Advanced Programming in the UNIX® Environment

ptg10805159

338 Signals Chapter 10

Also understand that the test for process existence is not atomic. By the time that

killreturns the answer to the caller,the process in question might have exited, so the

answer is of limited value.

If the call tokillcauses the signal to be generated for the calling process and if the

signal is not blocked, eithersignoor some other pending, unblocked signal is delivered

to the process beforekillreturns. (Additional conditions occur with threads; see

Section 12.8 for moreinformation.)

10.10 alarmandpause Functions

Thealarmfunction allows us to set a timer that will expire at a specified time in the

future. When the timer expires, theSIGALRMsignal is generated. If we ignore or don’t

catch this signal, its default action is to terminate the process.

#include <unistd.h>

unsigned int alarm(unsigned intseconds);

Returns: 0 or number of seconds until previously set alarm

Thesecondsvalue is the number of clock seconds in the futurewhen the signal should

be generated. When that time occurs, the signal is generated by the kernel, although

additional time could elapse beforethe process gets control to handle the signal,

because of processor scheduling delays.

Earlier UNIX System implementations warned that the signal could also be sent up to 1 second

early.POSIX.1 does not allow this behavior.

There is only one of these alarm clocks per process. If, when we callalarm,a

previously registered alarm clock for the process has not yet expired, the number of

seconds left for that alarm clock is returned as the value of this function. That

previously registered alarm clock is replaced by the new value.

If a previously registered alarm clock for the process has not yet expired and if the

secondsvalue is 0, the previous alarm clock is canceled. The number of seconds left for

that previous alarm clock is still returned as the value of the function.

Although the default action forSIGALRMis to terminate the process, most processes

that use an alarm clock catch this signal. If the process then wants to terminate, it can

perform whatever cleanup is required beforeterminating. If we intend to catch

SIGALRM, we need to be careful to install its signal handler beforecallingalarm.Ifwe

callalarmfirst and aresentSIGALRMbeforewecan install the signal handler,our

process will terminate.

Thepausefunction suspends the calling process until a signal is caught.

#include <unistd.h>

int pause(void);

Returns:−1witherrnoset toEINTR