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From: mjr@hussar.dco.dec.com (Marcus J. Ranum)
Newsgroups: comp.lang.c,comp.unix.programmer
Subject: Re: Request code for log-file mechanism
Keywords: c log-file source
Message-ID: <1990Oct17.213140.19516@decuac.dec.com>
Date: 17 Oct 90 21:31:40 GMT
References: <1990Oct17.094623.2381@westc.uucp>
Sender: news@decuac.dec.com (Network News)
Reply-To: mjr@hussar.dco.dec.com (Marcus J. Ranum)
Organization: Digital Equipment Corp., Washington Ultrix Resource Center
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In article <1990Oct17.094623.2381@westc.uucp> marco@westc.uucp (Marco Nijdam) writes:
>We are writing an application that must keep a log-file of the
>actions that where taken. It is possible that more than one
>process writes to a log file at the same time.

	Depending on how time-critical the logging function is, you might
want to either use a log server (like syslog(), which is already written
and comes with most bsd Unixes) - otherwise, you can roll your own by
just keeping the file open, attempting to lock it, seeking to the eof,
writing, and releasing the lock. Something like:

/*
don't take this as gospel! for the purpose of example, error checks are
omitted. besides, where do you log failures to write to your log ?
*/
#include	<stdio.h>
#include	<sys/file.h>

log(fd,txt)
int	fd;
char	*txt;
{
	int	rv;
	int	ln = strlen(txt);

	while(flock(fd,LOCK_EX))
		sleep(1);

	(void)lseek(fd,0L,SEEK_END);

	rv = (write(fd,txt,ln) == ln);

	(void)flock(fd,LOCK_UN);
	return(rv);
}