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Avoid using stdio streams after they have been closed. (#89)

* In closeall(), skip stdin and flush std{err,out} instead of closing.
Otherwise awk could fclose(stdin) twice (it may appear more than once)
and closing stderr means awk cannot report errors with other streams.
For example, "awk 'BEGIN { getline < "-" }' < /dev/null" will call
fclose(stdin) twice, with undefined results.

* If closefile() is called on std{in,out,err}, freopen() /dev/null instead.
Otherwise, awk will continue trying to perform I/O on a closed stdio
stream, the behavior of which is undefined.

--- a/run.c

+++ b/run.c

@@ -1846,7 +1846,10 @@

 			continue;

 		if (ferror(files[i].fp))

 			FATAL("i/o error occurred on %s", files[i].fname);

-		if (files[i].mode == '|' || files[i].mode == LE)

+		if (files[i].fp == stdin || files[i].fp == stdout ||

+		    files[i].fp == stderr)

+			stat = freopen("/dev/null", "r+", files[i].fp) == NULL;

+		else if (files[i].mode == '|' || files[i].mode == LE)

 			stat = pclose(files[i].fp) == -1;

 		else

 			stat = fclose(files[i].fp) == EOF;

@@ -1856,6 +1859,7 @@

 			xfree(files[i].fname);

 		files[i].fname = NULL;	/* watch out for ref thru this */

 		files[i].fp = NULL;

+		break;

  	}

  	tempfree(x);

  	x = gettemp();

@@ -1873,8 +1877,12 @@

 			continue;

 		if (ferror(files[i].fp))

 			FATAL( "i/o error occurred on %s", files[i].fname );

+		if (files[i].fp == stdin)

+			continue;

 		if (files[i].mode == '|' || files[i].mode == LE)

 			stat = pclose(files[i].fp) == -1;

+		else if (files[i].fp == stdout || files[i].fp == stderr)

+			stat = fflush(files[i].fp) == EOF;

 		else

 			stat = fclose(files[i].fp) == EOF;

 		if (stat)

-- 

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