Monday, October 25, 2010

Signal handler is not your regular function

Signal handler is a function which will be called when the program receives a signal. Most of the signals have their own default handlers and for few the parent program should handle/ignore this signal explicitly. In case of SIGCHLD, if the parent process should define the handler explicitly and wait for the child to exit (we can use SIGIGN too and no need to wait),otherwise the child process will become zombie.

Signal handler can not do everything what any normal function can do. It can call the functions and system calls which are async-safe.
This is the list of async-safe fuctions and system calls,

_Exit(), _exit(), abort(), accept(), access(), aio_error(), aio_return(), aio_suspend(), alarm(), bind(), cfgetispeed(), cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(), chmod(), chown(), clock_gettime(), close(), connect(), creat(), dup(), dup2(), execle(), execve(), fchmod(), fchown(), fcntl(), fdatasync(), fork(), fpathconf(), fstat(), fsync(), ftruncate(), getegid(), geteuid(), getgid(), getgroups(), getpeername(), getpgrp(), getpid(), getppid(), getsockname(), getsockopt(), getuid(), kill(), link(), listen(), lseek(), lstat(), mkdir(), mkfifo(), open(), pathconf(), pause(), pipe(), poll(), posix_trace_event(), pselect(), raise(), read(), readlink(), recv(), recvfrom(), recvmsg(), rename(), rmdir(), select(), sem_post(), send(), sendmsg(), sendto(), setgid(), setpgid(), setsid(), setsockopt(), setuid(), shutdown(), sigaction(), sigaddset(), sigdelset(), sigemptyset(), sigfillset(), sigismember(), sleep(), signal(), sigpause(), sigpending(), sigprocmask(), sigqueue(), sigset(), sigsuspend(), sockatmark(), socket(), socketpair(), stat(), symlink(), sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), timer_getoverrun(), timer_gettime(), timer_settime(), times(), umask(), uname(), unlink(), utime(), wait(), waitpid(), and write().

Ref : http://beej.us/guide/bgipc/output/html/singlepage/bgipc.html#signals