Signals and Signal Handling

There have been four different implementations of signals in the history of UNIX. The Interix Software Development Kit (SDK) supports only the POSIX.1 set of signal semantics. However, the Interix SDK does support several different sets of signal-handling APIs, as follows :

• American National Standards Institute (ANSI) C signals are supported by the function signal() .Though this function behaves exactly as required by the ANSI C standard, that definition leaves windows of time in which signals may be lostor mishandled. The sigaction () API allows applications to close those windows.

• POSIX.1 signals are supported by the functions sigaction() , sigpending () , sigprocmask () , sigsuspend() , sigemptyset() , sigfillset() , sigaddset() , sigdelset() , and sigismember() .

• BSD 4.3 signals are supported by the functions killpg() , sigsetmask() , sigblock() , and sigvec() . The signal mask for these functions is int , not sigset_t .

  (Although sigpause() is provided by the Interix SDK, it is provided as the System V call, which does not behave in the same way as the BSD call.)

  If a future release of the Interix SDK supports more than 32 signals, these functions will become obsolete. Therefore, rather than depend on these functions, it is better to convert code to use the POSIX.1 signal calls.

• System V signals are supported with the functions sighold() , sigignore() , sigpause() , sigrelse() , and sigset() .

Table 10.7 lists the POSIX-supported signals, all of which are supported by Interix.

Table 10.7: POSIX-supported Signals

Signal Name	Description	Default Action/Effect	Number
SIGABRT	Abnormal termination	Terminate process	6
SIGALRM	Time-out alarm	Terminate process	14
SIGBUS	Bus error	Terminate process	10
SIGCHLD	Change in status of child	Ignore	18
SIGCONT	Continues stopped process	Ignore	25
SIGFPE	Floating-point exception	Terminate process	8
SIGHUP	Hang up	Terminate process	1
SIGILL	Illegal hardware instruction	Terminate process	4
SIGINT	Terminal interrupt character	Terminate process	2
SIGIO	I/O completion outstanding	Ignore	19
SIGKILL	Termination	Terminate process (cannot be caught or ignored)	9
SIGPIPE	Write to pipe with no readers	Terminate process	13
SIGPOLL	Pollable event (Sys V) - synonym of SIGIO	Ignore	22
SIGPROF	Profiling timer alarm	Terminate process	29
SIGQUIT	Terminal quit character	Terminate process	3
SIGSEGV	Invalid memory reference	Terminate process	11
SIGSTOP	Stop process	Stop process (cannot be caught or ignored)	23
SIGSYS	Invalid system call	Terminate process	12
SIGTERM	Software termination	Terminate process	15
SIGTRAP	Trace trap	Terminate process	5
SIGTSTP	Terminal stop character	Stop process	24
SIGTTIN	Background read from control TTY	Stop process	26
SIGTTOU	Background write to control TTY	Stop process	27
SIGURG	Urgent condition on socket	Ignore	21
SIGUSR1	User -defined signal	Terminate process	16
SIGUSR2	User-defined signal	Terminate process	17
SIGVTALRM	Virtual time alarm	Terminate process	28
SIGXCPU	CPU time limit exceeded	Terminate process	30
SIGXFSZ	File size limit exceeded	Terminate process	31

Interix supports most of the signal handling functions. However, Interix doesnot support some nonstandard platform-specific implementations, such as sigfpe , signal handling for specific SIGFPE codes. Table 10.8 shows platform-specific functions not supported by Interix and Interix substitutes, if they exist.

Table 10.8: Platform-Specific Signal Functions Not Supported by Interix

Function Name	Description	Suggested Interix Replacement
bsd_signal	Simplified signal facilities.	See sample code in UNIX bsd_signal Code Replacement, immediately following this table.
getcontext	Gets current user context.	No support or equivalent in Interix.
gsignal	Software signals.	No support or equivalent in Interix.
makecontext	Manipulates user contexts.	No support or equivalent in Interix.
psiginfo	Software signals.	No support or equivalent in Interix.
psignal	System signal messages.	No support or equivalent in Interix.
setcontext	Sets current user context.	No support or equivalent in Interix.
sig2str	Translates the signal number signum to the signal name.	Write a simple table lookup routine.(See Table 10.7.)
sigaltstack	Sets or gets signal alternative stack context.	No support or equivalent in Interix.
sigfpe	Handles signals for specific SIGFPE codes.	_controlfp ( ,_MCW_EM)
siggetmask	Gets the current set of masked signals.	Use sigblock(mask) or sigsetmask(mask) with mask set to zero (0).
siginterrupt	Allows signals to interrupt functions.	Controlled by the SA_RESTART flag passed to sigaction().
sigsend	Sends a signal to a process or a group of processes.	No support or equivalent in Interix.
sigsendset	Sends a signal to a process or a group of processes.	No support or equivalent in Interix.
sigstack	Sets and/or gets alternative signal stack context.	No support or equivalent in Interix.
ssignal	Software signals.	No support or equivalent in Interix.
str2sig	Translates the signal name str to a signal number.	Write a simple table lookup routine.(See Table 10.7.)
swapcontext	Manipulates user contexts.	No support or equivalent in Interix.
sys_siglist	System signal messages.	Implement the signals shown in Table 10.7 as a vector of signal message strings.

UNIX bsd_signal Code Replacement

Code that uses the bsd_signal() function should be implemented by using other signal functions in Interix.

The function call bsd_signal(sig, func) can be implemented as follows:

 #include <signal.h> void (*bsd_signal(int sig, void (*func)(int)))(int) {     struct sigaction act, oact;     act.sa_handler = func;     act.sa_flags = SA_RESTART;     sigemptyset(&act.sa_mask);     sigaddset(&act.sa_mask, sig);     if (sigaction(sig, &act, &oact) == -1)          return(SIG_ERR);     return(oact.sa_handler); } 

The preceding code can support calls to bsd_signal in a migrated application; it can also be used to replace use of signal() in BSD-derived applications as long as the signal handler expected a single parameter of type int . If the handler expected any other parameter or parameters, the use of signal() must be modified to use sigaction() .