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libc: Don't use atexit() to flush streams

__putc() was using atexit() to install a handler to flush
all the output streams, but this could drive to wrong behavioud
because we cannot guarantee that handlers installed before calling
__putc() are not going to buffer anything to output streams.

--- a/src/libc/libc.h

+++ b/src/libc/libc.h

@@ -56,3 +56,4 @@

 extern void (*_exitf[])(void);

 extern unsigned _exitn;

+extern void (*_flushall)(void);

--- a/src/libc/stdio/__putc.c

+++ b/src/libc/stdio/__putc.c

@@ -13,8 +13,6 @@

 int

 __putc(int ch, FILE *fp)

 {

-	static int first = 1;

-

 	if (fp->flags & _IOERR)

 		return EOF;

@@ -29,15 +27,7 @@

 	if (fp->buf == NULL && _allocbuf(fp))

 		return EOF;

-

-	if (first) {

-		if (atexit(cleanup)) {

-			fp->flags |= _IOERR;

-			errno = ENOMEM;

-			return EOF;

-		}

-		first = 0;

-	}

+	_flushall = cleanup;

 	if (fp->flags & _IOLBF) {

 		if (fp->wp == fp->lp && _flsbuf(fp))

--- a/src/libc/stdlib/exit.c

+++ b/src/libc/stdlib/exit.c

@@ -1,11 +1,13 @@

 #include <stdlib.h>

 #include "../libc.h"

+#include "../syscall.h"

 #undef exit

 void (*_exitf[_ATEXIT_MAX])(void);

 unsigned _exitn;

+void (*_flushall)(void);

 void

 exit(int status)

@@ -12,6 +14,7 @@

 {

 	while (_exitn > 0)

 		(*_exitf[--_exitn])();

-

-	_Exit(status);

+	if (_flushall)

+		(*_flushall)();

+	_exit(status);

 }