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From: edward@ucbarpa.Berkeley.EDU (Edward Wang)
Newsgroups: comp.unix.questions
Subject: Re: reliable reads from pipes
Message-ID: <37970@ucbvax.BERKELEY.EDU>
Date: 4 Aug 90 10:55:48 GMT
References: <1990Aug3.233256.29659@NCoast.ORG>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: edward@ucbarpa.Berkeley.EDU.UUCP (Edward Wang)
Organization: University of California, Berkeley
Lines: 31

In article <1990Aug3.233256.29659@NCoast.ORG> atul@NCoast.ORG (Atul Parulekar) writes:
>The following program does not work all the time.
>. . .
>   int fifo[2],proc,n;
>   char line[81];
>
>   pipe (fifo);
>   [fork an exec of pwd]
>   n = read (fifo[0], line, 80);
>   line[n] = 0;
>   . . .

This may be of general interest.

The problem is that pwd may be writing less than the who path at a time.
The solution is to keep reading until end of file (n = 0) or error (n < 0).
There's no danger of looping because some progress is always made.

In the best Unix style,

	char *p, *q;
	for (q = (p = line) + sizeof line - 1;
	     p < q && (n = read(fifo[0], p, q - p)) > 0;
	     p += n)
		;
	*p = 0;
	/* if desired, check for overflow (p == q) or read error (n < 0) */

Something like this is also a good idea when writing,
because signals may cause partial writes to return.
Standard IO is well known for not doing this correctly.