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To wait for any child process. This is the same as

wait()

positive

To wait for the specific child whose process ID is equal to

pid

zero

To wait for any child process whose process group ID is equal to that of the calling

process.

less than

-1

To wait

for any child process whose process group ID is equal to the absolute value of

pid

Process groups are normally used only by shells supporting job control and not by ordinary

applications. This book does not discuss process groups.

The

options

argument is constructed from the bitwise OR of zero or more of the following

flags, defined in the header

age 108

WNOHANG

Causes the

waitpid()

function not to suspend execution of the calling process if

status is not immediately available for any of the child processes specified by

pid

WUNTRACED

Causes the status of any child processes specified by

pid

that are stopp

ed, and

whose status has not yet been reported since they stopped, to be reported to the

calling process. This is normally used only by the shell program to support job

control.

By the way, the name

WUNTRACED

comes from BSD. BSD supports several other

func

tions that are not part of POSIX, so the name made sense in the BSD context.

The

wait()

and

waitpid()

functions release any resources that the child was using. If you

do not care about the final status of the child, it is not good enough to simply omit the

wait()

waitpid()

call. A common way to produce a child that does not need

to be waited for is

fork()

a child and

wait()

on the child. The child performs another

fork()

to produce

a grandchild. The child then exits and the parent's wait returns. The grandchild is thus

disinherited by the grandparent. The spawned grandchild wi

ll release all its resources when it

terminates because there is no process left to wait for it. This technique is much more portable

than the alternative:

system("command &");

which depends on features that are outside the scope of POSIX.1.

Terminating the Current Process

There are four ways to terminate the current process:

Returning from

main()

Calling

exit()

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Calling

_exit()

Calling

abort()

Let's look at them in detail.

Returning from main()

The normal way for a program to terminate is to execute a return (

EXIT_SUCCESS

) statement

from the

main()

function. The action of returning a value from

main()

is exactly the same

as calling

exit()

with that value.

Executing a

return

with no value is not portable.

age 109

Calling exit()

The

exit()

function causes normal program termination. The

EXIT_SUCCESS

macro can

be used to indicate successful termination. Since the POSIX standard requires that

EXIT_SUCCESS

be defined as zero, it is safe to write

exit(0)

, keeping with historical

pract

ice. The call

exit(0)

is extremely portable.

The

exit()

function performs the following functions:

All functions registered by the Standard C

atexit()

function are called in the reverse

order of registration. If any of these functions calls

exit()

, the results are not portable.

All open output streams are flushed (data written out) and the streams are closed.

All files created by

tmpfile()

are deleted.

The

_exit()

function is called.

Calling _exit()

The

_exit()

function performs operating system-specific program termination functions.

These include:

All open file descriptors and directory streams are closed.

If the parent process is executing

a wait()

waitpid()

, the parent wakes up and

status is made available.

If the parent is not executing

a wait()

waitpid()

, the status is saved for return to

the parent on a subsequent

wait()

waitpid()

Children of the terminated process are assigned a new parent process ID. Note: the

termination of a parent does not directly terminate its children.

If the implementation supports the

SIGCHLD

signal, a

SIGCHLD

is sent to the parent.

Several job control signals are sent.

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All of the resources used by the process are returned.

Portable programs should use

exit()

instead of

_exit()

. The

_exit()

function exists

mainly because of the structure of traditional implementations and also the structure of

standards committees. The

exit()

function is defined by the C standard with some fe

atures

that are beyond the scope of POSIX. The only reason for an application to call

_exit()

is to

defeat the flushing of streams and the calling of functions registered by

atexit()

age 110

Calling abort()

The

abort()

function causes abnormal program termination. Exactly what that means is not

well-defined.

Portable applications should avoid using

abort()

except in the case of fatal errors. On some

systems, it may provide useful debugging information, such as a

core

file.

Terminating Another Process

The

kill()

function can be used to terminate another process. For example:

kill(pid,SIGKILL);

will kill the process identified by

pid

. It returns zero on success and

-1

on failure.

