Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.18 systemFunction 367

We first see whether the default action will occur; if so, we flush all the standardI/O

streams. This is not equivalent to callingfcloseon all the open streams (since it just

flushes them and doesn’t close them), but when the process terminates, the system

closes all open files. If the process catches the signal and returns, we flush all the

streams again, since the process could have generated moreoutput. The only condition

we don’t handle is the case wherethe process catches the signal and calls_exitor

_Exit.Inthis case, any unflushed standardI/O buffers in memory arediscarded. We

assume that a caller that does this doesn’t want the buffers flushed.

Recall from Section 10.9 that if callingkillcauses the signal to be generated for the

caller,and if the signal is not blocked (which we guarantee in Figure10.25), then the

signal (or some other pending, unlocked signal) is delivered to the process beforekill

returns. Weblock all signals exceptSIGABRT, so we know that if the call tokill

returns, the process caught the signal and the signal handler returned.

10.18 systemFunction

In Section 8.13, we showed an implementation of thesystemfunction. That version,

however,did not do any signal handling. POSIX.1 requires thatsystemignoreSIGINT

andSIGQUITand blockSIGCHLD.Beforeshowing a version that handles these signals

correctly,let’s see why we need to worry about signal handling.

Example

The program shown in Figure10.26 uses the version ofsystemfrom Section 8.13 to

invoke theed( 1 )editor.(This editor has been part of UNIX systems for a long time.We

use it herebecause it is an interactive program that catches the interrupt and quit

signals. If we invokeedfrom a shell and type the interrupt character, it catches the

interrupt signal and prints a question mark. Theedprogram also sets the disposition of

the quit signal so that it is ignored.) The program in Figure10.26 catches bothSIGINT

andSIGCHLD.If we invoke the program, we get

$./a.out

a append text to the editor’s buffer

Here is one line of text

. period on a line by itself stops append mode
1,$p print first through last lines of buffer to see what’sthere
Here is one line of text
wtemp.foo write the buffer to a file
25 editor says it wrote 25 bytes
q and leave the editor
caught SIGCHLD
When the editor terminates, the system sends theSIGCHLDsignal to the parent (the
a.outprocess). Wecatch it and return from the signal handler.But if it is catching the
SIGCHLDsignal, the parent should be doing so because it has created its own children,
so that it knows when its children have terminated. The delivery of this signal in the