Advanced Programming in the UNIX® Environment

ptg10805159

936 Solutions to Selected Exercises Appendix C

Then usefdopento associate the pipe descriptors with a standardI/O stream,

and set the streams to be line buffered. Do this beforethewhileloop that reads

from standardinput:

if ((fpin = fdopen(fd2[0], "r")) == NULL)

err_sys("fdopen error");

if ((fpout = fdopen(fd1[1], "w")) == NULL)

err_sys("fdopen error");

if (setvbuf(fpin, NULL, _IOLBF, 0) < 0)

err_sys("setvbuf error");

if (setvbuf(fpout, NULL, _IOLBF, 0) < 0)

err_sys("setvbuf error");

Thewriteandreadin thewhileloop arereplaced with

if (fputs(line, fpout) == EOF)

err_sys("fputs error to pipe");

if (fgets(line, MAXLINE, fpin) == NULL) {

err_msg("child closed pipe");

break;

}

15.6 Thesystemfunction callswait,and the first child to terminate is the child

generated bypopen.Since that’s not the child thatsystemcreated, it callswait

again and blocks until thesleepis done. Thensystemreturns. Whenpclose

callswait, an error is returned, since thereare nomorechildren towaitfor.

Thenpclosereturns an error.

15.7 Although the details vary by platform (see FigureC.19),selectindicates that

the descriptor is readable. After all the data has been read,readreturns 0 to

indicate the end of file. But withpoll,thePOLLHUPevent is returned, and this

can happen while there is still data to be read. Once we have read all the data,

however,readreturns 0 to indicate the end of file. After all the data has been

read, thePOLLINevent is not returned, even though we need to issue areadto

receive the end-of-file notification (the return value of 0).

FreeBSD Linux Mac OS X Solaris

Operation 8.0 3.2.0 10.6.8 10

selectonreadend of pipe withwriteend closed R/W/E R R/W R/W/E

pollonreadend of pipe withwriteend closed R/HUP HUP INV HUP

selectonwriteend of pipe withreadend closed R/W/E R/W R/W R/W

pollonwriteend of pipe withreadend closed R/HUP W/ERR INV HUP

Figure C.19 Pipe behavior withselectandpoll

The conditions shown in FigureC.19 include R (readable), W (writable), E

(exception), HUP (hangup), ERR (error), and INV (invalid file descriptor). With

an output descriptor that refers to a pipe that has been closed by the reader,

selectindicates that the descriptor is writable. But when we callwrite,the

SIGPIPEsignal is generated. If we either ignorethis signal or return from its