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From: levy@cbnewsc.ATT.COM (Daniel R. Levy)
Newsgroups: comp.lang.c
Subject: Re: I/O redirection
Summary: a few nit picks
Message-ID: <15008@cbnewsc.ATT.COM>
Date: 10 Apr 90 23:09:16 GMT
References: <1990Apr2.144841.12905@mentor.com> <1418@quando.UUCP>
Organization: AT&T Bell Laboratories, Skokie IL
Lines: 75

In article <1418@quando.UUCP>, cmueller@quando.UUCP (Christoph Mueller) writes:
< In article <1990Apr2.144841.12905@mentor.com> swhitchurch@mentor.com (Steve Whitchurch) writes:
< >I have a question: I need to change or redirect "stdout" from inside
< >a C function. What I want to do is capture the output of a printf and
< >put the results into a file. so it would go something like this:
< > redirect_stdout ();
< > call_function_that_prints_to_stdout ();
< > reset_stdout ();

< #include <stdio.h>
< static int filedescr;
< int redirect_stdout (filename)
< char *filename;
< {
< 	filedescr = dup (1);
< 	if (freopen (filename, "w", stdout) == (FILE *) 0)
< 	{
< 		return (-1); /* redirection failed */
< 	}
< 	else
< 	{
< 		return (0);  /* OK! */
< 	}
< }
< void reset_stdout ()
< {
	/*
< 	fclose (stdout);
	^^^^^^^^^^^^^^^^

	It is unsafe to use stdout after fclose()'ing it.  Better to
	*/

	fflush(stdout);
	close(1);

	/*
	You may want to be sure fd 0 is open first.  Maybe the program
	closed it somewhere along the line; you're OK if it was closed when
	the program started, because "filedescr" will be fd 0 in that event.
	Otherwise, the dup() will return fd 0, and stdio writes to "stdout"
	will go to a still-closed fd 1.
	*/

	{
#include <sys/types.h>	/* O.K., this should be at the top of the file, but */
#include <sys/stat.h>	/* it is more self-explanatory here <yes it works> */
	struct stat s;
	int fd0_open;
	if (!(fd0_open=(fstat(0,&s)==0))) open("/dev/null",0);
< 	dup (filedescr);
	if (fd0_open) close(0);
	}
< }
< main ()
< {
< 	char file [80];
< 	strcpy (file, "/tmp/out");
	/*
< 	redirect (file);

	Presumably redirect_stdout() was intended
	*/

	redirect_stdout(file);

< 	printf ("This will be written into a file.\n");
< 	reset_stdout ();
< 	printf ("This will be written to stdout.\n");
< }
-- 
Daniel R. Levy                     >>> God: just say "yes" <<<
AT&T Bell Laboratories     UNIX(R) mail:  att!ttbcad!levy, att!cbnewsc!levy
5555 West Touhy Avenue     Any opinions expressed in the message above are
Skokie, Illinois  60077    mine, and not necessarily AT&T's.