In general, it is safe and legal to kill your children and their children. It may be legal to kill

other processes in the system; however, ordinary applications should not kill any process that

they did not create (or cause to be created).

The

kill()

function can be used for other functions unrelated to terminating a process.

kill()

is discussed in greater detail later in this chapter.

Signals

Signals inform a process of the occurrence of an event. There are two general types of events:

Errors

For example, division by zero, illegal instruction, or an invalid memory

reference.

Asynchronous events

For example, termination of a child or parent process.

The general concept of signals is as old as UNIX. Early versions of UNIX had a number of

design flaws in the signal mechanism. The BSD system fixed many of these problems, and the

signals standardized by POSIX are very similar to BSD signals with a few imp

rovements.

Each process has an action to be taken in response to each signal defined by the system. A

signal is

delivered

to a process when the appropriate action is taken.

During the time between the generation of a signal and the delivery of that signal, the signal is

pending.

In most cases, this interval cannot be detected by an application. However, a signal

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can be

blocked.

Each process has a

signal mask

that defines the set of signals currently blocked from delivery

to it. The signal mask from a process is inherited from its parent. The

sigaction()

sigprocmask()

, and

sigsuspend()

functions control the manipulation of the si

gnal

mask.

age 111

One of several actions is taken when a signal is delivered:

The process is terminated.

The signal is ignored.

The process is stopped.

The process is continued.

The signal is

caught

by a signal-handling function in the application.

There is a set of standard signals which a process can use. These signals are:

SIGABRT

Abnormal termination signal caused by the

abort()

function. A portable program

should avoid catching

SIGABRT

SIGALRM

The timer set by the

alarm()

function has timed-out.

SIGFPE

Arithmetic exception, such as overflow or division by zero.

SIGHUP

Han

gup detected on controlling terminal or death of a controlling process.

SIGILL

Illegal instruction indicating a program error. Applications may wish to catch this signal

and attempt to recover from bugs. A portable program should not intentionally generate

illegal instructions.

After a

SIGILL

is caught, the only portable thing to do is to

siglongjmp()

back to

a known place in your program (or call

exit()

SIGINT

Interrupt special character typed on controlling keyboard.

SIGKILL

Termination signal. This si

gnal cannot be caught or ignored.

SIGPIPE

Write to a pipe with no readers.

SIGQUIT

Quit special character typed on controlling keyboard.

SIGSEGV

Invalid memory reference. Like

SIGILL

, portable programs should not intentionally

generate invalid memory refer

ences.

SIGTERM

Termination signal.

SIGUSR1

Application-defined signal 1.

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SIGUSR2

Application-defined signal 2.

Unless the application changes the action, any of the above signals cause the abnormal

termination of the process.

* Even non-portable programs should avoid intentionally generating illegal instructions. What

happens if a new model defines the instruction to do something?

age 112

There is also a set of job control signals. They are:

SIGCHLD

Child process terminated or stopped. By default, this signal is ignored.

SIGCONT

Continue the process if it is currently stopped; otherwise, ignore the signal.

SIGSTOP

Stop signal. This signal cannot be caught or ignored.

SIGTSTP

Stop special chara

cter typed on the controlling keyboard.

SIGTTIN

Read from the controlling terminal attempted by a member of a background process

group.

SIGTTOU

Write to controlling terminal attempted by a member of a background process group.

Most systems will have signals in addition to those listed here. The POSIX interface allows an

application to manipulate the signals it knows about without disturbing the signals it does not

know about.

Signal Actions

There are three types of actions that can be associated with a signal:

SIG_DFL

SIG_IGN

, or

a pointer to a function.

The actions for these values are:

SIG_DFL

Signal-specific default action.

SIG_IGN

Ignore the signal.

It is possible to ignore

SIGFPE, SIGILL

, and

SIGSEGV

; however, programs with

illegal instructions, erroneous arithmetic operations, and invalid memory references

are not portable.

