapter 13, "Synchronization and State"
[Greenman 1997]


[IEEE Std 1003.1:2013]	XSH, System Interfaces, `longjmp` XSH, System Interfaces, `raise`
[ISO/IEC 9899:2011]	7.14.1.1, "The `signal` Function"
[OpenBSD]	`signal()` Man Page
[VU #834865]


[Zalewski 2001]	"Delivering Signals for Fun and Profit"

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