Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.15 sigsetjmpandsiglongjmpFunctions 355

Example —signal_intrFunction

Figure10.19 shows a version of the signal function that tries to prevent any

interrupted system calls from being restarted.

#include "apue.h"

Sigfunc *

signal_intr(int signo, Sigfunc *func)

{

struct sigaction act, oact;

act.sa_handler = func;

sigemptyset(&act.sa_mask);

act.sa_flags = 0;

#ifdef SA_INTERRUPT

act.sa_flags |= SA_INTERRUPT;

#endif

if (sigaction(signo, &act, &oact) < 0)

return(SIG_ERR);

return(oact.sa_handler);

}

Figure 10.19Thesignal_intrfunction

For improved portability, we specify theSA_INTERRUPTflag, if defined by the system,

to prevent interrupted system calls from being restarted.

10.15 sigsetjmpandsiglongjmp Functions

In Section 7.10, we described thesetjmpandlongjmpfunctions, which can be used

for nonlocal branching. Thelongjmpfunction is often called from a signal handler to

return to the main loop of a program, instead of returning from the handler.Wesaw

this approach in Figures 10.8 and 10.11.

There is a problem in callinglongjmp,however.When a signal is caught, the

signal-catching function is entered, with the current signal automatically being added to

the signal mask of the process. This prevents subsequent occurrences of that signal

from interrupting the signal handler.Ifwelongjmpout of the signal handler,what

happens to the signal mask for the process?

Under FreeBSD 8.0 and Mac OS X 10.6.8,setjmpandlongjmpsave and restorethe signal

mask. Linux 3.2.0 and Solaris 10, however,donot do this, although Linux supports an option

to provide BSD behavior.FreeBSD and Mac OS X provide the functions_setjmpand

_longjmp,which do not save and restorethe signal mask.

To allow either form of behavior,POSIX.1 does not specify the effect ofsetjmpand

longjmpon signal masks. Instead, two new functions,sigsetjmpandsiglongjmp,

aredefined by POSIX.1. These two functions should always be used when branching

from a signal handler.