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[libc] Fix exit() and atexit()

Functions registered with atexit() must be called
in reverse order. This implementation also protects
against calling exit() from an atexit() handler.

--- a/libc/include/errno.h

+++ b/libc/include/errno.h

@@ -5,6 +5,7 @@

 #define EDOM   1

 #define EILSEQ 2

 #define ERANGE 3

+#define ENOMEN 4

 extern int errno;

--- a/libc/src/atexit.c

+++ b/libc/src/atexit.c

@@ -1,19 +1,19 @@

 /* See LICENSE file for copyright and license details. */

 #include <stdlib.h>

+#include <errno.h>

 #undef atexit

-extern void (*_atexitf[_ATEXIT_MAX])(void);

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

+extern unsigned _exitn;

 int

 atexit(void (*fun)(void))

 {

-	void (**bp)(void);

-

-	for (bp = _atexitf; bp < &_atexitf[_ATEXIT_MAX] && *bp; ++bp)

-		/* nothing */;

-	if (bp == &_atexitf[_ATEXIT_MAX])

-		return 0;

-	*bp = fun;

-	return 1;

+	if (_exitn == _ATEXIT_MAX) {

+		errno = ENOMEN;

+		return -1;

+	}

+	_exitf[_exitn++] = fun;

+	return 0;

 }

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

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

@@ -3,19 +3,13 @@

 #include <stdlib.h>

 #undef exit

-void (*_atexitf[_ATEXIT_MAX])(void);

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

+unsigned _exitn;

 void

 exit(int status)

 {

-	void (**bp)(void);

-	int i;

-

-	for (i = _ATEXIT_MAX-1; i >= 0; --i) {

-		if (bp = _atexit[i]) {

-			*_atexit[i] = NULL;

-			(*bp)();

-		}

-	}

+	while (_exitn > 0)

+		(*_exitf[--exitn])();

 	_Exit(status);

 }