Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!mailrus!cornell!uw-beaver!fluke!ssc-vax!voodoo!jdt
From: jdt@voodoo.UUCP (Jim Tomlinson)
Newsgroups: comp.unix.questions
Subject: Redirecting stdout and stderr (in a program)
Message-ID: <635@voodoo.UUCP>
Date: 19 Dec 89 21:17:08 GMT
Reply-To: jdt@voodoo.UUCP (Jim Tomlinson)
Distribution: na
Organization: BoGART Graphics In Your Face
Lines: 56


Sorry to ask a question that's no doubt been answered a bazillion
times, but the subject lines for last month suggest the answer hasn't
come across recently.  I'm trying to redirect stdout and stderr to a
file; we'll skip why I don't just do this from the shell; suffice to
say that the program is called by inetd, not to mention I'm detaching
the process from any controlling terminal, parent process group, etc.
To cut to the chase: we know stderr is unbuffered and stdout is
buffered.  The following code segment should unbuffer stdout, or at
least flush stdout before we fprintf "and to stderr":

#include <stdio.h>
main()
{
    FILE	*pr_file;
    
    /* close stdout and stderr */
    close(1);
    close(2);

    /* now let's point stdout and stderr at our logfile */
    if (pr_file = fopen("/tmp/testlog", "w"))
    {
	/* we got stdout (next avail. file descriptor); let's dup to
	 * point stderr at it */
	dup(1);
	/* specify unbuffered output */
	if (setvbuf(pr_file, (char *) NULL, _IONBF, 0))
	{
	    perror("setvbuff failed");
	    exit(-1);
	}
	fprintf(stdout, "Printing to stdout\n");
	fflush(pr_file);
	fprintf(stderr, "and to stderr.\n");

    }
    else
    {
	exit(-1);
    }
}

but /tmp/testlog looks like this after the code is run:

and to stderr.
Printing to stdout

Why isn't the output in the proper order?  BTW, this is SGI's
IRIX 3.2 (SYSV with BSD extensions).  Thanx for any help you
can give me on this.


Jim Tomlinson                                                  (206) 234-7741
BoGART Project                               ....uw-beaver!ssc-vax!voodoo!jdt
Boeing Computer Services, Renton, WA           "falling snow, excellent snow"