The def

ault for

SIGCHLD

is to ignore the signal. Applications that wish to ignore

SIGCHLD

should set the action to be

SIG_DFL

, not to

SIG_IGN

Pointer to a function to catch signal

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On delivery of the signal, the receiving process executes the signal-catching fu

nction.

After returning from the signal-catching function, the process resumes execution.

Signal-Catching Functions

A signal-catching function receives control when a signal is delivered. A signal is somewhat

like an unseen hand placing a call statement in the middle of our program—the signal-catching

function gets control and is able to do things. When the signal catch

er returns, the interrupted

program continues without a trace.

* If a process sets the action for the

SIGCHLD

signal to

SIG

IGN

, the behavior is unspecified.

age 113

There are some cautions that a signal-catching function must observe:

While a portable program can catch errors such as illegal instructions; it should not assume

that it can continue from a

SIGFPE, SIGILL

, or

SIGSEGV

signal. Thus a portable

program can establish signal catchers to be more robust, but it should not

depend

on illegal

instructions or invalid memory references.

The program may be in the middle of some function when the signal is delivered. It is not

safe to call arbitrary functions from a signal-catching function. The following library

functions are defined by the standard as safe:

_exit()

getegid()

rename()

tcdrain()

access()

geteuid()

rmdir()

tcflow()

alarm()

getgid()

setgid()

tcflush()

cfgetispeed()

getgroups()

setpgid()

tcgetattr()

cfgetospeed()

getpgrp()

setsid()

tcgetpgrp()

cfsetispeed()

getpid()

setuid()

tcsendbreak()

cfsetosp

eed()

getppid()

sigaction()

tcsetattr()

chdir()

getuid()

sigaddset()

tcsetpgrp()

chmod()

kill()

sigdelset()

time()

chown()

link()

sigemptyset()

times()

close()

lseek()

sigfillset()

umask()

creat()

mkdir()

sigismember()

uname()

dup2()

mkfifo()

sigpending()

unlink()

dup()

open()

sigprocmask()

ustat()

execle()

pathconf()

sigsuspend()

utime()

execve()

pause()

sleep()

wait()

fcntl()

pipe()

stat()

waitpid()

fork()

read()

sysconf()

write()

fstat()

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All other library functions (including

printf()

and friends) are

unsafe

and should not be

called from signal-catching functions.

Examine and Change Signal Action

Both Standard C and the POSIX standard define a set of signal-handling functions. The

Standard C functions are limited. They may be useful for programs that need to operate on

non-POSIX systems, such as MS/DOS or System V.3.

Standard C Signals

First, the C Standard defines only a subset of the POSIX signals. These signals are:

SIGABRT

Abnormal termination signal. This is caused by the

abort()

function. Standard C

suggests that other events may cause

SIGABRT

; however, it does not say what those

events might be.

SIGFPE

Arithmetic exception, such as overflow or division by zero.

* IEEE Std 1003.1-1988 defines

ustat()

as safe to call from signal-catching function. The POSIX

standard never defines

ustat()

, and it was deleted from the 1990 revision.

age 114

SIGILL

Illegal instruction.

SIGINT

Interrupt special character typed on controlling keyboard.

SIGSEGV

Invalid memory reference.

SIGTERM

Termination signal.

Standard C does not require that any of these signals be generated. An illegal memory

reference may, or may not, generate a

SIGSEGV

The Standard C function used to specify signal handling is called

signal()

and is defined

by:

void (*signal(int sig, void(*func)(int)))(int);

where

sig

is a signal number. The

func

argument is a pointer to a signal-catching function or

to one of the following macros:

SIG_DFL

Set the signal to the default action.

SIG_IGN

Ignore the signal.

For example:

signal(SIGINT,SIG_IGN);

will cause the interrupt key (usually Control-C) to be ignored, and:

signal(SIGSEGV,oops);

will cause the function

oops(SIGSEGV)

to be called on illegal memory references.

