Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.5 Interrupted System Calls 329

To prevent applications from having to handle interrupted system calls, 4.2BSD

introduced the automatic restarting of certain interrupted system calls. The system calls

that wereautomatically restarted areioctl,read,readv,write,writev,wait,and

waitpid.Aswe’ve mentioned, the first five of these functions areinterrupted by a

signal only if they areoperating on a slow device;waitandwaitpidarealways

interrupted when a signal is caught. Since this caused a problem for some applications

that didn’t want the operation restarted if it was interrupted, 4.3BSD allowed the

process to disable this feature on a per-signal basis.

POSIX.1 requires an implementation to restart system calls only when theSA_RESTARTflag is

in effect for the interrupting signal. As we will see in Section 10.14, this flag is used with the

sigactionfunction to allow applications to request that interrupted system calls be

restarted.

Historically,when using thesignalfunction to establish a signal handler,implementations

varied with respect to how interrupted system calls werehandled. SystemVnever restarted

system calls by default. BSD, in contrast, restarted them if the calls wereinterrupted by

signals. On FreeBSD 8.0, Linux 3.2.0, and Mac OS X 10.6.8, when signal handlers areinstalled

with thesignalfunction, interrupted system calls will be restarted. The default on Solaris 10,

however, is to return an error (EINTR)instead when system calls areinterrupted by signal

handlers installed with thesignalfunction. By using our own implementation of the

signalfunction (shown in Figure10.18), we avoid having to deal with these differences.

One of the reasons 4.2BSD introduced the automatic restart feature is that

sometimes we don’t know that the input or output device is a slow device. If the

program we write can be used interactively,then it might be reading or writing a slow

device, since terminals fall into this category.If we catch signals in this program, and if

the system doesn’t provide the restart capability,then we have to test every read or

write for the interrupted error return and reissue the read or write.

Figure10.3 summarizes the signal functions and their semantics provided by the

various implementations.

Automatic restart

of interrupted

system calls?

Functions System remains installedSignal handler block signalsAbility to

ISO C, POSIX.1 unspecified unspecified unspecified

V7, SVR2, SVR3 never

SVR4, Solaris never

4.2BSD • •always

4.3BSD, 4.4BSD,

FreeBSD, Linux,

signal

Mac OS X

• • default

POSIX.1, 4.4BSD,

SVR4, FreeBSD,

Linux, Mac OS X,

Solaris

sigaction • • optional

Figure 10.3 Features provided by various signal implementations