Advanced Programming in the UNIX® Environment

ptg10805159

354 Signals Chapter 10

Example —signalFunction

Let’s now implement thesignalfunction usingsigaction.This is what many

platforms do (and what a note in the POSIX.1 Rationale states was the intent of POSIX).

Systems with binary compatibility constraints, on the other hand, might provide a

signalfunction that supports the older,unreliable-signal semantics. Unless you

specifically requirethese older,unreliable semantics (for backwardcompatibility), you

should use the following implementation ofsignalor callsigactiondirectly.(As

you might guess, an implementation ofsignalwith the old semantics could call

sigactionspecifyingSA_RESETHANDandSA_NODEFER.) All the examples in this

text that callsignalcall the function shown in Figure10.18.

#include "apue.h"

/* Reliable version of signal(), using POSIX sigaction(). */

Sigfunc *

signal(int signo, Sigfunc *func)

{

struct sigaction act, oact;

act.sa_handler = func;

sigemptyset(&act.sa_mask);

act.sa_flags = 0;

if (signo == SIGALRM) {

#ifdef SA_INTERRUPT

act.sa_flags |= SA_INTERRUPT;

#endif

}else {

act.sa_flags |= SA_RESTART;

}

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

return(SIG_ERR);

return(oact.sa_handler);

}

Figure 10.18An implementation ofsignalusingsigaction

Note that we must usesigemptyset to initialize the sa_mask member of the

structure. We’renot guaranteed that act.sa_mask = 0 does the same thing.

We intentionally set theSA_RESTARTflag for all signals other thanSIGALRM,so

that any system call interrupted by these other signals will be automatically restarted.

The reason we don’t wantSIGALRMrestarted is to allow us to set a timeout for I/O

operations. (Recall the discussion of Figure10.10.)

Some older systems, such as SunOS, define theSA_INTERRUPTflag. These systems

restart interrupted system calls by default, so specifying this flag causes system calls to

be interrupted. Linux defines the SA_INTERRUPT flag for compatibility with

applications that use it, but by default does not restart system calls when the signal

handler is installed withsigaction.The Single UNIX Specification specifies that the

sigactionfunction not restart interrupted system calls unless theSA_RESTARTflag is

specified.