SEI CERT C Coding Standard

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Implementation Details

POSIX

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.

Asynchronous-Signal-Safe Functions Functions 

_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.

Subclause 7.14.1.1, paragraph 4, of the C Standard [ISO/IEC 9899:2011] states:

  If the signal occurs as the result of calling the abort or raise functionthe abort or raise function, the signal handler shall not call the raise functionthe raise function.

 However, in the description of signal(), POSIX states:

This restriction does not apply to POSIX applications, as POSIX.1-2008 requires raise() to be async-signal-safe

See also undefined behavior 131. 

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The OpenBSD signal() manual page lists a few additional functions that are asynchronous-safe in OpenBSD but "probably not on other systems," including snprintf(), vsnprintf(), and syslog_r() (but only when the syslog_data struct is initialized as a local variable).

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