Standard C also defines the

raise()

function as:

int raise(int sig);

to send signal

sig

to the executing program. The

raise()

function returns zero if successful

and non-zero if unsuccessful. The

raise()

function should be used only in programs that

need to meet the C standard and do not use any POSIX features. The

raise()

function is

more portable than

kill()

for non-POSIX systems that conform to the C standard. The

kill()

function is much more portable to older UNIX systems.

POSIX Signals

The Standard C

signal()

function has several problems:

There is no way to determine the current action for a signal. This means that a called

function cannot use the

signal()

function without disturbing the caller. There is no way

to save and restore signal state.

When a signal occurs, there is no way to block other signals to keep the signal handler from

being interrupted.

age 115

There is no way for an implementation to cleanly extend the signal mechanism.

The POSIX-defined signal functions correct these problems.

The main function for manipulating signals is

sigaction()

. It is defined as:

int sigaction(int sig, const struct sigaction *act,

struct sigaction *oact);

The

sigaction

structure is defined in the header

<signal.h>

to include the following

members:

Member Type

Member Name

Description

void(*)()

sa_handler

SIG

DFL

for the default action.

or:

SIG

IGN

to ignore this signal.

or:

pointer to the signal-catching function.

sigset_t

sa_mask

Additional signals to be blocked during the execution of the

signal-ca

tching function. (

sigset_t

and blocked signals will

be defined soon.)

int

sa_flags

This member is used only for the

SIGCHLD

signal. If the value

NOCLDSTOP

is used, then

SIGCHLD

will not be generated

when children stop.

There may be other flags defined b

y a particular implementation.

A portable program should not use them. It should not be

disturbed by them either.

The

sigaction()

function sets the structure pointed to by

oact

to the old action for signal

s ig and then takes the action indicated by the structure pointed to by act.

There may be additional members in a given implementation's

struct sigaction

Portable programs are guaranteed that these members will not affect them. To use

implementation-defined members, implementation-defined flags must be set.

age 116

Example: Read with a timeout

Before getting too deeply into signals, it would be useful to go through a complete example.

The following program reads a line from the user. If the user does not type anything for 30

seconds, the

SIGALRM

signal will interrupt the read and the

gettext()

function will return

zero. The caller of

gettext()

can then take some alternate action, such a

s, giving the user

some help.

The program looks like this:

#define _POSIX_SOURCE 1

/* System Headers */

#include <stdio.h>

#include <signal.h>

#include <unistd.h>

/* Local Headers */

#include "panic.h"

/* Macros */

#define TIMEOUT 30

#define TRUE 1

#define FALSE 0

/* File scope variables */

volatile int flag; /* The keyword volatile warns the

* compiler that the variable

flag

* may change in a way that is not

* predictable by the compiler.

/* External variab

les */

/* NONE */

/* External functions */

/* NONE */

/* Structures and Unions */

/* Signal Catching Functions */

* The ding() func

tion catches the SIGALRM signal and

* merely sets flag to FALSE.

void ding()

{

flag = FALSE;

return;

}

* The gettext function reads a line from the user's

* console. If the line is not typed within TIMEOUT

* seconds, the gettext() function aborts the read and

age 117

* returns zero.

int gettext(char *buffer,int bufsize)

{

struct sigaction act,oact;

int nchars;

act.sa_handler = ding; /* Call ding() when the

* alarm goes off

sigemptyset(&act.sa_mask);

act.sa_flags = 0;

if (sigaction(SIGALRM,&act,&oact) != 0) PANIC;

flag = TRUE;

(void)alarm(TIME

OUT);

nchars = read(STDIN_FILENO,buffer,bufsize);

(void)alarm(0); /* Cancel outstanding SIGALRM (if any) */

/* Restore previos signal handler for SIGALRM */

if (sigaction(SIGALRM,&oact,NULL) != 0) PANIC;

if (flag) return(nchars);

return(0);

}

Signal Sets

The POSIX standard allows a great deal of flexibility for an implementation while still

providing portable interfaces. This is evident in the type

sigset_t

, which holds some sets of

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